CYTOLOGY

CYTOLOGY. CYTOPLASM STRUCTURE. CELL NUCLEUS. CELL REPRODUCTION. AGING AND DEATH OF THE CELL.

Using lectures (on the web-page of the department posted the presentation text and lectures), books, additional literature and other sources, students must to prepare the following theoretical questions:

1.     Cytoplasm structural components.

2.     What does hyaloplasm consist of?

3.     Organelles: attribute and classification:

a) in accordance with its meaning;

b) accordingly to its size;

c) by the structure.

4.     Microscopic- and ultrastructure and functions of common meaning organelles (mitochondria, Golgi complex, endoplasmic reticulum, lysosomes, peroxysomes, ribosomes, cell center, filaments and microtubules).

5.     Special organelles: species, location and functions.

6.     Inclusions: attribute, differences from organelles, classification.

7.     Morphofunctional characteristic of inclusions.

8.     General morphology of the interphase cell nucleus and their biological significance.

9.     Nuclear envelope ultrastructure and functions.

10. Chromatin chemical compounds. Euchromatin, heterochromatin.

11. Chromosomes types and significance. Thin structure and chemical composition of chromosomes.

12. Human karyotype (chromosomal set).

13. Sex chromatin, its investigation significance in medicine.

14. Nucleolus and karyoplasma structure and compositions.

15. Cell cycle definition. Interphase stages characteristic features.

16. Main types of cell division.

17. Mitosis stages and reasons that force the cell to divide.

18. Nucleus and cytoplasm changes in different stages of mitosis.

19. Endomitosis morphologic features and examples.

20. Meiosis peculiarities.

21. Aging and death of the cell. Paranecrosis and degeneration.

 

Main tasks of histology

1.  Understanding cell, tissue, and organ structure at levels not visible to the unaided eye, including 3-dimensional relationships among their biochemical constituents.

2.           Understanding the relationship between the substructure and the normal functions of cell, tissues, and organs.

3.           Establishing a basis for learning histopathology – the relationship between abnormal tissue and organ structure and functional defects.

4.           Providing a basis for treating diseased and injured tissues and organs. In medicine, this is the ultimate goal.

 

Histology means “the study of tissues”.

Histology is classed as a subdiscipline of anatomy (“cutting apart”), because its methods involve dividing tissues and organs into pieces and preparing them for microscopic examination and chemical analyses. Two aspects of the subject are distinguished; special histology deals with the arrangement and special adaptations of tissues in the various organs whereas general histology deals with the components of the individual tissues.

Tissue structure and tissue function are so closely related that neither can be fully understood without an understanding of the other. Their relationship should be the main focus of both your initial study and your review.

 

GENERAL FEATURES OF CELL

 

Cells are the structural and functional units of life (and of disease processes) in all tissues, organs, and organ systems. There are 2 basic cell types. Prokaryotic are typically small, single-celled organisms (e.g., bacteria) that lack a nuclear envelope, histones, and membranous organelles. Eukaryotic cells exist primarily as components of multicellular organisms.

Cellular components: eukaryotic cells have 3 major components: The cell membranes separate a cell from its environment and form distinct functional compartments (nucleus, organelles) in the cell. The outer cell membrane is called the plasma membrane, or plasmalemma.

The membrane-bound nucleus contains a cell’s DNA, which encodes the genetic informatioeeded for protein synthesis and thus for all the activities of the cell. It also has components that help determine which parts of the genetic code are used and that deliver coded information to the cytoplasm.

The cytoplasm surrounds the nucleus and is enclosed by the plasma membrane. It contains the structures and substances needed to decode the instructions of DNA and carry on the activities of the cell.

Cellular Functions: The 3 major activities characteristic of most living organisms are nourishment, growth and development, and reproduction.

 

general structure of the cell

 

CELL MEMBRANE

Biochemical Components

Lipids. Lipids are present in cell membranes as phospholipids, sphingolipids, and cholesterol. Each phospholipid molecule has a polar (hydrophilic) phosphate-containing head group and a nonpolar (hydrophobic) pair of fatty-acid tails. Membrane phospholipids are arranged in a bilayer with their tails directed toward one another at the center of the membrane. In electron micrographs of osmium-stained tissue, a single membrane, or unit membrane, has 2 dark outer lines with a lighter layer between them.

Cholesterol is amphipatic and becomes intercalated between phospholipids in membranes. It increases the stability of the bilayers and prevents the loss of membrane liquidity at low temperatures. The concentration of phospholipids and cholesterol varies in the membranes of organisms that live at temperature extremes. Presumably, this variation maintains membrane fluidity above crucial threshold levels. If membrane fluidity falls below these hypothetical thresholds, vital functions such as selective membrane transport may cease and cell will die.

Proteins. Proteins may comprise over 50 % of membrane weight. Most membrane proteins are globular and belong to one of the following 2 groups: a. Integral membrane proteins are tightly lodged in the lipid bilayer; detergents are required to extract them. They are folded, with their hydrophilic amino acids in contact with the phosphate groups of the membrane phospholipids and their hydrophobic amino acids in contact with the fatty-acid tails. Some protrude from onlyone membrane surface, while others, called transmembrane proteins, penetrate the entire membrane and protrude from both sides.

b. Peripheral membrane proteins are more loosely associated with the inner or outer membrane surface; some are globular, some filamentous.

Carbohydrates. Carbohydrates occur on plasma membranes mainly as oligosaccharide moieties of membrane glycoproteins and glycolipids. Membrane oligosaccharides have a characteristic branching structure and project from the cell’s outer surface, forming a superficial coat called the glycocalyx that participates in cell adhesion and recognition.

 

 

Membrane Organization: The widely accepted fluid mosaic model describes biologic membranes as “protein icebergs in a lipid sea.” Integral proteins exhibit lateral mobility and may undergo rearrangement determined by their association with peripheral proteins, cytoskeletal filaments within the cell, membrane components of adjacent cells, and components of the extracellular matrix. Integral proteins not anchored by such associations sometimes diffuse to and accumulate in one membrane region, a process termed capping.

The membrane consists of a phospholipid double layer (belayer) with proteins inserted in it (integral proteins) or bound to the cytoplasmic surface (peripheral proteins). Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called transmembrane proteins, whereas others are embedded in either the outer or inner leaflet of the lipid bilayer. The dotted line in the integral membrane protein is the region where hydrophobic amino acids interact with the hydrophobic portions of the membrane. Many of the proteins and lipids have externally exposed oligosaccharide chains. B: Membrane cleavage occurs when a cell is frozen and fractured (cryofracture). Most of the membrane particles (1) are proteins or aggregates of proteins that remain attached to the half of the membrane adjacent to the cytoplasm (P, or protoplasmic, face of the membrane). Fewer particles are found attached to the outer half of the membrane (E, or extracellular, face). For every protein particle that bulges on one surface, a corresponding depression (2) appears in the opposite surface. Membrane splitting occurs along the line of weakness formed by the fatty acid tails of membrane phospholipids, since only weak hydrophobic interactions bind the halves of the membrane along this line.             

 

Membrane Functions:

1. Selective permeability. The cell membrane forms an effective seal between a cell or organelle’s internal and external environment, preventing intrusion of harmful substances, dispersion of macromolecules, and dilution of enzymes and substrates. However, membranes display selective permeability, essential to maintaining the functional steady state, or homeostasis, required for cell survival. Homeostatic mechanisms attributable to the cell membrane maintain optimal intracellular concentrations of ions, water, enzymes, and substrates. Three mechanisms allow passage of selected molecules.

a. Passive diffusion. Certain substances (e.g., water) can cross the membrane in either direction, following a concentration gradient. Passive diffusion does not require energy expenditure.

b. Facilitated diffusion. Certain molecules (e.g., glucose) cannot freely diffuse across membranes but must be helped across by a membrane component. This facilitated diffusion is often unidirectional, but it follows a concentration gradient and requires no energy.

c. Active transport. Some nondiffusible molecules can move into or out of cell either along or against a concentration gradient. Such movement requires energy, usually as ATP. An example of this active transport is the sodium pump (Na⁺/K⁺-ATPase), which can expel sodium ions from a cell even when the external sodium concentration is higher than the internal one.

2. Transduction of signals. Receptors with strong binding affinities for extrinsic signals such as hormones are located at the cell surface. The signal molecule to which a receptor binds specifically is called its ligand. Once receptors bind their signal molecules, they may transmit the signal to the cell interior (a phenomenon called transduction of signals) by one of a variety of mechanisms:

a. Receptors may transmit the signal through their association with cytoskeletal components at the inner surface of the membrane.

b. Receptors may interact with other membrane components to produce second-messenger molecules, which then transmit the message to the cell’s interior.

c. Signal-receptor complexes may be moved into the region of a coated pit and hi endocytosed, carving the signal into the cell.

d. The receptor itself may have enzyme activity (stimulated by binding the signal molecule) and transmit the signal by enzymatically altering intracellular proteins

3. Endocytosis. Cells engulf extracellular substances and bring them into the cytoplasm in membrane-limited vesicles by mechanisms described collectively a endocytosis.

a. In phagocytosis (“cell eating”), the cell engulfs insoluble extracellular substances such as large macromolecules or entire bacteria. The vesicles formed are termed phagosomes.

b. In pinocytosis (“cell drinking”), the cell engulfs small amounts of intercellular fluid, which may contain a variety of solutes. Pinocytotic vesicles are usually smaller than phagosomes. 4. Exocytosis. Exocytosis removes substances from the cell. Cells use this process both for secretion and for excretion of undigested material. A membrane-limited vesicle or secretory granule fuses with the plasma membrane and releases its contents into the extracellular space, without disrupting the plasma membrane.

