Connective tissues
Using lectures (on the web-page of histology ndepartment), lecture presentations textbooks, additional literature and other nresourses, students should prepare such theoretical questions:
1. nStructural and functional characteristics of connective tissue that distinguish nit from the other basic tissue type.
2. nFunctions carried out by connective tissue.
3. nFundamental components of loose connective tissue.
4. The nbiochemical composition and the sites of synthesis of extracellular matrix components.
5. nComparison of the collagen, elastic and reticular fibers in terms of their nstructure and function.
6. nStructure and function of the various cell types found in the loose connective ntissue.
7. nStructural and functional characteristics of connective tissue that distinguish nit from the other basic tissue type.
8. nComparison of the different types of dense connective tissues in terms of the ntypes, relative amounts, and arrangement of cells, fibers, and ground nsubstance.
9. nRelations of the composition of the various connective tissues with special nproperties to their specific functions.
10. nName body organs where each connective tissue type may be found and relate nthese locations to the tissue’s function.
Connective ntissue is a term applied to a basic type nof tissue of mesodermal origin, which provides structural and metabolic support nfor other tissues and organs throughout the body. Connective ntissue usually contains blood vessels and mediates the exchange of nutrients, nmetabolites and waste products between tissues and the circulatory system. The ntraditional term “connective tissue” thus badly does justice to the wide rang nof functions of this type of tissue and it is now probably more appropriate to nuse the term supporting tissue. Connective tissue occurs in many different nforms with diverse physical properties.
1. Visceral organs (e.g., kidneys, lungs) contain an abundance of nconnective tissue that holds the parenchymal epithelial cells together to form nthe organ.
2. The cardiovascular system is rich in connective tissues. Here, they tie nmuscle cells and endothelial cells together into a functionally integrated nsystem.
3. Skeletal muscles are bound together by connective tissues and attached nto bones by ligaments and tendons, which are types of specialized connective ntissue.
4. The central nervous system (CNS) contains less connective tissue than other systems. nLittle connective tissue is associated with the nervous system parenchyma of nneurons and glair cells; however, collagenous fibers and connective tissue ncells form a sheath around the brain and spinal cord. Also, some connective ntissue exists in and around peripheral nerves and ganglia.
Organs ncan be divided into parenchima, which is composed of the cells nresponsible for the main functions typical of the organ, and stroma, nwhich is supporting tissue. Except in the brain and spinal cord, the stroma is nmade of connective tissue.
The functions of connective tissues, determined chiefly by their mechanical properties, ninclude the binding together, compartmentalization, support, and physical and nimmunologic protection of other tissues and organs, as well as storage.
Support. Structural support is the major function of nconnective tissue, which forms the framework upon which all other body tissues nare assembled. Its physical properties allow it to bind, to fill spaces, and to nseparate functional units of other tissues and organs. It thus maintains nfunctional units in their proper 3-dimensional relationships, allowing nmaintenance and coordination of all body functions.
Defense physical. The viscosity of the extracellular matrix, due nlargely to hyaluronic acid, slows the progress of many bacteria and foreigparticles. Sheets of tightly packed and often interwoven collagen fibers, as iorgan capsules, help to confine local infections. However, some bacteria nsecrete enzymes that break down matrix components; eg, staphylococci, nclostridia, streptococci, and pneumococci secrete hyaluronidase.
Defense immunologic. Foreign bodies that successfully penetrate epithelia nare intercepted by immunoresponsive cells that inhabit the underlying nconnective tissue. These cells not only activate a local immune response n(inflammation) but mobilize the immune system to supply additional cells via nthe bloodstream. Recruited cells migrate through capillary and venule walls ninto the connective tissue, a process called diapedesis.
Repair. Rapidly closing any breaches in the body’s nprotective barriers is an important function of connective tissue. Injury nstimulates invasion of the site by immunocom-petent cells and the proliferatioof fibroblasts. Macrophages remove clotted blood, damaged tissue, and foreigmaterial, while fibroblasts secrete extracellular matrix materials to fill the nbreach. Rapidly formed collagenous matrices that close wounds are often less nwell organized than the original tissues and form scars. Small scars may neventually be completely remodeled; larger scars are only partially remodeled.
Storage. Reserves of water and electrolytes, especially nsodium, are stored in the extracellular matrix, owing to the high polyamonic ncharge density of glycosammoglycans. Energy reserves in the form of lipids are nstored in adipocytes.
Transport. Except in the central nervous system, most blood and nlymphatic vessels are surrounded by loose connective tissue, which is thus a ncrossroads for transporting substances to and from other tissues.
All connective ntissue cell types derive from cells of the embryonic mesenchyme. Mesenchyme derives from embryonic mesoderm, except nhead mesenchyme, which derives from the neural crest (mesectoderm).
Simplified representation of the connective tissue cell nlineage derived from the multipotential embryonic mesenchyme cell. Dotted narrows indicate that intermediate cell types exist between the examples nillustrated. Note that the cells are not drawn in proportion to actual sizes, neg, adipocyte, megakaryocyte, and osteoclast cells are significantly larger nthan the other cells illustrated.
All connective tissues have two nmajor constituents, cells and extracellular material. Extra cellular material nis the constituent, which determines the physical properties of each type of nconnective tissue. Extra cellular material consists of a matrix of organic nmaterial called ground substance within which are embedded a variety of fibers.
Connective tissue types and subtypes nare classified according to the amounts, types, and proportions of these ncomponents.
There are three nmain types of connective tissue:
1. nConnective tissue proper.
2. nConnective ntissue with special properties.
3. nSkeletal tissues(supporting nconnective).
PROPER CONNECTIVE TISSUE
Connective tissue proper, found in most norgans, is characterized by a predominance of fibers (mainly type I collagen) nin the extracellular matrix. Its varied functions are chiefly related to nbinding cells and tissues into organs and organ systems. Its subclasses are nbased on the type, density, and orientation of its fibers.
1. Loose connective tissue Loose connective tissue or areolar tissue ngenerally appears very disorganized. It consists of a loosely arranged network nof different types of fibers, upon which many kinds of fixed and wandering ncells are suspended. The ground substance is nabundant and only moderately viscous. This flexible yet delicate tissue nsurrounds and suspends vessels and nerves as they traverse most organs, nunderlies and supports most epithelia, and fills spaces between other tissues n(e.g., between muscle fibers and their dense connective tissue sheaths). It nalso supports the serous membranes (mesothelia) of the pleura, pericardium, and nperitoneum. Always well vascularized, areolar tissue conveys oxygen and nnutrients to avascular epithelia. Its cells function in immune surveillance for nforeign substances entering the body through the blood or epithelia.
Simplified nrepresentation of the loose connective tissue cells
Loose nconnective tissue (surface view)
Stained nwith iron haematoxylin. Medium magnification
1. Fibroblasts.
2. Collagefibers.
3. Elastic nfibers.
2. Dense connective tissue Fibers are the npredominant component of dense connective tissue. Nearly all the fibers nare of type I collagen. The cells are predominantly mature fibroblasts n(fibrocytes). The ground substance is essentially identical to that of areolar ntissue but is present in lesser quantities. There are 2 types of dense nconnective tissue: regular, with a ropelike arrangement of fiber bundles, and nirregular, with a fabriclike arrangement.
a. Dense regular connective tissue The fibers of this tissue are ntightly packed into parallel bundles, between which are a few highly nattenuated, spindle-shaped fibroblasts. The small, cigar-shaped nuclei of the fibroblasts are noriented parallel to the fibers; the cytoplasm is difficult to distinguish with nthe light microscope. There is little room for the ground substance, which nnevertheless permeates the tissue. The tensile strength of the packed collagefibers makes them ideal for transmitting mechanical force over long distances nusing a minimum of material and space, while resisting mechanical forces from nother directions. This tissue therefore serves to transmit the force of muscle ncontraction, to attach bones to one another, and to protect other tissues and norgans. It is found in tendons, ligaments, periosteum, perichondrium, deep nfascia, and some organ capsules.
Longitudinal section of dense regular connective tissue n(tendon). Bundles of collagen fibers fill the spaces between the elongated nfibroblasts. H&E stain. Medium magnification.
Longitudinal section of dense regular connective tissue from na tendon. A: Thick bundles of nparallel collagen fibers fill the intercellular spaces between fibroblasts. Low nmagnification. B: Higher nmagnification view of a tendon of a young animal. Note active fibroblasts with nprominent Golgi regions and dark cytoplasm rich in RNA. PT stain.
Electron micrograph of a fibrocyte in dense regular nconnective tissue. The sparse cytoplasm of the fibrocytes is divided into nnumerous thin cytoplasmic processes that interdigitate among the collagefibers. x25,000.
b. Dense irregular connective tissue The components of this tissue are identical to those nfound in dense regular connective tissue. At first glance, dense irregular nconnective tissue seems poorly organized, but its collagebundles have a complex woven pattern that provides resistance to tensile stress nfrom any direction. Dense irregular connective tissue has numerous extra ncellular fibers in dense random arrays. Its functions include covering the more nfragile tissues and organs and protecting them from multidirectional nmechanical stresses. It is found in the reticular layer of the dermis and imost organ capsules.
Dense irregular connective tissue from human dermis ncontains thick bundles of collagen fibers, fibroblast nuclei (arrowheads), and na few small blood vessels (bv). H&E stain. Medium magnification.
Section of immature dense irregular collagen tissue. This nfigure shows numerous fibroblasts (arrow) with many thin cytoplasmic extensions n(arrowheads). As these cells are pressed by collagen fibers, the appearance of ntheir cytoplasm depends on the section orientation; when the section is nparallel to the cell surface, parts of the cytoplasm are visible. PT stain. nMedium magnification.
Dense irregular connective tissue contains many nrandomly oriented bundles of collagen fibers. H&E stain. Medium nmagnification.
Loose and dense irregular connective tissue of dermis nof the skin. H&E stain. Low magnification.
CELLULAR ELEMENTS
The loose connective tissue ncontains the following cell types: fibroblasts, undiferentiated (mesenchymal), nmacrophages, and a varying, but much smaller, number of lymphoid wandering ncells, mast cells, eosinophils, plasma cells, pigment cells and fat cells.
Fibroblasts These are the predominant connective tissue cells nand are ubiquitous in connective tissue proper. They synthesize, secrete, and nmaintain all the major components of the extra cellular matrix, bind extra ncellular matrix constituents to form tissue, and facilitate wound healing. nFor example, when the skin is cut, fibroblasts proliferate and migrate toward nthe wound to fill the gaps in the tissue. As proliferation continues, they nsecrete large amounts of extra cellular matrix. Fibroblasts form the scar that ncloses the wound.
Section of rat skin. A nconnective tissue layer (dermis) shows several fibroblasts (F), which are the nelongated cells. H&E stain. Medium magnification.
Structurally, nfibroblasts are of 2 types, one of which resembles mesenchymal ncells. This type is stellate, with long cytoplasmic processes and a large, novoid, pale-staining nucleus. The cytoplasm contains abundant rough endoplasmic nreticulum and Golgi complex, and this cell type is important in the production of ncollagen and other matrix components. Also, lysosomes and vacuoles of secretioproduct are prominent features of fibroblasts. Fibroblast secretions include ncollagen, fibronectin, glycoproteins, and proteoglycans. Fibroblasts have many nactin-containing microfilaments because they are highly motile cells. nFibroblasts also contaiumerous microtubules, which probably help maintaithe fibroblastic cell morphology.
Quiescent fibroblasts are elongated cells with thicytoplasmic extensions and condensed chromatin. Pararosaniline-toluidine blue n(PT) stain. Medium magnification.
Cells of the second type are nless active and are sometimes termed fibrocytes, nbecause they are believed to be more mature. Fibrocytes are smaller and nspindle-shaped, with a dark, elongate nucleus and fewer cytoplasmic norganelles. They can revert to the fibroblast state and participate in tissue nrepair.
Active (left) nand quiescent (right) nfibroblasts. External morphologic characteristics and ultrastructure of each ncell are shown. Fibroblasts that are actively engaged in synthesis are richer nin mitochondria, lipid droplets, Golgi complex, and rough endoplasmic reticulum nthan are quiescent fibroblasts (fibrocytes).
Electron micrograph revealing portions of several flattened nfibroblasts in dense connective tissue. Abundant mitochondria, rough nendoplasmic reticulum, and vesicles distinguish these cells from the less nactive fibrocytes. Multiple strata of collagen fibrils (C) lie among the nfibroblasts. x30,000.
Macrophages These are large, stellate cells derived from cells of nthe blood monocyte lineage that infiltrate connective tissue and develop into nphagocytes. They have a highly variable shape nbecause they move throughout connective tissues. They have a single irregularly shaped nucleus with none or two prominent indentations and a conspicuous mass of euchromatin. nResident macrophages can proliferate and form additional macrophages. Dye nparticles injected into the body are engulfed by these cells and accumulate icytoplasmic granules. Macrophages contain many lysosomes, which aid idigesting phagocytosed materials, and a well-developed Golgi complex. nMacrophage lysosomes contain specific bacteriocidal agents including nlysozyme, a hydrolytic enzyme for degrading bacteria cell walls containing noligosaccharide, and superoxide dismutase, an enzyme that helps generate nbacteriocidal oxygen-free radicals and hydrogen peroxide. They help maintaithe integrity of connective tissues by removing foreign substances and cellular ndebris, and they participate in the immune response by presenting phagocytosed nantigens to lymphocytes. Macrophages also have a prominent cortical array of nactin-contaimng microfilaments and many microtubules and intermediate filaments nfor locomotion and phagocytosis.
Section of pancreas from a rat injected with the vital dye ntrypan blue. Note that 3 macrophages (arrows) have engulfed and accumulated the ndye in the form of granules. H&E stain. Low magnification.
Accumulation of the dust by macrophages. H&E stain. nMedium magnification.
Electron micrograph of a macrophage. Note the secondary nlysosomes (L), the nucleus (N), and the nucleolus (Nu). The arrows indicate nphagocytic vacuoles.
To remove large nforeign objects such as splinters, macrophages may fuse to form multinuclear ngiant cells. Monocyte-derived phagocytes, which together constitute the nmononuclear phagocyte system, include:
(1) Bone marrow stem cells
(2) Monocyte precursors in bone marrow
(3) Monocytes in peripheral blood
(4) Fixed nmacrophages in connective tissues n(histiocytes)
(5) Phagocytic nKupffer cells lining the liver nsinusoids
(6) Alveolar nmacrophages and free nmacrophages in serous cavities
(7) Free nand fixed macrophages in the nspleen, lymph nodes, and thymus
(8) Microglia in the CNS. n(Other CNS glia such as astrocytes and oligodendroglial cells are derived from nembryonic neuroectoderm; however, convincing evidence exists that mi-croglial ncells are derived from mesenchyme and are part of the mononuclear phagocyte nsystem.)
(9) Osteoclasts nin bone. (Osteoclasts are derived from monocytes and share many of the nmorphologic and functional characteristics of other mononuclear phagocyte nsystem components.)
Macrophages also nexhibit chemotaxis. They can move up a concentration gradient of certaicomplement system components (materials released during an inflammatory nresponse) and bacterial components. Chemotaxis is important for attracting nphagocytic cells and increasing their number in areas of bacterial invasion and ninflammation.
Electron micrograph of several macrophages and 2 eosinophils nin a region adjacent to a tumor. This figure illustrates the participation of nmacrophages in tissue reaction to tumor invasion.