 

CYTOPLASM

Cytoplasm is inner compound of the cell, it consists of hyaloplasm and structural components (organelles and inclusions).

Cytoplasm structures can be divided into 3 groups:

1. Organelles are membrane-bound, enzyme-containing, permanent subcellular compartments.

2. Cytoplasmic inclusions are structures, membrane-bound or not, that are generally more transient than organelles and less actively involved in cell metabolism.

3. The cytoskeleton is composed of proteinaceous elements that form a supporting network within the cytoplasm; some of these elements (microtubules) also form discrete cytoplasmic structures such as centrioles.

 

ORGANELLES

Organelles are permanent structures of cytoplasm, which have exact typical structure and perform exact functions.

General organelles

1. Microscopic (mitochondrion, Golgi apparatus) and submicroscopic.

2. Membrane bounded and membraneless.

Special organelles

–       Cilia

–       Flagella

–       Fibrilles (myo-, tono-, neurofibrilles)

 

Mitochondria

The largest of the cytoplasmic organelles, mitochondria are the energy providers (“powerhouses”) of the cell.

Structure. Mitochondria are microscopic general organelles comparable in size to bacteria (usually 2-6 mm in length and 0.2 mm in diameter but quite variable) and have various shapes: spherical, ovoid, filamentous. Each mitochondrion is bounded by 2 unit membranes.

a. The outer mitochondrial membrane has a smooth contour and forms a continuous but relatively porous covering. It is freely permeable to various small molecules.

b. The inner mitochondrial membrane is less porous and is therefore semipermeable. It has numerous infoldings, or cristae, that project into the

 

 

mitochondrion’s interior. The mitochondrial cristae of most cells are sheltlike, but those in steroid-secreting cells are typically more tubular.

c. The mitochondrial membranes create 2 membrane-limited spaces. The intermembranous space is located between the inner and outer membranes and is continuous with the intercristal space that extends into the cristae. The intercristal space, or matrix space, is enclosed by the inner membrane and contains the mitochondrial matrix

d. The mitochondrial matrix contains water, solutes, and large matrix granules, believed to be concerned with mitochondrial calcium-ion concentrations. It also contains circular DNA and mitochondrial ribosomes similar to those of bacteria. The matrix contains numerous soluble enzymes involved in such specialized mitochondrial functions as the Krebs cycle (citric acid cycle, tricarboxylic acid cycle), b-oxidation of lipids, and mitochondrial DNA synthesis. Function. Mitochondria provide the cell with the energy for chemical and mechanical work by storing energy generated from cellular metabolites in the high-energy bonds of ATP.

 

Details of mitochondria. 1 – External envelope; 2 – Cristae; 3 – Matrix (the more electron-dense material); 4 – Granules within the matrix.

 

Location. Mitochondria are found iearly all eukaryotic cells, and in most they are dispersed throughout the cytoplasm. However, they accumulate in the highest concentrations in cell types and intracellular regions with the highest energy requirements. Cardiac muscle cells are notable for the abundance of their mitochondria. Epithelial cells lining the kidney tubules have abundant mitochodria interdigitated between basal plasma membrane infoldings where active transport of ions and water occurs.

 

Ribosomes

The ribosomes are protein-synthesizing general submicroscopic organelles. There are 2 basic types. Mitochondrial (like prokaryotic) ribosomes are smaller (20 nm) than the cytoplasrnic ribosomes of eukaryotes (25 nm).

Structure. Each type of ribosome has 2 unequal ribosomal subunits. Cytoplasmic ribosomes are composed of ribosomal RNA (r-RNA) synthesized in the nucleolus and associated proteins synthesized in the cytoplasm. They are intensely basophilic. Light microscopy reveals cytoplasmic accumulations of ribosomes as basophilic patches, formerly termed ergastoplasm in glandular cells and Nissl bodies in neurons. In electron micrographs, ribosomes appear as small, electron-dense cytoplasmic granules.

Location and function. Cytoplasmic ribosomes occur in 2 forms. Free ribosomes are individual ribosomes dispersed in cytoplasm. Polyribosomes, or polysomes, are groups of ribosomes evenly distributed along a single strand of messenger RNA (mRNA), an arrangement that permits synthesis of multiple copies of a protein from the same message. Polysomal ribosomes read (translate) the mRNA code and thus play a critical role in assembling amino acids into specific proteins. Polysomes are found free in the cytoplasm (free polysomes) and attached to membranes of the rough endoplasmic reticulum (polysomes of the endoplasmic reticulum). Free polysomes are involved in the synthesis of structural proteins and enzymes for intracellular use. Polysomes of the rough endoplasmic reticulum are involved in synthesizing proteins that are secreted or isolated.

 

 

Diagram illustrating (A) the concept that cells synthesizing proteins (represented here by spirals) that are to remain within the cytoplasm possess (free) polyribosomes (ie, nonadherent to the endoplasmic reticulum). In B, where the proteins are segregated in the endoplasmic reticulum and may eventually be extruded from the cytoplasm (export proteins), not only do the polyribosomes adhere to the membranes of rough endoplasmic reticulum, but the proteins produced by them are injected into the interior of the organelle across its membrane. In this way, the proteins, especially enzymes such as ribonucleases and proteases, which could have undesirable effects on the cytoplasm, are separated from it.

 

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a complex general submicroscopic organelle involved in the synthesis, packaging, and processing of various cell substances. It is a freely anastomosing network (reticulum) of membranes that form vesicles, or cisternae; these may be elongated, flattened, rounded, or tubular. Transfer vesicles (transitional vesicles) are small, membrane-limited vesicles that bud from the ER and cross the intervening cytoplasm to reach the Golgi complex for further processing or packaging of their contents. In fully differentiated cells, ER occurs in 2 forms, called rough and smooth.

The endoplasmic reticulum is an anastomosing network of intercommunicating channels and sacs formed by a continuous membrane. Note that the smooth endoplasmic reticulum (foreground) is devoid of ribosomes, the small dark dots that are present in the rough endoplasmic reticulum (background). The cisternae of the smooth reticulum are tubular, whereas in the rough reticulum they are flat sacs.

 

 

Rough endoplasmic reticulum

Structure. The rough endoplasmic reticulum (RER), also called granular endoplasmic reticulum, is studded with ribosomes, many of them in polysomal clusters. RER cisternae are typically parallel, flattened, and elongated, especially in cells specialized for protein secretion (eg, pancreatic acinar cells, plasma cells), in which RER is particularly abundant. The ribosomes give RER basophilic staining properties. The fine structure of RER (membranes and individual ribosomes) is visible only with the electron microscope.

Function. RER is mainly concerned with the synhtesis of proteins for sequestration from the rest of the cytoplasm, ie, secretory proteins such as collagen, proteins for incorporation into cell membranes, and lysosomal enzymes (separated from the rest of the cytoplasm to prevent autolysis). RER in protein-secreting epithelial cells often lies in the basal cytoplasm, between the plasma membrane and the nucleus.

 

Part of a lymphocyte showing continuity of the rough endoplasmic reticulum (rer) with the nuclear envelope (at arrow).

 

Smooth endoplasmic reticulum

Structure. The smooth endoplasmic reticulum (SER) lacks ribosomes and thus appears smooth in electron micrographs. SER cistemae are more tubular or vesicular than those of RER. SER stains poorly, if at all, so with the light microscope it is indistinguishable from the rest of the cytoplasm.

Function. Because it lacks ribosomes, the Ser cannot synthesize proteins. It has many

enzymes, important in lipid metabolism, steroid hormone synthesis, glycogen synthesis (glucose-6-phosphatase), and detoxification.

Location. The SER is suspended in the cytoplasm of many cells and is especially abundant in cells that synthesize steroid hormones (e.g., cells of the adrenal cortex and the gonads). It is also abundant in liver cells (hepatocytes), where it is involved in

glycogen synthesis and drug detoxification. Specialized SER termed sarcoplasmic reticulum is found in striated muscle cells, where it helps to regulate muscle contraction by sequestering and releasing calcium ions.

 

High magnification of a network of smooth endoplasmic reticulum. Unlike rough endoplasmic reticulum, which usually occurs in flat sheets, this organelle comprises interconnected tubules (1). 2 – Mitochondrion; 3 – Free ribosomes, seen either singly or as Polyribosomes (polysomes).

 

Golgi Complex

The Golgi complex (Golgi apparatus) participates in many activities, particularly those associated with secretion. It has an essential role in coordinating membrane flow and vesicle traffic among organelles.

Structure. This membranous general microscopic organelle is composed of 3 major compartments:(l) a conspicuous stack of 3-10 discrete, slightly curved, flattened cisternae; (2) numerous small vesicles peripheral to the stack; and (3) a few large vacuoles, sometimes called condensing vacuoles, at the concave surface of the stack. The cis face (convex face, forming face) of the stack is usually closest to adjacent dilated ER cisternae and is surrounded by transfer vesicles. Its cisternae stain more darkly with osmium. The trans face (concave face, maturing face) often harbors several condensing vacuoles and generally faces away from the nucleus.

 

 

Three-dimensional representation of a Golgi complex. Through transport vesicles that fuse with the Golgi cis face, the complex receives several types of molecules produced in the rough endoplasmic reticulum (RER). After Golgi processing, these molecules are released from the Golgi trans face in larger vesicles to constitute secretory vesicles, lysosomes, or other cytoplasmic components.