Mast cells are a distinct cell type but are morphologically nsimilar to basophils in peripheral blood. Mast cells are widely distributed iloose areolar connective tissues and are especially plentiful in the lamina npropria, which is the areolar connective tissue beneath the moist mucosal nepithelium of many visceral organs. These derive from bone marrow precursors nand are characterized by abundant basophihc cytoplasmic granules. At the EM nlevel, these granules appear as electron-dense granules. Other features of mast ncells at this level are many small plasma membrane folds and a well-developed nGolgi complex.
Mast ncells are involved in inflammatory reactions and immediate hypersensitivity nallergic reactions and are relatively common in the lamina propria of the nrespiratory and digestive systems.
Section of rat tongue. Several mast cells in the connective ntissue surround muscle cells and blood vessels. PT stain. Medium magnification.
Mast cells in the connective ntissue. Methylene blue. Medium magnification.
(1) Mast cells bind a portion of the nimmunoglobulin E (IgE) molecules released into the serum during exposure to nantigens such as ragweed pollen.
(2) Upon re-exposure to the antigen, nthe IgE bound to the surface of the mast cells facilitates release of nhistamine (which promotes capillary leakage and edema), slow reacting nsubstance (which promotes smooth muscle contraction and blood vessel nleakage), eosinophil chemotactic factor (which attracts eosinophils), nand heparin (which inhibits coagulation).
Electron micrograph of a human mast cell. The granules n(G) contain heparin and histamine. Note the characteristic scroll-like nstructures within the granules. M, mitochondrion; C, collagen fibrils; E, nelastic fibril; N, nucleus. x14,700. Inset: nHigher magnification view of a mast cell granule. x44,600.
Mast-cell nsecretion.
1: IgE molecules are bound nto the surface receptors.
2: After a second exposure nto an antigen (eg, bee venom), IgE molecules bound to surface receptors are ncross-linked by the antigen. This activates adenylate cyclase and results ithe phosphorylation of certain proteins.
3: At the same time, Ca2+ nenters the cell.
4: These events lead to nintracellular fusion of specific granules and exocytosis of their contents.
5: In addition, nphospholipases act on membrane phospholipids to produce leukotrienes. The nprocess of extrusion does not damage the cell, which remains viable and nsynthesizes new granules. ECF-A, eosinophil chemotactic factor of anaphylaxis.
Mesenchymal cells These are the precursors of most cells indigenous to nconnective tissues, including fibroblasts and adipose cells. Embryonic nmesenchyme consists of a loose network of stellate mesenchymal cells and nabundant intercellular fluid. Some mesenchymal cells remain undifferentiated iadult connective tissue and constitute a reserve population of stem cells ncalled adventitial cells, which are ndifficult to distinguish from some fibroblasts and pericytes in the basement membrane nof capillary wall.
Plasma cells These differentiate nfrom antigen-stimulated B lymphocytes and are the primary producers of ncirculating antibodies. They are sparsely distributed throughout the nbody but abundant in areas susceptible to penetration by bacteria. Plasma cells nare large and ovoid, with an eccentric nucleus and abundant rough endoplasmic nreticulum. The characteristic “clock face” of the nucleus results nfrom a large, central nucleolus and several large heterochromatin clumps nregularly spaced inside the nuclear envelope. These cells usually exhibit a nclear juxtanuclear area (cytocenter) containing a well-developed Golgi complex nand centrioles.
Portion of a chronically inflamed intestinal villus. The nplasma cells are characterized by their size and abundant basophilic cytoplasm n(rough endoplasmic reticulum) and are involved in the synthesis of antibodies. nA large Golgi complex (arrows) is where the terminal glycosylation of the nantibodies (glycoproteins) occurs. Plasma cells produce antibodies of nimportance in immune reactions. PT stain. Medium magnification.
Plasma ncells with a “clockface nucleus” in loose connective tissue. PT stain. High nmagnification.
Ultrastructure of a plasma cell. The cell contains a nwell-developed rough endoplasmic reticulum, with dilated cisternae containing nimmunoglobulins (antibodies). In plasma cells, the secreted proteins do not naggregate into secretory granules. Nu, nucleolus.
Electron micrograph of a plasma cell showing aabundance of rough endoplasmic reticulum (R). Note that many cisternae are ndilated. Four profiles of the Golgi complex (G) are observed near the nucleus n(N). M, nmitochondria.
Section of an inflamed intestinal lamina propria. nInflammation was caused by nematode parasitism. Aggregated eosinophils and nplasma cells function mainly in the connective tissue by modulating the ninflammatory process. Giemsa stain. Low magnification.
Reticular cells These reticular connective tissue cells make up a nfunctionally diverse yet morphologically similar group. They produce the reticular fibers that form the netlike stroma of nhematopoietic and lymphoid tissues. Some apparently can phagocytose nantigenic material and cellular debris. Others (antigen-presenting cells) collect nantigens on their surfaces and help activate immunocompetent cells to mount aimmune response. Reticular cells are typically stellate wth long, thicytoplasmic processes. Each has a central, pale, irregularly rounded nucleus nand a prominent nucleolus. In the cytoplasm, the number of mitochondria and the ndegree of development of the Golgi complex and rough endoplasmic reticulum is nvariable. Some reticular cells, particularly those with less developed norganelles, may be stem cells of various blood types.
Adipose cells Adipose cells, or adipocytes, are mesenchymal nderivatives specialized as storage depots for lipids. They are found scattered nsingly or in groups in the loose connective tissue, especially along the blood nvessels. When they accumulate in large numbers and crowd out the other cells, nthe tissue is transformed into adipose tissue.
Other blood-derived nconnective tissue cells Many nwandering cell types originate in the bone marrow and are carried to connective ntissue by the blood and lymph. Blood-derived cells found in connective tissues ninclude the leukocytes (white blood cells, i.e., lymphocytes, monocytes, neutrophils, eosinophils, nand basophils) which have roles in the immune response. When injury or ninflammation damages tissue, leukocytes (e.g., monocytes), lymphocytes, and nphagocytic granulocytes (e.g., neutrophils, eosinophils, basophils) leave the ncirculation and join fibroblasts and other connective tissue resident cells to repair ndamage and combat microorganisms that cause inflammation.
EXTRACELLULAR MATRIX
The fibers and nground substance constitute the extra cellular matrix. Connective tissues contain abundant matrix, which nlargely determines their mechanical properties. The fibers are of 3 types, ncollagen fibers, elastic fibers and reticular fibers. The ground substance, iwhich the fibers and cells are embedded, is composed mainly of nglycosaminoglycans (GAGs) dissolved in tissue fluid. Matrix nviscosity and rigidity are determined by the amount and types of GAGs, the nassociation of GAGs with core proteins to form proteo- glycans, GAG-fiber associations, and GAG-GAG associations. nComponents of the fibers and ground nsubstance are synthesized and secreted by connective tissue cells (mostly by nfibroblasts), and the fibers are assembled in the extra cellular space.
1. Proteoglycans are composed of a core protein to which GAGs nare attached. The GAGs of proteoglycans are straight-chain polymers of nrepeating sugar heterodimers made up of hexosamine (glucosamine or ngalactosamine) and uronic acid (glucuronic or iduronic acid). Five major nclasses of GAGs, differing in their sugars, exist in connective tissues: nhyaluronic acid (which does not form proteoglycans), chondroitisulfate, dermatan sulfate, keratan sulfate, and heparan sulfate.
2. Glycoproteins are proteins to which shorter, branched noligosaccharide chains are co-valently bound. Glycoproteins of ground substance nare much smaller than proteoglycans. Examples: fibronectin, which nmediates the attachment of cells to the extracellular matrix; laminin, a ncomponent of basal lamina that mediates attachment of epithelial cells; and nchondronectin, a component of cartilage matrix that mediates the attachment nof chondrocytes to their matrix.
The characteristics of the amorphous nground substance are as varied as the connective tissues themselves.
a. nIn loose areolar connective tissue, it is sparse and watery.
b. nThe amorphous ground substance in cartilage contains nproteoglycan, which provides structural rigidity and flexibility yet permits nnutrients to diffuse from surrounding blood vessels.
c. nIn bones and teeth, the amorphous ground substance is ncalcified, providing these specialized connective tissues with increased ntensile strength.
d. nIn blood, the amorphous ground substance is a complex mixture nof glycoproteins dissolved in liquid.
Amorphous ground substance is a nwatery open mesh that allows water and metabolites to diffuse freely through nconnective tissues. It contains proteins that bind cells to fibers, and it nprovides connective tissues with tensile strength and flexibility appropriate nfor their functions.
COLLAGEN FIBERS
Collagen and elastic fibers in loose connective ntissue. H&E stain. Medium magnification.
The protein collagen is the most abundant nin the body. There are many collagen types, some of which form fibers. Collagefibers often collect to form fiber bundles. The main function of collagefibers is the provision of tensile strenght.
1. Synthesis and assembly
a. Intracellular steps Free polysomes reading collagen mRNA attach to the nrough en-doplasmic reticulum, and protocollagen polypeptides are ndeposited in the cisternae. Each protocollagen chain, or alpha chain, nhas a molecular weight of about 28,000 daltons and about 250 amino acids; every nthird amino acid is glycine. Proline and lysine residues within the nchains are then hydroxylated by proline and lysine hy-droxylases (possibly ismooth endoplasmic reticulum) to form hydroxyproline and nhydroxylysine, unusual amino acids present in relatively large amounts icollagen. Core sugars (galactose and glucose) attach to the hydroxylysine nresidues in the endoplasmic reticulum. With the aid of registration peptides nat the ends of the alpha chains, 3 chains coil around one another to form a ntriple helical molecule called procollagen. Further glycosylation may noccur in the Golgi complex, where procollagen is packaged for secretion. Golgi nvesicles release procollagen into the extracellular space by exocytosis.
b. Extra cellular steps In the extra cellular space, the enzyme nprocollagen peptidase cleaves the registration peptides from procollagen, nconverting it to tropocollagen. Tro-pocollagen molecules become aligned nin staggered fashion to form collagen fibrils, possibly under the control of the nadjacent cell.
Collagetypes Not all collagen types nare well characterized. A few examples of collagens whose biochemical nstructure, function, and location have been studied in some detail are ndescribed here.
a. Type I collagen, the most abundant and widespread, forms large fibers nand fiber bundles. It is found in tendons, ligaments, bone, dermis, orgacapsules, and loose connective tissue.
b. Type II collagen is found in adults only in the cartilage matrix (some noccurs in the embryonic notochord) and forms only thin fibrils.
c. Type III collagen is similar to type I, but it is more heavily nglycosylated and stains with silver. Often found in association with type I, ntype III forms networks of thin fibrils that surround and support soft flexible ntissues (adipocytes, smooth muscle cells, nerve fibers). It is the major fiber ncomponent (reticular fibers) of hematopoietic tissues (eg, bone marrow, spleen) nand of the reticular laminae underlying epithelial basal laminae.
Section of a muscular artery stained with picro-sirius and nobserved with polarization optics. The upper tunica media (muscular layer) ncontains reticular fibers consisting mainly of collagen type III. The lower nlayer (tunica adventitia) contains thick fibers and bundles of collagen type I. nDeficiencies of collagen type III may result in rupture of the arterial wall. nMedium magnification.
d. Type IV collagen is the major collagen type in basal laminae. It does nnot form fibers or fibrils.
e. Type V collagen is present in placental basement membranes and blood nvessels and in small amounts elsewhere. Its structure and function are poorly ncharacterized.
f. Type X collagen is found in the matrix surrounding hypertrophic nchondrocytes of degenerating growth plate cartilage in sites of future bone formation.
Electron micrograph of human collagen fibrils in cross and nlongitudinal sections. Each fibril consists of regular alternating dark and nlight bands that are further divided by cross-striations. Ground substance ncompletely surrounds the fibrils. x100,000.
In the most abundant form of collagen, type I, neach molecule (tropocollagen) is composed of two alpha1 and one alpha2 peptide nchains, each with a molecular mass of approximately 100 kDa, intertwined in a nright-handed helix and held together by hydrogen bonds and hydrophobic ninteractions. Each complete turn of the helix spans a distance of 8.6 nm. The nlength of each tropocollagen molecule is 280 nm, and its width is 1.5 nm.
Schematic drawing of an aggregate of collagen molecules n(tropocollagen), fibrils, fibers, and bundles.
There is a stepwise overlapping arrangement of nrodlike tropocollagen subunits, each measuring 280 nm (1).
This arrangement results in the production of nalternating lacunar and overlapping regions (2)
that cause the cross-striations characteristic nof collagen fibrils and confer a 64-nm periodicity of dark and light bands whethe fibril is observed in the electron microscope (3).
Fibrils aggregate to form fibers (4),
which aggregate to form bundles (5)
routinely called collagen fibers. Collagen type nIII usually does not form bundles.
Collagesynthesis. The assembly of the triple helix and the hydroxylation and nglycosylation of procollagen molecules are simultaneous processes that begin as nsoon as the 3 chains cross the membrane of the rough endoplasmic reticulum n(RER). Because collagen synthesis depends on the expression of several genes nand on several post-translation events, many collagen diseases have beedescribed.
Histologic appearance
Light microscopy Collagen occurring in large or small bundles of nfibrils or as individual fibrils exhibits acidophilic staining properties iH&E-stained sections. In sections stained with Masson’s trichrome, collagefibers stain green. Thin fibers (eg, type III) stain darkly with silver stains, nbut thicker bundles do not. Collagen molecules that do not form fibers or nfibrils (eg, type IV) cannot be distinguished from the surrounding ground nsubstance except by immunohistochemistry.
Electron microscopy All collagen fibrils and fibers have stripes at nintervals of 64 nm along their length. This periodicity reflects the staggering nof tropocollagen molecules.
Mechanical properties Collagen fibers’ most important mechanical property nis their tensile strength, which is (weight for weight) greater than that of nsteel.
Location Collagefibers are found in all connective tissues and in the reticular laminae of ncertain basement membranes. In bone, its lacunar regions (spaces betweeoverlapping tropocollagen units) may act as nucleation sites for the nhydroxyapatite crystals of bone matrix.
Collagen Renewal: Collageis a very stable protein, and its turnover is quite slow—slowest in tendons and nother dense connective tissues, fastest in loose connective tissue. Macrophages nand neutrophils release collagenase, which breaks down old collagen, and new ncollagen is synthesized by fibroblasts. As humans age, their extra cellular ncollagen becomes increasingly cross-linked, and its turnover slows in all nconnective tissues.
ELASTIC FIBERS
Elastic fibers consist of an amorphous nalbuminoid protein called elastin and numerous proteinaceous nmicrofibrils that become embedded in the elastin.
Total preparation of young rat mesentery showing red npicrosirius-stained nonanastomosing bundles of collagen fibers, while the nelastic fibers appear as thin, dark anastomosing fibers stained by orcein. nCollagen and elastic fibers provide structure and elasticity, respectively, to nthe mesentery. Medium magnification. B: nThe same preparation observed with polarizing microscopy. Collagen bundles of nvarious thicknesses are observed. In the superimposed regions, the bundles of ncollagen are a dark color. Medium magnification.
Skin dermis, selectively stained for elastic fibers. Dark nelastic fibers are interspersed with pale red collagen fibers. The elastic nfibers are responsible for skin’s elasticity. Medium magnification.