Electron micrograph of a Golgi complex of a mucous cell. To the right is a cisterna (arrow) of the rough endoplasmic reticulum containing granular material. Close to it are small vesicles containing this material. This is the cis face of the complex. In the center are flattened and stacked cisternae of the Golgi complex. Dilatations can be observed extending from the ends of the cisternae. These dilatations gradually detach themselves from the cisternae and fuse, forming the secretory granules (1, 2, and 3). This is the trans face. Near the plasma membrane of two neighboring cells is endoplasmic reticulum with a smooth section (SER) and a rough section (RER). x30,000. Inset: The Golgi complex as seen in 1-micrometer sections of epididymis cells impregnated with silver. x1200.

 

Main events occurring during trafficking and sorting of proteins through the Golgi complex. Numbered at the left are the main molecular processes that take place in the compartments indicated. Note that the labeling of lysosomal enzymes starts early in the cis Golgi network. In the trans Golgi network, the glycoproteins combine with specific receptors that guide them to their destination. On the left side of the drawing is the returning flux of membrane, from the Golgi to the endoplasmic reticulum.

Functions

a. Polysaccharide synthesis. The Golgi complex contains various glycotrans-ferases that initiate or lengthen polysaccharide or oligosaccharide chains, adding one sugar at a time.

b. Modification of secretory products. Products synthesized by the ER are packaged in vesicles by the Golgi complex. The Golgi complex is thus important in the synthesis glycoproteins, proteoglycans, glycolipids, and sulfated GAGs.

c. Packaging of secretory products. Products synthesized by the ER are packed in vesicles by the Golgi complex. These secretory vesicles, or secretory granules, are transported to the plasma membrane for exocytosis.

d. Concentration and storage of secretory products. The Golgi complexes of some cells concentrate and store secretory products prior to secretion. Such concentration is a major function of the condensing vacuoles on the trans face of the Golgi complex, which also often serve as precursors to secretory granules.

Location. The Golgi complex is typically near the nucleus (juxtanuclear) and is often found near centrioles (which may also have an important role in directing vesicle traffic). Golgi complexes are best developed ieurons and glandular cells, which are specialized for secretion.

 

lysosomes

Lysosomes are spherical, membrane-limited vesicles that may contain more than 50 enzymes each and function as the cellular digestive system. They are general submicroscopic organelles. Their characteristic enzyme activities distinguish them from other cellular granules. The enzyme most widely exploited for their identification is acid phosphate, because it occurs almost exclusively in lysosomes. Other enzymes common in lysosomes are ribonucleases, deoxyribonucleases, cathepsins, sulfatases, b-glucoronidase, and phospholipases and other proteases, glucosidases, and lipases. Lysosomal enzymes usually occur as glycoproteins and are most active at an acidic pH. Lysosomes occur in various sizes and electron densities, depending on their level of activity.

1. Primary lysosomes are small (5-8 nm in diameter), with electron-dense contents; they appear as black circles in electron micrographs. They are the storage form of lysosomes, and their enzymes are mostly inactive. Lysosomes enzymes synthesized in the RER are transferred to the Golgi complex for further glycosylation; it is uncertain whether their final packaging as primary lysosomes occurs in the Golgi complex or in GERL. The primary lysosomes disperse through the cytoplasm. They are found in most cells but are most abundant in phagocytic cells (eg, macrophages, neutrophils).

2. Secondary lysosomes are larger and less electron-dense and have a mottled appearance in electron micrographs. They are formed by the fusion of one or more primary lysosomes with a phagosome. Their primary function is the digestion of products of heterophagy and autophagy; when the lysosomal enzymes mix with the phagosome contents, they become active. Lysosomal enzymes also catabolize certain products of cell synthesis, thus regulating the quality and quantity of secretory material. Secondary lysosomes occur throughout the cytoplasm in many cells, iumbers that reflect the cell’s lysosomal and phagocytic activity.

 

Detail of secondary lysosome with engulfed material within it. 1 – Limiting membrane; 2 – Matrix; 3 – partly digested material.

 

3. Residual bodies are membrane-limited inclusion of varying size and electron density associated with the terminal phases of lysosome function. They contain undigestible materials such as pigments, crystals, and certain lipids.

F. Peroxisomes are membrane-limited, enzyme-containing vesicles somewhat larger than primary lysosomes. Peroxisomes function in hydrogen peroxide metabolism. They contain urate oxidase, hydroxyacid oxidase, and d-amino acid oxidase, which produce hydrogen peroxide capable of killing bacteria; they also contain catalase, which oxidizes various substrates and uses the hydrogen removed in the process to convert the toxic hydrogen peroxide to water. Peroxisomes also participate in gluconeogenesis by assisting in the b-oxidation of fatty acids. They are found dispersed in the cytoplasm or in association with the SER.

Phagosomes. Phagosomes are membrane-limited vesicles of varying size containing material destined for lysosomal digestion. Two major types are known. Heterophagosomes contain the products of heterophagy, material of extracellular origin ingested by phagocytosis. Autophagosomes contain the products of autophagy, i.e., material of intracellular origin such as worn or damaged organelles. The digestion of phagosomal contents begins when a phagosome fuses with one or more primary lysosomes to form a secondary lysosome.

Electron micrograph showing 4 dark secondary lysosomes surrounded by numerous mitochondria.

 

 

Current concepts of the functions of lysosomes. Synthesis occurs in the rough endoplasmic reticulum (RER), and the enzymes are packaged in the Golgi complex. Note the heterophagosomes, in which bacteria are being destroyed, and the autophagosomes, with RER and mitochondria in the process of digestion. Heterophagosomes and autophagosomes are secondary lysosomes. The result of their digestion can be excreted, but sometimes the secondary lysosome creates a residual body, containing remnants of undigested molecules. In some cells, such as osteoclasts, the lysosomal enzymes are secreted to the extracellular environment. Nu, nucleolus.

 

CYTOPLASMIC INCLUSIONS

Inclusions are temporary structures of cytoplasm, which are passive and have no exact structure.

Inclusions are divided in secretory, excretory, pigmental and trophic.

Due to the origin there are exogenous (synthesized out of cell) and endogenous (produced by the cell) inclusions.

Inclusions may consist of proteins, lipids, carbohydrates and mineral.

Prominent among inclusions serving as storage depots are spherical lipid droplets, which differ in appearance depending upon the type of histologic preparation.

Section of adrenal gland showing lipid droplets (L) and abundant anomalous mitochondria (M).

 

Glycogen granules are inclusions that are PAS-positive in light microscopy and appear in electron micrographs as rosettes of electron-dense particles. Both lipid droplets and glycogen granules lack a limiting membrane. Melanin is a brownish pigment widely distributed in vertebrates, often found in electron-dense, membrane-limited granules termed melanosomes. It is particularly abundant in epidermal cells and in the pigment layer of the retina.

 

Electron micrograph of a section of a liver cell showing glycogen deposits as accumulations of small electron-dense particles (arrows). The dark structures with a dense core are peroxisomes. Mitochondria (M) are also shown. x30,000.

 

Electron micrograph of a pancreatic acinar cell from the rat. Numerous mature secretory protein granules (S) are seen in association with condensing vacuoles (C) and the Golgi complex (G). x18,900.

 

CYTOSKELETON

The cytoskeleton, a mesh of filamentous elements called microtubules, microfilaments, and intermediate filaments, provides structural stability for the maintenance of cell shape. It is important in cell movement and in the rearrangement of cytoplasmic components.

Microtubules

Structure. Microtubules are the thickest components of the cytoskeleton, with diameters of 24 nm. They are fine tubular structures of variable lenght, with dense walls (5 nm thick) and a clear internal space (14 nm in internal diameter). The walls are composed of subunits called tubulin heterodimers, each of which consists of one a-tubulin and one b-tubulin protein molecule. The tubulin heterodimers are arranged in protofilaments. Thirteen of these threadlike polymers of a- and b-tubulin align parallel to one another to form the wall of each microtubule. Microtubules increase in length by adding new heterodimers to one end, called the nucleation site.

Function. Microtubules have roles in maintenance of cell shape, axoplasmic transport ieurons, melanin dispersion in pigment cells, chromosome movements during mitosis, and the shuttling of vesicles within the cell. Unlike microfilaments, microtubules are unable to contract.

Location. Microtubules are found throughout the cytoplasm of most cells and in highly organized groupings in centrioles, cilia, flagella, basal bodies, and the mitotic spindle apparatus.

 

Centrioles

Structure. A centriole is a cylindrical group of microtubules, 150 nm in overall diameter and 350-500 nm long, containing 9 microtubule triplets in a pinwheel array. Each microtubule in a triplet shares a portion of the wall of the neighboring microtubule. An interphase (nondividing) cell has a pair of adjacent centrioles with perpendicular long axes, each surrounded by several electron-dense satellites, or pericentriolar bodies. Other cytoplasmic microtubules originate from the pericenriolar bodies and radiate into the cytoplasm.

Part of a lymphocyte showing a centriole (C) cut transversely. Note the triplet arrangement of microtubules cut in cross-section. GA – Golgi apparatus (body); PR – Polyribosomes (polysomes); NS – Perinuclear space (of the nuclear envelope).

 

Drawing of a centrosome with its granular protein material surrounding a pair of centrioles, one shown at a right angle to the other. Each centriole is made of 9 bundles of microtubles, with 3 microtubules per bundle.

 

Function. Centrioles are the structural organizers of the cell. Centriole duplication is a prerequisite for cell division, and during mitosis the centrioles organize the microtubules of the mitotic spindle.

Location. Between cell divisions, centrioles are near the nucleus, often surrounded by Golgi complexes. The centrioles and associated Golgi complexes constitute the cell cytocenter, which appears as a clear zone near the nucleus. During the S phase of interphase, each centriole duplicates by giving rise to a procentriole that grows at right angels to the original. During mitosis, the new centriole pairs migrate to opposite cell poles to organize the spindle.