1. Synthesis and assembly
a. Intracellular steps Microfibrillar proteins and proelastin are nsynthesized on ribo-somes of the RER and secreted separately. Proelasticontains large amounts of the hydrophobic amino acids glycine, proline, and nvaline, accounting for elastin’s insolubility. Microfibrillar protein contains nmostly hydrophilic amino acids.
b. Extra cellular steps Proelastin molecules polymerize extracellularly to nform elastin chains. Lysyi oxidases then catalyze the conversion of ncertain lysine residues of elastin to aldehydes, 3 of which condense with a nfourth, unaltered lysine residue to form desmosine and isodesmosine. nThese amino acids, very rare except in elastin, cross-link individual elastichains. Elastin then associates with numerous microfibrils to form a branching nand anastomosing network of elastic fibers.
Histologic appearance Elastin occurs as short branching fibers which form nan irregular network throughout the tissue. Elastin contains few charged amino nacids, so it stains poorly with standard ionic dyes and elastin fibers are ndifficult to demonstrate in histological sections. Special stains, such as norcein, Verlhoeff’s stain or Weigert’s resorcin-fuchsin stain, are used ilight microscopic preparations. In EM preparations, both amorphous elastin and nmicrofibrils can be visualized.
Mechanical properties Elastic fibers are extremely pliable and elastic. nThey can be stretched to 150% of their length without breaking and then returto their original length.
Location Elastic fibers are found where their mechanical nproperties are necessary to allow tissues to stretch or expand and then returto their original shape, eg, in arterial walls, interalveolar septa, bronchi nand bronchioles of the lungs, vocal ligaments, and ligamenta flava of the nvertebral column.
CONNECTIVE nTISSUE WITH SPECIAL PROPERTIES
Connectine tissue with nspecial properties includes reticular connective tissue, mucous connective ntissue, adipose tissue and pigmented tissue.
RETICULAR CONNECTIVE TISSUE
The reticular fibers (type III ncollagen) of this tissue form delicate 3-dimensional, netlike scaffolding upowhich cells, the predominant element, are suspended.
There is very little ground nsubstance. Reticular connective tissue functions in the support of motile cells nand filtration of body fluids. It is found mainly in hematopoietic tissues such as bone marrow, spleen, and lymph nodes. n
Usually reticular tissue has a lot nof different macrophages (free, dendritic, interdigital), which promote nprotective function and control of blood cells maturation and interaction. Sinusoidal hemocapillaries are typical for this ntissue. Their permeable wall allow mature blood formed elements to enter the nblood stream, fixed macrophages in their wall control this process.
Reticular connective tissue showing only the attached cells nand the fibers (free cells are not represented). Reticular fibers are enveloped nby the cytoplasm of reticular cells; the fibers, however, are extracellular, nbeing separated from the cytoplasm by the cell membrane. Within the sinuslike nspaces, cells and tissue fluids of the organ are freely mobile.
Reticular cells are firmly attached nto the fibers, which may be mostly covered by the long attenuated reticular ncell processes. Reticular fibers are argyrophilic fibers. When treated with nsilver salts, they reduce the silver salts, creating silver metal deposits that nstain the fibers black. Other cell types, such as lymphocytes, are suspended ithe spaces of the network.
Reticular connective tissue showing reticular fibers nof reticular cells. Silver impregnation. Medium magnification.
MUCOUS CONNECTIVE TISSUE
This tissue has small numbers of ncells and fibers distributed randomly in the abundant ground substance, which nhas a syrupy to jellylike consistency and is composed chiefly of hyaluronic nacid. Mucous tissue yields readily to pressure but can return to its original nshape, so it is useful for protecting nunderlying structures from excess pressure. It is the predominant ncomponent (Wharton’s jelly) of the umbilical cord, of the nucleus pulposis of nthe intervertebral disks, and of the pulp of young teeth. Mucous tissue has now fibers, it is rich with nmust cells and macrophages (Caschenko-Hofbayer cells).
Mucous tissue of an embryo showing fibroblasts immersed in a nvery loose extracellular matrix composed mainly of molecules of the ground nsubstances. H&E stain. Medium magnification.
Mucous tissue of umbilical cord named “Wharton’s jelly”.
H&E stain. Low magnification.
ADIPOSE TISSUE
Adipose tissue, nor fat, is a connective tissue specialized to store fuel. If we were not nequipped to store fuel, all of our time would have to be spent obtaining food.
Distribution of adipose tissue. In a humaewborn, nmultilocular adipose tissue constitutes 2–5% of the body weight and is ndistributed as shown. The black areas indicate multilocular adipose tissue; nshaded areas are a mixture of multilocular and unilocular adipose tissue.
The cytoplasm of fat cells, or nadipocytes, contains large triglyceride deposits in the form of one or more nlipid droplets with no limiting membranes. Together, the clusters of adipocytes nscattered throughout the body constitute an extremely important metabolic orgathat varies widely in size and distribution, depending on such factors as age, nsex, and nutritional status.
Clusters of nadipocytes are separated into lobes and lobules by septa of collagenous nconnective tissue of variable density. A network of reticular fibers surrounds nindividual cells. The ground substance is sparse.
There are 2 basic types of adipose ntissue, termed white adipose tissue, or white fat, and brown adipose ntissue, or brown fat. A white fat adipocyte has a single large lipid ndroplet; a brown fat adipocyte has many small droplets.
WHITE ADIPOSE TISSUE
This type of adipose tissue comprises up to 20% of total nbody weight iormal, well-nourished male adults and up to 25% in females.
Distinguishing Features: White nadipose tissue, the more abundant of the 2 types, is also termed unilocular adipose tissue, a reference to nthe single fat droplet in each of its cells. In mature adipocytes, the droplet nis so large that it displaces the nucleus and remaining cytoplasm to the ncellular periphery. Cell diameter varies from 50 to 150 nm. Adipocytes ihistologic sections have a signet-ring appearance because most of the nlipid is washed away during preparation, leaving only a flattened nucleus and a nthin rim of cytoplasm. The cytoplasm near the nucleus contains a Golgi napparatus, mitochondria, a small amount of rough endoplasmic reticulum, and nfree ribosomes. The cytoplasm in the thin rim contains smooth endoplasmic nreticulum and pinocytotic vesicles. This tissue is sometimes termed yellow nadipose tissue or yellow fat; dietary carotenoids accumulate in the nlipid droplets, making the tissue yellow. White fat is richly vascularized but nnot as richly as brown fat.
White adipose tissue
Sudan III stain. Medium magnification.
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Photomicrograph nof unilocular adipose tissue of a young mammal. Arrows show nuclei of nadipocytes (fat cells) compressed against the cell membrane. Note that, nalthough most cells are unilocular, there are several cells (asterisks) with nsmall lipid droplets in their cytoplasm, an indication that their ndifferentiation is not yet complete. Pararosaniline–toluidine blue (PT) stain. nMedium magnification.
White adipose tissue
H&E stain. Medium magnification.
The nprocess of lipid storage and release by the adipocyte. Triglycerides are ntransported in blood from the intestine and liver by lipoproteins known as nchylomicrons (Chylo) and very low-density lipoproteins (VLDL). In adipose ntissue capillaries, these lipoproteins are partly broken down by lipoproteilipase, releasing free fatty acids and glycerol. The free fatty acids diffuse nfrom the capillary into the adipocyte, where they are re-esterified to glycerol nphosphate, forming triglycerides. These resulting triglycerides are stored idroplets until needed. Norepinephrine from nerve endings stimulates the cyclic nAMP (cAMP) system, which activates hormone-sensitive lipase. Hormone-sensitive nlipase hydrolyzes stored triglycerides to free fatty acids and glycerol. These nsubstances diffuse into the capillary, where free fatty acids are bound to the nhydrophobic moiety of albumin for transport to distant sites for use as aenergy source.