 

Basal bodies

In cell bearing cilia or flagella, centrioles migrate to the apical plasma membrane and give rise to basal bodies in a manner similar to centriole self-duplication. Basal bodies are structurally similar to centrioles, with 9 microtubule triplets. They occur in the cytoplasm, one at the base of each cilium or tlagellum, and serve as the anchoring points and microtubule organizers for these structures.

A ciliated cell usually has hundreds of cilia, motile 5-10 urn long, 0.2 um wide, cell-surface evaginations covered by plasma membrane. Each cilium contains a core, or axoneme, composed of 9 peripheral microtubule doublets surrounding a pair of enjoined microtubules (the “9 + 2” arrangement). Partners in a doublet are called subfibers A and B. Subfiber A is a complete microtubule, having 13 tubulin protofilaments. Subfiber B has 10 or 11 protofilaments and is completed by shing part of its partner’s wall. A pair of arms made of dynein (a protein with ATPse ctivity) extends from the wall of each subfiber A toward the adjacent doublet. Protein bridges called nexins link adjacent doublets, and radial spokes link the doublets to thesheath surrounding the central microtubule pair. Each axoneme is organized by and anchored in a basal body.

 

Special organelles

Cilia

Cilia are the special organelles, which contain 9 pairs of microtubules inside

Photomicrograph of the epithelium covering the inner surface of the respiratory airways. Most cells in this epithelium contaiumerous cilia in their apices (free upper extremities). N, cell nuclei; M, cytoplasmic mucus secretion, which appears dark in this preparation. H&E stain. High magnification.

 

EM of cilia cut longitudinally. (A few microvilli are on the neighboring cell to the left, for a size comparison.) Notice that each cilium is rooted in a barrel-like basal body. The dense lines extending from the basal bodies and up into the cilia are microtubules. The unit membrane of the cell continues up over each cilium.

 

Cross-cuts of cilia showing the typical 9X2 +2 arrangement of microtubules within the cytoplasm ( ring of 9 doublets plus 2 single microtubules in the center). The cell membrane envelopes each cilium.

 

Tangential section of cilia showing the structural transitions that occur between the shaft of the cilia (upper right) and the basal bodies (lower left) which give rise to the cilia.

 

Flagella

A flagellum is like a cilium, but it is longer, and there is usually only one or 2 flagella on a cell. In humans and other mammals, flagella occur only in the tails of spermatozoa, which are typically 50-55 um long and 0.2-0.5 um thick along most of their length. The axoneme of a flagellum is identical to that of a cilium.

EM of microtubules, seen as fine parallel lines when cut longitudinally (lower panel) or circles when cut transversely (upper panel). Images are from dendrites and axons of neurons.

 

 

Schematic representation of microtubules, cilia, and centrioles. A: Microtubules as seen in the electron microscope after fixation with tannic acid in glutaraldehyde. The unstained tubulin subunits are delineated by the dense tannic acid. Cross sections of tubules reveal a ring of 13 subunits of dimers arranged in a spiral. Changes in microtubule length are due to the addition or loss of individual tubulin subunits. B: A cross section through a cilium reveals a core of microtubules called an axoneme. The axoneme consists of 2 central microtubules surrounded by 9 microtubule doublets. In the doublets, microtubule A is complete and consists of 13 subunits, whereas microtubule B shares 2 or 3 heterodimers with A. When activated by ATP, the dynein arms link adjacent tubules and provide for the sliding of doublets against each other. C: Centrioles consist of 9 microtubule triplets linked together in a pinwheel-like arrangement. In the triplets, microtubule A is complete and consists of 13 subunits, whereas tubules B and C share tubulin subunits. Under normal circumstances, these organelles are found in pairs with the centrioles disposed at right angles to one another.

 

Microfilaments

Microfilaments are general submicroscopic organelles. There are three types of filaments:

a.     myofilaments, which are typical for muscular tissue;

b.     neurofilaments ierve cells;

c.     tonofilaments in epithelia

 

The cytosolic actin filament. Actin dimers are added to the plus (+) end and removed at the minus (–) end, dynamically lengthening or shortening the filament, as required by the cell.

 

Electron micrograph of fibroblast cytoplasm. Note the microfilaments (MF) and microtubules (MT). x60,000.

 

Structure. Microfilaments are the thinnest cytoskeletal components (5-7 nm wide). They are usually composed of one of several types of actin protein. In striated muscle cells, actin filaments form a stable paracrystalline array in association with filaments of myosin. Actin filaments in other cells are less stable and can dissociate and reassemble. These changes are regulated in part by calcium ions and cyclic AMP and by actin-binding proteins in the cytoplasm.

Function. Microfilaments are contractile, but to contract, they usually interact with myosin. In muscle cells, myosin forms thick filaments. Ionmuscle cells, it exists in soluble form.

Location. Ionmuscle cells, microfilaments are generally distributed as an irregular meshwork throughout the cytoplasm.

 

Intermediate filaments

Structure. Intermediate filaments are intermediate in thickness (10.-12 nm) between microtubules and microfilaments. They are composed of proteins that are structurally related to nuclear laminas and differ depending on the cell type. Examples of intermediate filament proteins: cytokeratins in epithelial cells, vimentin in mesenchymal derived cells (e.g., fibroblasts, chondrocytes), desmin in muscle cells, glial fibrillary acidic protein in glial cells, neurofilaments (intermediate filament bundles) ieurons.

 

Electron micrograph of a skin epithelial cell showing intermediate filaments of keratin associated with desmosomes.

 

NUCLEUS

Nucleus is an important part of eukaryotic cell.

Nuclei vary in appearance from tissue to tissue and cell to cell, but they generally have a nuclear envelope, chromatin, nucleoplasm and one to several nucleoli. Nuclei display wide variations in (1) size, both absolute and relative to the amount of cytoplasm (nucleocytoplasmic ratio); (2) number per cell, allowing classification of cells as enucleate, mononucleate, binucleate, or multinucleate; (3) chromatin pattern, i.e., the amount and distribution of heterochromatin; and (4) location, eg, basal, central, eccentric.

Schematic representation of a cell nucleus. The nuclear envelope is made of 2 membranes of the endoplasmic reticulum, enclosing a perinuclear cisterna. Where the two membranes fuse, they form nuclear pores. Ribosomes are attached to the outer nuclear membrane. Heterochromatin clumps are associated with the nuclear lamina, whereas the euchromatin (EC) appears dispersed in the interior of the nucleus. In the nucleolus, note the associated chromatin (arrows), heterochromatin (Hc), the pars granulosa (G), and the pars fibrosa (F).

 

Nuclear Envelope. The nuclear contents are set apart from the cytoplasm by a double membrane called the nuclear envelope and a narrow (40-70 nm) intermembranous space called the perinuclear cisterna, or perinuclear space.

 

Three-dimensional representation of a cell nucleus to show the distribution of the nuclear pores, the heterochromatin (dark regions), the euchromatin (light regions), and a nucleolus. Note that there is no chromatin closing the pores. The number of nuclear pores varies greatly from cell to cell.

 

EM of the nuclear envelope. Dense chomatin material (heterochromatin) (1) is distributed along the nuclear envelope except in the region of the nuclear pores (2). 3 – euchromatin; 4 – smooth endoplasmic reticulum; 5 – Golgi body.

 

Nuclear envelope is often considered an extension of the RER, because its outer surface is often peppered with ribosomes and shows occasional continuities with the RER. The inside of the inner membrane is lined with the fibrous lamina, a layer consisting of proteins called lamins. The envelope is perforated by many nuclear pores, each of which has a diameter of about 70 nm and is bounded by 8 globular subunits called annular proteins, which present an octagonal appearance in some preparations. Each pore is covered by a pertinacious diaphragm that is thinner than the envelope. The pores provide a channel for the movement of important molecules between the nucleus and cytoplasm; these molecules include nucleic acids synthesized in the nucleus and used in the cytoplasm (mRNA, rRNA, and tRNA) and proteins synthesized in the cytoplasm and used in the nucleus (histones, polymerases).

 

Electron micrograph obtained by cryofracture of a rat intestine cell, showing the two components of the nuclear envelope and the nuclear pores.

 

Illustration to show the structure, the localization, and the relationship of the nuclear lamina with chromosomes. The drawing also shows that the nuclear pore complex is made of 2 protein rings in an octagonal organization. From the cytoplasmic ring, long filaments penetrate the cytosol, and from the intranuclear ring arise filaments that constitute a basketlike structure. The presence of the central cylindrical granule in the nuclear pore is not universally accepted.

 

Electron micrographs of nuclei showing their envelopes composed of 2 membranes and the nuclear pores (arrows). The two upper pictures are of transverse sections; the bottom is of a tangential section. Chromatin, frequently condensed below the nuclear envelope, is not usually seen in the pore regions. x80.000.

 

Electron micrograph of a nucleus, showing the heterochromatin (HC) and euchromatin (EC). Unlabeled arrows indicate the nucleolus-associated chromatin around the nucleolus (NU). Arrowheads indicate the perinuclear cisterna. Underneath the cisterna is a layer of heterochromatin, the main component of the so-called nuclear membrane seen under the light microscope. x26.000.

 

Chromatin. Nuclear chromatin is an intensely basophilic substance consisting of DNA and associated histone and nonhistone proteins. Nuclei containing highly coiled chromatin, termed heterochromatin, stain darkly with basic dyes. Because the DNA of chromatin must uncoil to be transcribed, cells with dark-staining (heterochromatic) nuclei are less active in DNA transcription than other cells, using a smaller portion of their total genome. Uncoiled chromatin, termed euchromatine, stains poorly and is difficult to distinguish even with electron microscopy. Large, pale-staining (euchromatic) nuclei usually indicate greater transcriptional activity and faster cell division.

 

Chromosomes

The chromosomes, the most highly condensed form of chromatin, are visible during mitosis. Chromosomal set of human cell (karyotype) includes 46 chromosomes (23 pairs). 44 of them are autosomes and the last pair is gonosomes or sex chromosomes. In females, only one X chromosome (either of the 2) is used by each cell; the inactive X chromosome is often visible as a clump of heterochromatin termed sex chromatin, or the Barr body. In most cells (epithelial cells of oral mucosa, nerve cells of spinal cord), the Barr body is attached to the inner surface of the nuclear envelope. In a neutrophilic leukocyte, it may appear as a drumstick-shaped appendage of the lobulated nucleus.

 

 

The orders of chromatin packing believed to exist in the metaphase chromosome. Starting at the top, the 2-nm DNA double helix is shown; next is the association of DNA with histones to form filaments of nucleosomes of 11 nm and 30 nm. Through further condensation, filaments with diameters of 300 nm and 700 nm are formed. Finally, the bottom drawing shows a metaphase chromosome, which exhibits the maximum packing of DNA.

Nucleosome

Schematic representation of a nucleosome. This structure consists of a core of 4 types of histones (2 copies of each)—H2A, H2B, H3, and H4—and one molecule of H1 or H5 located outside the DNA filament.

 

Human karyotype preparation made by means of a banding technique. Each chromosome has a particular pattern of banding that facilitates its identification and also the relationship of the banding pattern to genetic anomalies. The chromosomes are grouped iumbered pairs according to their morphologic characteristics.

 

Nucleolus

During interphase (between mitoses), each nucleus usually has at least one (or 2) intensely basophilic body called a nucleolus. Nucleoli are the synthesis sites for most ribosomal RNA (rRNA), which are connected with nucleolar organizers. The nucleolus disappears in preparation for mitosis and reappears after mitosis is completed.

Nucleolus is the darker structure, which could be seen in the nucleus. Usually amount of nucleoli directly depends on activity of cell and chromosomal set of the cell; more active ones have two, sometime three and even four nucleoli.

Main compounds of the nucleolus are the next: fibrillar compound (r-RNA filaments, trascripted from adequate part of DNA), granular one consist of ribosomes, and amorphous part contains nearest condensed chromatin.

 

Detailed EM of nucleolar structure, showing fibrillar (1), granular (2), and amorphous (3) portions.

 

The term nucleolonema is used to refer to a threadlike basophilic substructure of the nucleolus. The nucleolonema contains 2 rRNA-rich components distinguishable by electron microscopy.

a. The pars fibrosa consists of densely packed ribonucleoprotein fibers, 5-10 nm in diameter. These fibers consist of the newly synthesized primary transcripts of the rRNA genes and associated proteins.

b. The pars granulosa contains dense granules, 15-20 nm in diameter that represents maturing ribosomal subunits during assembly for export to the cytoplasm.

 

Nucleoplasm

         The nucleoplasm is the matrix in which the other intranuclear components are embedded. It is composed of enzymatic and nonenzymatic proteins, metabolites, ions, and water. It includes the nuclear matrix – a fibrillar “nucleoskeletal” structure that appears to bind certain hormone receptors-and newly synthesized DNA.

 

Cells reproduction

Cells reproduction – one of the most important biologic phenomena, which promote necessary conditions for biologic system existence during enough long period of time. Cells reproduction is realized by division of initial cell. This is the basic point of cellular theory.

There are such principal types of cells reproduction: mitosis, amitosis, meioses and endomitosis.

Cell cycle (cyclus сеllularis). All the period of cell existence from division to the next one or from division to the death of the cell is termed cell cycle. In adult vertebrates animals and human being cells of different organs and tissues have different possibility for division and thus different length of cell cycle. Cell cycle consists of interphase (period out of division) and the proper cell division.

Cell division is preceded by duplication of all important cells compounds, first of all its chromosomal set (DNA). Exact copy is synthesized along the initial chromosome of chemical compounds, which are present in the cell. Doubled chromosome consists of two similar portions – chromatides, each of them contains one DNA molecule. This duplication occurs at exact period of interphase and only after that cell division begins. Special protein cyclin, which regulates cells mitosis, was described recently. Deceleration of cyclin synthesis elongates interphase duration.

All cellular cycle is subdivided into four periods: actually mitosis, presynthetic (G1), synthetic (S) and post-synthetic (G2) periods of interphase. Denotation of synthetic period by the G letter originates from English «grow» — to grow. It is the period of the increased growth of young cell, mainly due to accumulation of cellular proteins. Preparation of cell to the DNA synthesis, which takes place iext S-period, begins in this period. If in the experiment to cause oppression of synthesis of proteins or m-RNA in G1-periode, transition of cell in S-period is blocked. In the G1 period enzymes, necessary for the formation of the DNA precursors, the RNA metabolism and proteins, are synthesized.

If a cell is going to be some time divided, interphase will consist of 3 periods. Right after mitosis is finished a cell enters synthetic or G1-period, further passes to synthetic or S-period and then — in postsynthetic or G2-period. G2-period finishes interphase and after that a cell enters next mitosis.

 

The 4 phases of the cell cycle. In G₁ the cell either continues the cycle or enters a quiescent phase called G₀. From this phase, most cells can return to the cycle, but some stay in G₀ for a long time or even for their entire lifetime. The checking or restriction point (R) in G₁ stops the cycle under conditions unfavorable to the cell. When the cell passes this restriction point, it continues the cycle through the synthetic phase (S) and the G₂ phase, originating 2 daughter cells in mitosis (M) except when interrupted by another restriction point (not shown) in G₂.

 

If a cell does not going for new division, she left a cellular cycle and enters the period of rest, or G₀-period. If the cell in G₀-periode again will “want to be divided”, she goes out from G₀-period and returns to the G₁-period. Thus, if a cell is in G₁-periode, she necessarily will divide early or late, cells in S- and G₂-periods will divide at the nearest time.

There are few types of cells in G₀ period. 1. First of all, the most numerous group of cells, which are not dividing because they perform their principal functions for multicellular organism: hepatocytes produce bile, myocytes contract and so on. These cells from time to time may return to the G₁ period, producing new cells. 2. Cells, which after the birth can never divide. A – group of cells whose life span is equal for the life of human being (nerve cells, cardiac myocytes), B – group of cells whose life span is enough short, they can’t divide and are replaced by new once.

G₁-period can proceed from 2–4 hours to a few weeks or even months. The S-period duration varies from 6 to 8 hours, and G₂-period — from a few hours to the half-hour. Mitosis lasts – from 40 to 90 minutes. Thus the shortest phase of mitosis is anaphase. It takes a few minutes only.

G₁-period is characterized by high synthetic activity, during which a cell must increase its size and amount of organells to the size of maternal one. Incomprehensible why, but cell at the beginning of mitosis must have a size equal to the maternal cell. And while it will not happen, a cell continues to remain in G1-periode. Cleavage is the unique exception at which blastomers are dividing with decrease of cell size.

At the end of G1-period there is the special moment, point R (restriction point), after which a cell enters S-period during a few hours (usually 1–2). Period of time between R-point and the S-period beginning is time of cell preparation for transition in S-period.

Main process, which occurs in S-period is duplication of DNA. All the other reactions at this time are directed on providing of the DNA synthesis — synthesis of histone proteins, synthesis of enzymes, which regulate and provide the synthesis of nucleotides and formation of new DNA filaments. Centrioles of cell center are also duplicated in S-period.

The essence G₂-period is not quite clear presently, but in this period there is formation of matters, which are necessary for the process of mitosis (proteins of microtubules of spindle of division – tubulines, the ATF molecules). Therefore the postsynthetic period of the G₂ cellular cycle is named premitotic. At the end of G₂-period the RNA synthesis sharply decreases and is fully stopped during mitosis. The synthesis of proteins during mitosis goes down also, and then achieves a maximum in G₀-periode, repeating the character of the RNA synthesis at all.

Cell changes in all periods of cellular cycle are strictly controlled by the special regulatory molecules, which provide: 1) passing of cell at the certain period of cellular cycle and 2) transition from one period to other. Thus each period passage and also transition from one period in other is controlled by different matters. Cyclin-dependent proteinkinazes (cdp) are one of the participants of the regulatory system Exactly they regulate activity of genes responsible for passing of cell in that or other period of cellular cycle. There are a few their varieties, and all of them are present in a cell constantly, regardless of period of cellular cycle. But cyclin-dependent proteinkinazes require the special activators. Cyclins perform this function. Cyclins are not constantly present in cells, they appear periodically. It depends on their synthesis and rapid destruction. A lot of types of cyclins are known. Synthesis of each cyclin takes place at exact period of cellular cycle. Some cyclins appear at one period of cycle another – in other. Thus, system of “cyclins — cyclin-dependent proteinkinazes” regulates cellular cycle

 

 

Phases of the cell cycle in bone tissue. The G₁ phase (presynthetic) varies in duration, which depends on many factors, including the rate of cell division in the tissue. In bone tissue, G₁ lasts 25 h. The S phase (DNA synthesis) lasts about 8 h. The G₂-plus-mitosis phase lasts 2.5–3 h.