Development nof fat cells. Undifferentiated mesenchymal cells are transformed into nlipoblasts that accumulate fat and thus give rise to mature fat cells. When a nlarge amount of lipid is mobilized by the body, mature unilocular fat cells nreturn to the lipoblast stage. Undifferentiated mesenchymal cells also give nrise to a variety of other cell types, including fibroblasts. The mature fat ncell is larger than that shown here in relation to the other cell types.
Distribution:
Subcutaneous fat Subcutaneous fat (hypodermis) is the layer of white nadipose tissue found just beneath the skin except in the eyelids, penis, nscrotum, and most of the external ear. (There is some fat in the earlobe.) Iinfants, it forms a continuous thermal insulating layer of uniform thickness ncovering the entire body and is termed the panniculus adiposus. Iadults it becomes thicker or thinner in selected areas, depending upon the nperson’s age, sex, and dietary habits. Where it becomes thinner, the tissue ntakes on the appearance of areolar tissue. In males, the fat layer thickens nover the nape of the neck, deltoids (shoulders), triceps brachii (back of the nupper arm), lumbosacral region (lower back), and buttocks. In females, nadditional fat is deposited in the breasts and buttocks and over the nepitrochanteric region (hips) and anterior aspect of the thighs.
Intraabdominal fat Fat deposits of varying sizes surround blood and nlymphatic vessels in the omentum and mesenteries suspended in the abdominal ncavity. Additional accumulations occur in retroperitoneal areas, such as around nthe kidneys on the posterior abdominal wall.
Other locations Other prominent accumulations of fat are found withithe eye orbits and surrounding major joints (e.g., knees). Such accumulations nalso form pads in the palms and soles.
Functional nCharacteristics: Adipocytes store nfatty acids in triglycerides (esters of glycerol and 3 fatty acids). The ntriglycerides stored in both white and brown fat undergo continuous turnover. nTheir released fatty acids serve as a source of chemical energy for cells (the npredominant source in resting muscle) and as a source of the raw materials for nmaking phospholipids (the predominant component of biologic membranes). nTurnover is regulated by several histophysiologic factors, which shift the nequilibrium toward fat uptake or fat mobilization, depending on the body’s nlevel of, and need for, circulating fatty acids.
Histogenesis: The unilocular adipocytes of white fat derive from nmesenchymal precursor cells that resemble fibroblasts. The appearance of nnumerous small lipid droplets in the cytoplasm signals the transformation of nthese cells into lipoblasts. As lipid accumulation continues, the small ndroplets fuse until a single large lipid droplet forms.
n
BROWN ADIPOSE TISSUE
Distinguishing Features: Brown fat is called multilocular adipose tissue nbecause of the multiple small lipid droplets in its adipocytes. Browadipocytes are smaller than white adipocytes and have a spheric, centrally nlocated nucleus. They contain large numbers of mitochondria; the tan to nreddish-brown tissue color is due chiefly to mitochondrial cytochromes. Loose nconnective tissue septa give brown adipose tissue a lobular appearance like nthat of a gland in histologic section. The vascular supply (partly responsible nfor the color) is very rich, as is the autonomic nerve supply. Many unmyelmated nnerve fibers contact the adipocytes.
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Photomicrograph nof multilocular adipose tissue (lower portion) with its characteristic cells ncontaining central spherical nuclei and multiple lipid droplets. For ncomparison, the upper part of the photomicrograph shows unilocular tissue. PT nstain. Medium magnification.
Multilocular nadipose tissue. Note the central nucleus, multiple fat droplets, and abundant nmitochondria. A sympathetic nerve ending is shown at the lower right.
Brown adipose tissue
H&E stain. Low magnification.
Distribution: Brown fat is less abundant than white at all ages. nYoung and middle-aged adults have little or none, but fetuses, newborns, and nthe elderly have accumulations in the axilla, the posterior triangle of the nneck (near the carotid artery and thyroid gland), and around the renal hilus.
Functional nCharacteristics: Brown fat has nessentially the same functional capabilities as white, but its metabolic activity is more intense and can lead nto generation of heat. Under conditions of excessive cold, autonomic nstimulation can cause oxidative phosphorylation in the numerous mitochondria to nbe uncoupled from adenosine triphosphate (ATP) synthesis, and the released nenergy dissipates as heat. The numerous vessels supplying this tissue carry the nheat to the body. Brown fat is important in hibernating animals and in humainfants before other thermoregulatory mechanisms are well developed.
Histogenesis: The multilocular adipocytes of brown fat derive from nmesenchymal precursors that assume an epithelial shape and arrangement. The nmultiple small fat droplets that appear during development do not coalesce nduring maturation.
Differences between brown and white adipose tissue
PIGMENT TISSUE
In the tunica suprachoroidea and in the lamina nfucsa of the sclerae of the eye, the majority of the cells in the loose nconnective tissue are melanocytes. Such a tissue can be termed “pigment ntissue”. The large cells of the pigmented epithelium are filled with coarse, ndark brown melanin granules (endogenous pigmental protein inclusions), which nobscure their outlines and their nuclei. nAggregations of pigmental cells could be found in the areola of mammary ngland and around the anus.
Melanocytes of the skiwithout staining. High magnification.
The regenerative capacity of the connective tissue is nclearly observed when tissues are destroyed by inflammation or traumatic ninjury. In these cases, the spaces left after injury to ntissues whose cells do not divide (eg, cardiac muscle) are filled by connective ntissue, which forms a scar. The healing of surgical incisions depends on the nreparative capacity of connective tissue. The main cell type involved in repair nis the fibroblast.
When it is adequately stimulated, such as during wound nhealing, the fibrocyte reverts to the fibroblast state, and its synthetic nactivities are reactivated. In such instances the cell reassumes the form and nappearance of a fibroblast. The myofibroblast, a ncell with features of both fibroblasts and smooth muscle cells, is also nobserved during wound healing. These cells have most of the morphological ncharacteristics of fibroblasts but contain increased amounts of actimicrofilaments and myosin and behave much like smooth muscle cells. Their nactivity is responsible for wound closure after tissue injury, a process called nwound contraction.
When adequately stimulated, macrophages may increase nin size and are arranged in clusters forming epithelioid ncells (named for their vague resemblance to epithelial cells), or several may nfuse to form multinuclear giant cells. Both cell types are usually found nonly in pathological conditions.
Macrophages act as defense elements. They phagocytose ncell debris, abnormal extracellular matrix elements, neoplastic cells, nbacteria, and inert elements that penetrate the organism. Macrophages are also nantigen-presenting cells that participate in the processes nof partial digestion and presentation of antigen to other cells (see Chapter n14). A typical example of an antigen-processing cell is the macrophage present nin the skin epidermis, called the Langerhans cell (see Chapter 18). Although nmacrophages are the main antigen presenting cells, under certain circumstances nmany other cell types, such as fibroblasts, endothelial cells, astrocytes, and nthyroid epithelial cells, are also able to perform this function. Macrophages nalso participate in cell-mediated resistance to infection by bacteria, viruses, nprotozoans, fungi, and metazoans (eg, parasitic worms); in cell-mediated nresistance to tumors; and in extrahepatic bile production, iron and fat nmetabolism, and the destruction of aged erythrocytes.
When macrophages are stimulated (by injection of nforeign substances or by infection), they change their morphological ncharacteristics and metabolism. They are then called activated nmacrophages and acquire characteristics not present in their nonactivated nstate. These activated macrophages, in addition to showing an increase in their ncapacity for phagocytosis and intracellular digestion, exhibit enhanced nmetabolic and lysosomal enzyme activity. Macrophages also have an important nrole in removing cell debris and damaged extracellular components formed during nthe physiological involution process. For example, during pregnancy the uterus nincreases in size. Immediately after parturition, the uterus suffers ainvolution during which some of its tissues are destroyed by the action of nmacrophages. Macrophages are also secretory cells that produce an impressive narray of substances, including enzymes (eg, collagenase) and cytokines that nparticipate in defensive and reparative functions, and they exhibit increased ntumor cell–killing capacity.