 

Cellular cycle regulation

 

MITOSIS

 

Mitosis or karyokinezis, or indirect division, is the universal method of reproduction of cells. The name the «mitosis» originates from the Greek word of «mitos» — filament, which the filaments of chromosomes are understood under.

During mitosis a cell passes the some phases, as a result of which every daughter’s cell gets the same set of chromosomes, which was in a maternal cell.

Mitosis – is basic type of division of eukaryotic cells. The biological value of mitosis consists that it provides constancy of number of chromosomes in all cells of organism. In mitosis there is the division of the DNA chromosomes of maternal cell strictly equally between two daughter’s cells arising out of her. As a result of mitosis all cells of body, except for germ cells, have the same genetic information. Such cells are named somatic (from Greek “soma” – body).

In 1874 I. D. Chistyakov described the stages (phases) of mitosis in the disputes of plaunov, not yet clear by submitting to itself their sequence. The detailed researches of morphology of mitosis in the first time were performed by E. Strasburger on the plants (1876-1879) and V. Fleming on animals (1882). The mitosis of different types of cells lasts a different time, but in most cases does not exceed 1-2 hours. Duration of process of division depends also from the conditions of external environment (temperature, light mode and other indexes).

 

Phases of mitosis

In the process of mitosis a few stages are known, which replace one another: prophase, metaphase, anaphase and telophase. Duration of stages of mitosis is different and relies on the type of tissue, physiological state of organism, external factors; first and last phases are longer.

 

Phases of mitosis.

 

During mitosis as a result of euchromatine condensation doubled chromosomes become already visible in a nucleus and move to the poles of cell by an achromatic mitotic spindle, and then a cellular body is divided. At the beginning of mitosis each chromosome already is a double structure. But in prophase this duality does not yet seen because of tight adjoining of sisterly chromatides to each other. At the end of prophase nucleolus disappears as a result of ribosomal genes inactivation in the area of nucleolar organizers. A nuclear envelope disintegrates simultaneously on fragments, and then on small membrane vesicles. In addition, the amount of elements of granular endoplasmic reticulum (both cisterns, and ribosomes), which corresponds considerable reduction of level of proteins synthesis.

During prophase there is another process very important for the division of cell— formation of spindle of division as a result of divergence of centrioles to the poles of cell. Double centriole — diplosome will move to each pole. With diplosome divergence microtubules arise from the peripheral areas of maternal centrioles. The formed spindle of division in animal cells is fusiform and centrosphere consists of the two centrioles and fibers of spindle which lie between them. All these three structures are built from microtubules, which appear as a result of polymerization of tubulines in the area of centriole. In addition, the special structures of chromosomes are the centers of organization of microtubules of spindle — centromere (kinetochore), located in the areas of the primary chromosomal strangulation.

 

Electron micrograph of kinetochore – place of microtubules attachment on a chromosome. Metaphase of a human lung cell in tissue culture. Note the insertion of microtubules in the centromeres (arrows) of the densely stained chromosomes. Reduced from x50.000.

 

As a result two types of fibers appear in the spindle of division: central, that go from poles to the center, and chromosomal, which connect chromosomes with one of poles of cell and arise up later.

Metaphase begins from that moment, when chromosomes are freely located in a cytoplasm after dissolution of nuclear envelope, and begin to move to the equator of cell. This process carries the name of metakinesis. In the middle of metaphase chromosomes lie in the equatorial plate of spindle, producing so-called metaphase plate, or maternal star, in which the centromeres of chromosomes are turned to the center, and their shoulders — to periphery. It is possible to see at the end of metaphase, that each chromosome consists of the two chromatides, the shoulders of which lie parallely, they are separated by a crack, and they remain united only in the area of centromere. Metaphase occupies third of time of all mitosis.

 

Electron micrograph of a section of a rooster spermatocyte in metaphase. The figure shows the two centrioles in each pole, the mitotic spindle formed by microtubules, and the chromosomes in the equatorial plate. The arrows show the insertion of microtubules in the centromeres. Reduced from x19.000.

 

Anaphase. All sisterly chromatides simultaneously in all chromosomes lose communication between itself in the area of centromeres and synchronously begin to move to the opposite poles of cell at a speed of 0,2-0,5 mkm/min. They are oriented by centromeres to the poles, and shoulders — to the equator. It is the shortest stage of mitosis, which occupies just a few percents from all time of cellular division. Anaphase – very important stage of mitosis, exactly at this stage there is the division of two identical sets of chromosomes and their moving to the opposite ends of cell. Except for motion of chromosomes to the poles, poles themselves go away to. The mechanism of motion of chromosomes is exactly unknown. Lately most researchers adhere to the hypothesis of «sliding filaments», due to which the neighboring microtubules of spindle, cooperating between it and retractive proteins, pull chromosomes to the poles. Violation of passing of anaphase results in formation of aneuploids – genome mutation, which means the aliquant change of chromosomes amount in daughter’s cells.

Telophase begins when two diploid chromosomal sets stop in the poles. The orientation of chromosomes remains the same, as well as in anaphase with centromeres to the poles. Decondensation of chromosomes begins; they increase in a volume, in the places of their contacts with the cytoplasm membrane vesicles a nuclear envelope recommences. New nucleoli are forming. In telophase also there is the division of cellular body which has the name of cytotomy, or cytokinesis. In animal cells the division of cytoplasm takes place by formation of strangulation in the area of former equator; plasmalemma incurves into a cell. Thus in the cortical layer of cytoplasm in the area of equator circular actin fibrilles are disposed. Contraction of such ring results in division of cellular body. New G₁-neriod begins iew cells.

Images obtained with a confocal laser scanning microscope from cultured cells. An interphase nucleus and several nuclei are in several phases of mitosis. DNA appears red, and microtubules in the cytoplasm are blue. Medium magnification. A: Interphase. A nondividing cell. B: Prophase. The blue structure over the nucleus is the centrosome. Note that the chromosomes are becoming visible because of their condensation. The cytoplasm is acquiring a round shape typical of cells in mitosis. C: Metaphase. The chromosomes are organized in an equatorial plane. D: Anaphase. The chromosomes are pulled to the cell poles through the activity of microtubules. E: Early telophase. The two sets of chromosomes have arrived at the cell poles to originate the two daughter cells, which will contain sets of chromosomes similar to those in the mother cell. F: Telophase. The cytoplasm is being divided by a constriction in the cell equator. Note that the daughter cells are round and smaller than the mother cell. Soon they will increase in size and become elongated.

 

Always it should be remembered that before a cell enters into mitosis or meiosis she passes G₁-, S-, G₂-periods of cellular cycle. In S-period there is the synthesis or DNA duplication. As a result of it the amount of the DNA molecules is doubled. Thus, a cell always enters into mitosis with the already doubled amount of the DNA molecules, at a man — 92 (46х2).

 

 

Scheme of mitosis:

A — metaphase — chromosomes lie in the equator of cell in the process of spindle of division formation; the spindle of division consists of 3 types of microtubules: 1 — pole microtubules (prove the divergence of poles), 2 —microtubules of kinetochore (prove divergence of chromatides), 3 — astral microtubules (are connected with internal surface of cell membrane and fix the poles of division);

B — early anaphase — beginning of chromatides and poles divergention;

C— late anaphase — completion of process of chromatides and poles divergention in the cell; two independent processes occur in anaphase: 1) divergention of cell poles, 2) divergention of chromatides to the poles of cell.

 

Photomicrograph of cultured cells to show cell division. Picrosirius-hematoxylin stain. Medium magnification. A: Interphase nuclei. Note the chromatin and nucleoli inside each nucleus. B: Prophase. No distinct nuclear envelope, no nucleoli. Condensed chromosomes. C: Metaphase. The chromosomes are located in a plate at the cell equator. D: Late anaphase. The chromosomes are located in both cell poles, to distribute the DNA equally between the daughter cells.

 

The described cellular cycle is typical to the cells which are able for the division. But at the same time in an organism there are cells, which “go out” of cellular cycle. It is the so-called cells of G₀-period. They do not pass S-period and are not divided, being in a state of rest. The cells of G₀-period may lose their ability for division temporary or finally.

The steam cells of different tissues belong to the first type (for example, hemopoietic). They are the low differentiated cells, which, keeping a capacity for the division, for a long time move out cell cycle. Cells, which, losing a capacity for the division, are specialized, belong to the second type of differentiated cells. Among the cells of this type there are two subtypes. First ones becoming on the way of differentiation, forever lose a capacity for the division, some time they perform their functions and then dye. Such cells – the mature cells of blood, cells of epidermis and so on. The cells of second subtype after differentiation do not lose a capacity for the division and, at the proper time, can “return” to the cycle. For example, the cells of liver at deleting of part of organ begin to synthesize DNA and enter mitosis. Third type of the cells of G₀-period — are the high-differentiated cells, which in an adult organism irreversibly lose a capacity for the division and have the term of life, equaling the term of life of whole organism. These are, for example, nervous cells and cardiomyocytes.

 

Structure of mitotic chromosom

Chromosomes — are dense fibers-like bodies with a diameter 0,2-2 mkm and long at a man from 1,5 to 10 mkm, which are well painted by basic dyes and noticeable in a cell during the mitotic division. They were named by Valdeer. Condensation of chromatin with formation of chromosomes is the brightest sign of cell division. It is known, that chromosomes do not disappear with ending of mitosis, and exist in a nucleus during interphase too, but thanks to decondensation they acquire other kind and they are not visible as separate bodies. Main structure of both interphase, and mitotic chromosomes the molecules of deoxyribonucleoproteins lie — DNP.

Last time it is thought, that each chromosome is built from one giant molecule DNP, packed in a relatively short little body — actually mitotic chromosome. It is exposed, that length of the individual linear molecules DNA can achieve the hundred of micrometers and even a few centimeters. Among the chromosomes of man the first chromosome is greatest. She contains DNA molecule about 7 cm by general length. Total length of the DNA molecules in all chromosomes of one cell of man makes about 170 sm.

It is discovered, that in a mitotic chromosome the giant molecule of deoxyribonucleoprotein forms lateral loops. The typical chromosome of man can contain 2600 loops, each of which is formed by the area of chromatin fibril with middle length about 400 nm (0,4 mkm). Loops compactization conduces to formation of such structures as a chromonema fibril.

Best of all to study morphology of mitotic chromosomes in metaphase and at the beginning of anaphase, when they are condensed most of all. In each chromosome it is possible to mark the narrowed place — primary strangulation (centromere), which divides a chromosome into two shoulders.

Chromosomes, which have the shoulders of same length, are named metacentric. If one shoulder is shorter, a chromosome is submetacentric. The third variety of chromosomes has centromere, located almost at the end of shoulder (acrocentric).

In the area of the primary strangulation kinetochore (centromere) is located, which is the center of organization of microtubules, which constitute chromosomal filaments of spindle of division. Some chromosomes have also the secondary strangulation, which lie near one of ends of chromosome and chromosomes separating the so-called satellite. The secondary strangulation is yet named the organizers of nucleoli, because exactly in these areas nucleolus appears at the beginning of interphase. The end areas of shoulders are named telomeres.

Each type of organisms’ vegetable and animal has the specific number, sizes and structure of chromosomes. The aggregate of these signs of chromosomal set is named karyotype. Human karyotype characterized by the presence of 23 pairs of chromosomes among which 22 pairs of autosomes and one pair of sex chromosomes. Among last ones, gonosomes, there are X- and U-chromosomes.

 

 

Human karyotype. Structure of mitotic chromosomes

 

The chromosomes of man are dividing on sizes on seven groups — А, В, С, D, Е, F, G. The amount of chromosomal sets in a cell is designated by the term of ploidy and letter of n. Somatic cells have the double (diploid) set of chromosomes (2 n), gametes — single haploid (n) set. If a cell has a 3set of chromosomes, it is triploid, if 4n – tetraploid and others like that. A plenty of chromosomal sets is reflected by the term of polyploidy.

Considerable successes on the way of study of morphology of chromosomes were attained as a result of application of the special methods of their painting — the so-called differential colouring, in the first time offered T. Kasperson. It appeared that chromosomes were painted heterogeneously. Important there is that each chromosome at the such differential colouring has the unique picture. The last enables expressly to identify each chromosome in the set, and also make the so-called chromosomal cards with finding of localization in them out certain genes. In 2001 by efforts of american scientists, headed by prof. Watson, a project «Human genome» was completed – the sequence of nucleotides was deciphered and chromosomal localization of all genes of is known.

MEIOSIS

Original form of cellular reproduction, typical for the process of formation of gametes. Meiosis includes two successive mitotic divisions, without interphase. One maternal cell produces 4 daughter cells. Cells with the haploid chromosomal set appear as a result of meiosis. Crossingover is the characteristic feature of meiosis prophase — exchange by the homologous areas of chromosomes, providing changeability of organisms.

 

THE MEIOSIS MAIN FEATURES

·         typical only for gametes (germ cells)

• consists of two successive divisions with short interphase between them

• prophase of first division is enough complicated and consists of 5 stages

• in zigonema of 1^st division prophase homologous chromosomes (bivalents or tetrads) are connected and remain linked between itself to the 1^st division anaphase

• in anaphase of 1^st division whole chromosomes consisting of the two chromatides are moving to the poles instead of chromosomes division on separate chromatides, as in a mitosis

• in interphase between 1^st and 2^nd divisions of meiosis S-period is absent and before the 2^nd division DNA is not duplicated

• in the process of meiosis daughter’s cells are not fully separated, and remain thin cytoplasmic bridges

 

Meiosis occurs of two types: spermatogenesis and ovogenesis.

Spermatogenesis takes place in testis of men and result in spermatozoa development. It begins in pubertation and lasts permanently all the life. One spermatogone gives rise for 4 mature spermatozoa almost of the same size: 2 androspermia (with Y gonosomes) and 2 gynecospermia (with X gonosomes). Billions of spermatozoa are developing every day in seminiferous tubuli of testis. Duration of spermatogenesis – 75 days (72 days in testis and then 3 more days in epididymis).

Ovogenesis begins in pubertation too, but usually it lasts till menarche (50-52 years). This is periodic process of oocyte development each menstrual cycle. Usually 1 oocyte is ovulated every month. 1 oocyte and 2-3 residual bodies appear in ovogenesis from 1 ovogony.

 

 

Part of a seminiferous tubule with its surrounding tissues. The seminiferous epithelium is formed by 2 cell populations: the cells of the spermatogenic lineage and the supporting or Sertoli cells.

Both spermatogenesis and ovogenesis include 3 main stages: reproduction (mitotic division of spermatogonia and ovogonia), growth and maturation with two divisions, which result in appearance of four spermatids and one ovum with residual (polar) bodies. Spermatogenesis has one more stage – spermatozoa formation of spermatids in which cells absolutely change their shape from round to flagellated. This is necessary for future motility of spermatozoa.

Diagram showing the clonal nature of the germ cells. Only the initial spermatogonia divide mitotically and produce separate daughter cells. Once committed to differentiation, the cells of all subsequent divisions stay connected by intercellular cytoplasmic bridges. Only after they are separated from the residual bodies can the spermatozoa be considered isolated cells.

Interstitial cells and cells of the seminiferous epithelium. H&E stain. High magnification.

 

It is necessary to note that process of spermatogenesis is regulated by endocrine system (follicle stimulying hormone of adenopityitary) and actively controlled by Sertoli cells, which sustain, protect and select normal cells.

The Sertoli cells form the blood-testis barrier. Neighbor Sertoli cells are attached by occluding junctions that divide the seminiferous tubules into 2 compartments and impede the passage of substances between both compartments. The basal compartment comprises the interstitial space and the spaces occupied by the spermatogonia. The adluminal compartment comprises the tubule lumen and the intercellular spaces down to the level of the occluding junctions (OJ). In this compartment are spermatocytes, spermatids, and spermatozoa. Cytoplasmic residual bodies from spermatids undergo phagocytosis by the Sertoli cells and are digested by lysosomal enzymes. The myoid cells surround the seminiferous epithelium.

 

The principal changes occurring in spermatids during spermiogenesis. The basic structural feature of the spermatozoon is the head, which consists primarily of condensed nuclear chromatin. The reduced volume of the nucleus affords the sperm greater mobility and may protect the genome from damage while in transit to the egg. The rest of the spermatozoon is structurally arranged to promote motility. Bottom: The structure of a mature spermatozoon.

 

 

Amitosis

Amitosis is direct division of cell, at which a cell is divided without the preceding duplication of the DNA molecules. Therefore daughter’s cells have a different amount DNA. The spindle of division does not appear at amitosis, often there is the division of nucleus without the division of cytoplasm. Amitosis is observed in senescent cells, at different pathological processes, and, as a rule, results in formation of nonviable cells. This is the basic point of cells malformation and neoplasm appearance.

At the same time such type of cells division is necessary at damage (wound) of cells tissues and organs. It allows producing a lot of new cells in short time, because amitosis requires less time as compare to mitosis (S period is absent). Being controlling by macrophages this process is suitable for reparative regeneration but overproduction of abnormal cells causes’ malformation.

 

Section of a malignant epithelial skin tumor (squamous cell carcinoma). An increase in the number of cells in mitosis (rapid mitoses) and diversity of nuclear morphology (nuclei have different sizes and shapes) are signs of malignancy. PT stain. Medium magnification.

 

Section of a fast-growing malignant epithelial skin tumor showing an increased number of cells in mitosis in the low layers of epidermis and great diversity of nuclear morphology. Cells have different sizes smaller as normal one with higher nuclear-cytoplasmic ratio. PT stain. Medium magnification.

 

Endomitosis

Process at which a cell passes the cellular cycle S-period with the subsequent division of nucleus, but without the division of cytoplasm or without the division of both nucleus and cytoplasm (G₁ — S — G₂ — G₁; Metaphase is absent). Formation of polyploid cells is the result of endomitosis, with a multiple 2 increase of chromosomal set. On the basis of mitotic cycle special mechanisms appeared by which the amount of the inherited material can be enlarged at saving of constancy of number of cells. Endomitosis or endoreproduction — this is formation of cells with the DNA amount increase, which takes place as a result of mitosis blocking on certain stages. It means that in endomitosis normal karyokinesis is not accompanied with cytokinesis.

The stop of mitosis is possible after G₁-period, and then a cell can pass a next cycle of DNA replication. It will stipulate growth of amount of chromosomal sets in the four—eight and more times. Morphologically such nucleus is similar to another ones except of larger size. The stop of mitosis is possible also in prophase or metaphase, when the function of spindle of division is violated. Finally, passing by the cell of all phases of mitosis is possible; including telophase, but the division of cellular body is blocked (cytokinesis). So, there are cells with two nuclei (for example, cells of liver, smooth myocytes in uterus, cardiomyocytes often have two nuclei). As a result of series of endomitoses there are the giant polyploid cells of red bone marrow megakaryocytes which may have the number of chromosomal sets to 64 n. Multinucleated osteoclasts are typical for bony tissue, where they solve the bony matrix.

It is necessary to note that polyploid cells can be found only among the specialized differentiated cells, they never divide and can’t produce similar to them daughter cells. Having more hereditary information such cells may perform their functions much better.