Collagen synthesis depends on the nexpression of several genes and several posttranslational events. It should not nbe surprising, therefore, that a large number of pathological conditions are ndirectly attributable to insufficient or abnormal collagesynthesis.
Certain mutations in the 1 (I) or 2 (I) genes lead to osteogenesis nimperfecta. Many cases of osteogenesis imperfecta are due to deletions of nall or part of the 1 (I) gene. However, a single amino acid nchange is sufficient to cause certain forms of this disease, particularly nmutations involving glycine. Glycine must be at every third position for the ncollagen triple helix to form.
In addition to these disorders, several ndiseases result from an over-accumulation of collagen. In progressive nsystemic sclerosis, almost all organs may present an excessive accumulatioof collagen (fibrosis). This occurs mainly in the skin, digestive tract, nmuscles, and kidneys, causing hardening and functional impairment of the nimplicated organs.
Keloid nis a local swelling caused by abnormal amounts of collagen that form in scars nof the skin. Keloids, which occur most often in individuals of black Africadescent, can be a troublesome clinical problem to manage; not only can they be ndisfiguring, but excision is almost always followed by recurrence.
Vitamin C (ascorbic acid) deficiency nleads to scurvy, a disease characterized by the degeneration of connective ntissue. Without this vitamin, fibroblasts synthesize defective collagen, and nthe defective fibers are not replaced. This process leads nto a general degeneration of connective tissue that becomes more pronounced iareas in which collagen renewal takes place at a faster rate. The periodontal nligament that holds teeth in their sockets has a relatively high collageturnover; consequently, this ligament is markedly affected by scurvy, which nleads to a loss of teeth. Ascorbic acid is a cofactor for proline hydroxylase, nwhich is essential for the normal synthesis of collagen. Table 5–4 lists a few nexamples of the many disorders caused by failure of collagen biosynthesis.
Examples of clinical disorders resulting nfrom defects in collagen synthesis.
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Collagen turnover and nrenewal iormal connective tissue is generally a very slow process. In some norgans, such as tendons and ligaments, the collagen is very stable, whereas iothers, as in the periodontal ligament surrounding teeth, the collagen turnover nrate is very high. To be renewed, the collagen must first be degraded. nDegradation is initiated by specific enzymes called collagenases, which nare members of an enzyme class called matrix metalloproteinases or MMPs. nCollagenases clip collagen molecules in such a way that they are thesusceptible to further degradation by nonspecific proteases.
Ehlers–Danlos type IV ndisease, a deficiency of collagen type III, is characterized by nruptures in arteries and the intestine (Table 5–4), both structures rich ireticular fibers.
Fibrillin is a family of proteins nrelated to the scaffolding necessary for the deposition of elastin. Mutations nin the fibrillin gene result in Marfan syndrome, a disease characterized nby a lack of resistance in the tissues rich in elastic fibers. Because the nlarge arteries are rich in components of the elastic system and because the nblood pressure is high in the aorta, patients with this disease ofteexperience aortic swellings called aneurysms, a life-threatening condition.
MEDICAL APPLICATION
The degradation of proteoglycans is carried out by nseveral cell types and depends on the presence of several lysosomal enzymes. nSeveral disorders have been described in which a deficiency in lysosomal nenzymes causes glycosaminoglycan degradation to be blocked, with the consequent naccumulation of these compounds in tissues. The lack of specific hydrolases ithe lysosomes has been found to be the cause of several disorders in humans, nincluding Hurler, Hunter, sanfilippo, and Morquio syndromes.
Because of their high viscosity, intercellular nsubstances act as a barrier to the penetration of bacteria and other nmicroorganisms. Bacteria that produce hyaluronidase, an enzyme that nhydrolyzes hyaluronic acid and other glycosaminoglycans, have greater invasive npower because they reduce the viscosity of the connective tissue ground nsubstance.
Multiadhesive glycoproteins have carbohydrates attached, but in contrast to nproteoglycans the protein moiety usually predominates. The carbohydrate moiety nof glycoproteins is frequently a branched structure. Several such glycoproteins nhave important roles in the adhesion of cells to their substrate. Fibronectin n(L. fibra, fiber, + nexus, interconnection) is an important nexample synthesized by fibroblasts and some epithelial cells. This dimeric nmolecule, with a molecular mass of 222–240 kDa, has binding sites for ncollagens, certain GAGs, and integrins of cell membranes, ie, it is nmultiadhesive. Interactions at these sites help to mediate normal cell adhesioand migration and cause fibronection to be distributed as a network in the nintercellular spaces of many tissues (Figure 5–18a). Another multiadhesive nglycoprotein, laminin is a larger, trimeric, cross-shaped glycoproteithat participates in the adhesion of epithelial cells to the basal lamina, with nbinding sites for type IV collagen, GAGs, and integrins. All basal laminae are nrich in laminin (Figure 5–18b).
MEDICAL APPLICATION
The participation of fibronectin and laminin in both nembryonic development and the increased ability of cancer cells to invade other ntissues has been well-studied. The importance of fibronectin is shown by the nfact that mice whose fibronectin gene has been inactivated die during early nembryogenesis.
In connective tissue, in addition to the hydrated ground nsubstance, there is a small quantity of free fluid—called interstitial nor tissue fluid—that is similar to blood plasma in its content of ions nand diffusible substances. Tissue fluid contains a small percentage of plasma nproteins of low molecular weight that pass through the capillary walls as a nresult of the hydrostatic pressure of the blood. Although only a small nproportion of connective tissue proteins are plasma proteins, it is estimated nthat because of its volume and wide distribution, as much as one third of the nplasma proteins of the body are stored in the intercellular connective tissue nmatrix.
MEDICAL APPLICATION
Edema is promoted by the accumulation of water in the nextracellular spaces. Water in the extracellular compartment of connective ntissue comes from the blood, passing through the capillary walls into the nextracellular compartment of the tissue. The capillary wall is only slightly npermeable to macromolecules but permits the passage of water and small nmolecules, including low-molecular-weight proteins. In several pathologic nconditions, the quantity of tissue fluid may increase considerably, causing nedema.
Edema may result from venous or lymphatic obstructioor from a decrease in venous blood flow (eg, congestive heart failure). It may nalso be caused by the obstruction of lymphatic vessels due to parasitic plugs nor tumor cells and chronic starvation; protein deficiency results in a lack of nplasma proteins and a decrease in colloid osmotic pressure. Water therefore naccumulates in the connective tissue and is not drawn back into the ncapillaries.
Another possible cause of edema is increased npermeability of the blood capillary endothelium resulting from chemical or nmechanical injury or the release of certain substances produced in the body n(eg, histamine).
Blood vessels bring to connective tissue the various nnutrients required by its cells and carry away metabolic waste products to the ndetoxifying and excretory organs, the liver and kidneys.
Two forces act on the water contained in the ncapillaries: the hydrostatic pressure of the blood caused by the pumping naction of the heart, which forces water out across the capillary wall; and the ncolloid osmotic pressure of the blood plasma, which draws water back ninto the capillaries (Figure 5–20). Osmotic pressure is due mainly to plasma nproteins. (Because the ions and low-molecular-weight compounds that pass easily nthrough the capillary walls have approximately the same concentration inside nand outside these blood vessels, the osmotic pressures they exert are napproximately equal on either side of the capillaries and cancel each other.) nThe colloid osmotic pressure exerted by the blood proteins—which are unable to npass through the capillary walls—is not counterbalanced by outside pressure and ntends to bring water back into the blood vessel (Figure 5–20).