 

Necrosis

The series of cellular cycles are completed by death of cell. There are two basic forms of cellular death – necrosis and apoptosis.

Necrosis is the death of cell as a result of damaging influence of external factors (physical, chemical or biogenic) changing penetration of membranes and processes of cellular metabolism. It is always accompanied by inflammation of surrounding tissues.

Necrosis is not genetically controlled cellular death under influence of external factors. Iecrosis the following stages are selected: phase of early changes (paranekrosis or necrobiosis) and proper necrosis. In the period of early changes a cell is morphologically unchanging. 1-3 h must pass the changes recognizable at an electron microscopy or histochemically will appear, and at least 6-8 h will pass till the changes seen at a light microscopy will appear; the macroscopic changes develop yet later. For example, if a patient with the heart attack of myocardium dies a few minutes after a moment of heart attack beginning (pain because of insufficient blood supply of myocardium), on autopsy no structural certificate of necrosis will be exposed; if death treads on a 2^nd day after a sharp attack, the changes will be obvious.

The morphological changes characteristic for necrosis are the next:

• At first, these are the changes iucleus. Chromatin in dead cell is condensed in large lumps and a nucleus becomes diminished in a volume, wrinkled, dense, and prominent basophilic. There is karyopyknosis. The pyknotic nucleus can be fragmented on numerous little basophilic particles (karyorrhexis) or exposed to the lyses (to dissolution) as a result of lysosomal deoxyribonuclease action (karyolysis). Then it is multiplied in a volume, is poorly stained with haematoxylin and the contours of nucleus are gradually lost. At quickly developing necrosis a nucleus is exposed to the lyses without a pyknotic stage.

• Secondly, there are the cytoplasmic changes: it is approximate in 6 h since a cell was exposed to necrosis, its cytoplasm becomes homogeneous and is prominent acidophilic, that is painted by acidic dyes intensively, for example, in a rose color at coloring by eosin. This is first change exposed by a light microscopy, which arises up as a result of coagulation of cytoplasmic proteins and destruction (disappearances) of ribosomes. Ribosomal RNA gives a basophily to the normal cytoplasm. Specialized cells organells, for example, myofibrils in cardiomyocytes, disappear above all things. Swelling of mitochondria and destruction (destruction) of organells membranes cause vacuolization of cytoplasm. Finally, selfdigestion of cell by enzymes, which free oneself from own lysosomes, causes the lyses of cell (autolysis).

• Thirdly, it is the changes of intercellular substance, which engulf both the ground substance and fibers. More frequent the following changes develop than all: elastic and reticulin fibers are transformed into rose dense, homogeneous, sometimes basophilic masses, which can be exposed to fragmentation, disintegration or lyses. Rarer there can be the edema, lyses and mucous transformation of fibers structures.

At the first stage of necrosis because of damaging pathogenic influence there is dysfunction of cell membrane, that results in the loss by her membrane potential, as a result concentration of calcium ions, which move in a cell on the gradient of concentration, increases in her sharply, as in a cell a normal concentration of these ions is 1000 times less than in an intercellular environment. It results in activating of lysosomal enzymes and inhibition of respiratory mitochondrial enzymes.

At the 2^nd stage of necrosis endonucleases are activated at first, which results in a hydrolysis and breaking up of DNA, because of what specialized organells are destroying, there is karyopyknosis, karyorrhexis, karyolysis. Proteases are activated too; afterwards there is destruction and self digestion of endoskeleton. Increase of phospholipases activity results in destruction of membranes and output of enzymes with the fragments of organells in the intercellular space, and as a result inflammatory process begins.

 

Karyorrhexis. Karyolysis.

 

Apoptosis

The programmed death of cells, which is started by the products of expression of own genes of self-destruction. Apoptosis is the variant of cellular death, which takes place iormal physiological terms, and a cell is the active participant of the own death. Apoptosis is most often observed during the ordinary cellular update, at maintenance of tissue homeostasis, in embryogenesis, at induction and maintenance of immunological tolerance, tissue atrophy.

Apoptosis – the genetically controlled death of cell, takes place in 2 stages: the first stage is the stage of reversible changes, during which the process of apoptosis can be stopped and cellular structures will be reparated. Apoptosis lasts from initiation to beginning of the DNA fragmentation.

Phase second, irreversible, and during which cellular structures collapse and a cell forms an apoptotic bodies.

The first stage can pass on different scenarios depending on the mechanism of induction of apoptosis. Both a return to the normal state of cell and delay of apoptosis or transition iecrosis at lack of power resources is here possible (for example, delay of apoptosis in cardiomyocytes at the ATF failing as a result of ischemia). Direction and sequence of events of the second stage do not rely on the initiator of process.

In the course of apoptosis there are the following morphological changes: at first, cell compression. A cell diminishes in sizes; a cytoplasm is made more compact; organells, which look relatively normal, are disposed more compact.

It is assumed, that violation of form and volume of cell takes place as a result of activating of transglyutaminase in the apoptotic cells. This enzyme causes progressive formation of cross communications in cytoplasmic proteins, which result in forming of original shell under a cellular membrane, like in the keratinized cells of epithelium.

Secondly, it is condensation of chromatin. These are most characteristic evidences of apoptosis. Chromatin is condensed on periphery, under the membrane of nucleus, expressly outlined dense masses of a different form and sizes appear here. A nucleus can be torn on the two or a few fragments.

The mechanism of chromatin condensation is enough well studied. He is conditioned by breaking up of nuclear DNA in places, linking separate nucleosomes, that results in development of a plenty of fragments in which the number of pairs of grounds is divided on 180-200. At electrophoreses fragments give the characteristic picture of stair. This picture differs from such at necrosis of cells, where length of the DNA fragments varies. The DNA fragmentation iucleosomes takes place under the influence of calcium sensitive endonucleases. Endonuclease in some cells is found constantly (for example, in thymocytes), where it is activated by appearance in the cytoplasm of free calcium, and in other cells is synthesized before the beginning of apoptosis. However it is not yet set, how does chromatin condensation occur after breaking up of DNA by endonuclease?

Thirdly, the forming of cavities in the cytoplasm, and also subsequent formation of apoptotic bodies. In an apoptotic cell deep invaginations of surface with formation of cavities are originally formed, which results in fragmentation of cell and formation of membranebounded apoptotic bodies, consisting of cytoplasm and densely located organells, with or without the fragments of nucleus.

After formation an apoptotic bodies they are phagocized by surrounding healthy cells, either parenchimatous cells, or by macrophages. Apoptotic bodies are quickly collapsed in lysosomes, and surrounding cells either migrate, or are dividing, to fill free after death of cell space.

Phagocytosis of an apoptotic bodies by macrophages or other cells is activated by receptors on these cells.

At the 1^st phases of apoptosis jun-N-terminal kinase is activated because of cellular stress action, that results in the calcium expelling and cytochrome C output from mitochondria, thus kaspase 9 (kaspase -activator) is activated, which activates kaspase 3 (kaspase-effektor), as a result proteins proteolysis and the DNA fragmentation begins.

Since happened proteins proteolysis, DNA loses communication with karyolemma and is fragmented, whereupon chromatin is nebulized in a cytoplasm and the DNA molecule collapses to 180-200 p.o. All these processes carry the name of karyopyknosis. Further there is the break of nucleus (karyorrhexis). Simultaneously with it there is the water loss by the cell, that’s why it shrivels. At the finishing stage there is dysfunction of cell membrane, appearance of vesicles on the surface of cell and their separation, and then phagocytosis of apoptotic bodies.

Apoptosis is induced by physiological stimuli and is regulated by the special genes and their products, for example Cip1, Bax, Daax, FAF-1, FADD, TRADD, RAIDD, RIP, SIVA, FLIP, CAS, TIA-1/TIAR, TDAG51, p53, Fas/Apo-1, Bcl-2, Bad, Bag1, fos, myc.

 

Section of a mammary gland from an animal whose lactation was interrupted for 5 days. Note atrophy of the epithelial cells and dilation of the alveolar lumen, which contains several detached cells in the process of apoptosis, as seen from the nuclear alterations. PT stain. Medium magnification.

 

Electron micrograph of a cell in apoptosis showing that its cytoplasm is undergoing a process of fragmentation in blebs that preserve their plasma membranes. These blebs are phagocytized by macrophages without eliciting an inflammatory reaction. No cytoplasmic substances are released into the extracellular space.

 

References:

a) main

1.     Practical classes materials from theme “Cytology. Cytoplasm structure. Cell nucleus. Cell reproduction. Aging and death of the cell” (Intranet).

2.     Lecture presentations from theme “Introduction in histology. Cytology and embryology subject and tasks” (Intranet).

3.     Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 14–32.

4.     Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 6–44.

5.     Inderbir Singh Text book of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002. – P. 14–36.

6.     Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 22–25, 35–97.

 

b) additional

1.     Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. – Lippincott Williams and Wilkins, 2008. – P. 10–13, 19–25.

2.     Junqueira L. Basic Histology / L. Junqueira, J. Carneiro, R. Kelley. – [seventh edition]. – Norwalk, Connecticut : Appleton and Lange, 1992. – P. 27–56.

3.     Disk:

http://intranet.tdmu.edu.ua/data/teacher/video/hist/  

4.     Volkov K. S. Ultrastructure of cells and tissues / K. S. Volkov, N. V. Pasechko. – Ternopil : Ukrmedknyha, 1997. – P. 26–47.

http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf

http://en.wikipedia.org/wiki/Histology

http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html

http://www.udel.edu/biology/Wags/histopage/histopage.htm