In addition to leptin, white adipose tissue secretes nnumerous other cytokines and other factors with paracrine nand autocrine activity, including many proinflammatory cytokines. It is not nclear whether these are produced by adipocytes or other cells of the tissue nsuch as macrophages or fibroblasts. With its increased amounts of white adipose ntissue, obesity is characterized by a state of chronic mild inflammation. nCytokines and other factors released from visceral fat are being investigated nfor links to the inflammation-related disorders associated with obesity such as ndiabetes and heart disease.
Humans are one of the few mammals born with fat stores, which nbegin to accumulate at week 30 of gestation and are well-developed by birth iboth the visceral and subcutaneous compartments. After birth, the development nof new adipocytes is common around small blood vessels, nwhere undifferentiated mesenchymal cells are fairly abundant.
Excessive formation of adipose tissue, or obesity, occurs nwhen energy intake exceeds energy expenditure. Although fat cells cadifferentiate from mesenchymal stem cells throughout life, nadult-onset obesity is generally believed to involve largely increased size or nhypertrophy in existing adipocytes (hypertrophic obesity). Childhood obesity ncan involve both increased adipocyte size and formation of new adipocytes by ndifferentiation and hyperplasia of preadipocytes from mesenchymal cells. This nearly increase in the number of adipocytes may predispose an individual to nhyperplastic obesity in later life.
Unilocular adipocytes can generate very common benigtumors called lipomas. Malignant adipocyte- derived ntumors (liposarcomas) are infrequent in humans.
Connective tissue has a large variety of functions in the body, and cabe as different as blood and bone! However every connective tissue is made up nof two basic elements – cells and which of the following?
Matrix. The matrix is the substance that exists nbetween the cells in the connective tissue. The main constituents of the matrix nare fibers, and ground substance, which is the material existing between the ncells and fibers. In contrast to epithelial tissues, connective tissues nnormally do not cover surfaces (although areolar connective tissue is aexception: it lines joint cavities). Instead, connective tissues have a wide nvariety of functions, including support (e.g. the skeleton), energy reserves n(e.g. fat), and iutrition (e.g. blood).
The suffix “-blast” (e.g. nosteoblast) refers to the mature form of the cell, while “-cyte” n(e.g. osteocyte) refers to the immature form.
It is actually the other way around. The nsuffix “-blast” means “to bud” or “to sprout”. nThe blasts actually produce the matrix, and matures into cytes when the matrix nis sufficient. They retain the ability for cell division. Cytes, on the other nhand, have a much reduced capacity for cell division. Their main role is to nmaintain the matrix. From the question: Osteoblasts and osteocytes are cells nfound in bone.
Connective ntissue contains various types of cells, one of which is mast cells. What are nthese cells principally involved in?
Mediating allergic reactions. Mast cells are large, and have a ngranulated cytoplasm. They are important in allergic reactions; they secrete nhistamine, which dilates small blood vessels in an immune response. This allows nblood to reach the site of damage, and causes the redness seen with ninflammation. You may have heard of anti-histamines being used to treat nallergies, especially hay fever; these help to decrease the response of mast ncells. Mast cells also release heparin, which is thought to defend against ninvading pathogens such as bacteria. Connective ntissues also contain fibroblasts/-cytes, adipocytes, and macrophages.
Three types of fibers are found iconnective tissue matrix. Which of these is NOT one of those?
Muscle fibers. Muscle is in fact another tissue type naltogether, which will be covered in the next quiz of the series: Human Tissue nTypes III. Collagen fibers are very strong but flexible fibers that make up naround 30% of your dry body weight! There are different types of collagen (at least n15), which can each confer different properties to the tissue it is present in. nFor example, cartilage contains Type II collagen, which is particularly good at nretaining water, which makes cartilage quite spongy compared to bone. Reticular nfibers are found primarily in the spleen, liver, and lymph nodes. They also ncontain collagen protein (mainly Type III) as collagen fibers do. They provide na more delicate branched network, and provide support and strength to the ntissue. They also help to form the basement membrane, which underlies nepithelia. Elastic fibers contain elastin as the main protein, and their maiproperty is that of stretching – they can stretch up to about 1.5 times their nlength. They form irregular branched networks, and are found primarily in the nlung and aorta.
What is the abbreviation commonly used to ndescribe glycosaminoglycans, a component found in the ground substance of the nmatrix?
GAG & GAGs. Glycosaminoglycans are polysaccharides nthat trap water, forming hydrated gels. Most exist as proteoglycans, which nmeans they are associated with proteins. The proteins form a spine, and the nGAGs project from the spine like bristles of a brush – this conformation is nvery good at holding water. GAGs include chondroitin sulfate (found in cartilage nand bone) and dermatan sulfate (found in skin, tendons, and blood vessels). As nwell as making the connective tissue more spongy, GAGs provide strength under ncompression.
Which of these diseases is a disorder of nthe elastic fibers found in connective tissues? Sufferers tend to be tall and nhave long limbs and digits.
Marfan’s syndrome. Marfan’s syndrome is inherited from ngeneration to generation, and is caused by a mutation in the gene for the nprotein fibrillin, found in elastic fibers. Tissues that have abundant elastic nfibers are thus affected; these include the covering layer of bone n(periosteum), the ligament that supports the lens in the eye, and the walls of narteries. Symptoms can therefore include blurred vision (due to the ndisplacement of the lens) and, more seriously, the aorta can weaken, which calead to sudden bursting and subsequent death.
Connective tissue can be classified naccording to the relative amount of ground substance and the orientation of nfibers. The classifications are loose (or areolar), dense irregular, and dense nregular. Which of the following has a dense regular classification?
Tendons and ligaments. In dense regular tissues there is very nlittle ground substance and few cells (generally fibrocytes are present). There nare many fibers (mainly collagen) arranged in regular, parallel bundles, which ngives a high tensile strength to the tissue – the tissue can withstand pulling nalong the axis of the fibers. This tissue is normally silvery white. Dense nirregular tissue also has little ground substance and high proportions of nfibers. However, here the bundles are in all directions, and this means that nstress can be loaded on the tissue in a non-specific direction. Examples of nwhere it is found are in the periosteum and perichondrium. Finally, loose nconnective tissue is mostly made up of ground substance, and has a high water ncontent. It has relatively few fibers, but has some cells. This can be found iblood vessels and nerves.
Student’s Practical Activities
Task No 1. Students must know and illustrate such nhistologic specimens.
Specimen 1. Loose connective tissue (surface view). nIron haematoxylin
This specimedemonstrates the typical histological appearance of mature fibroblasts, nmacrophages, mast cells, plasma cells in loose connective tissue. The nfibroblast nuclei are condensed and elongated in the direction of the nextracellular fibers. The cytoplasm is reduced and spindle-shaped, with long ncytoplasmic processes extending into the matrix to meet up with those of other nfibroblasts; the cytoplasmic extensions are usually difficult to see with the nlight microscope.
Macrophages are large, nstellate cells, cytoplasm of which contain many lysosomes and well developed nGolgi complex. Mast cells are found in connective tissue, particularly in associatiowith blood vessels. The characteristic feature of mast cells is an extensive ncytoplasm packed with large granules, which are similar in composition to nbasophil granules, containing histamine and heparin. Plasma cells are large and novoid with eccentrically nuclei and well developed rough endoplasmatic nreticulum. The characteristic “clock face” of the nucleus results from a large, ncentral nucleolus and several large heterochromatin clumps regularly spaced ninside the nuclear envelope.
Collagen fibers are nstained dark in this specimen, they are thicker and nonbranched in opposite to nelastic fibers, which are thin and branhed.
Illustrate and indicate: 1. Fibroblasts. 2. Collagen fibers. 3. nElastic fibers. 4.Macrophages (histiocytes). 5.Mast cells. 6.Plasma cells. n7.Ground substance.
Specimen 2. Accumulation of dye by macrophages. nMethylene blue
On the large magnification in the ncytoplasm of macrophages you can see a material with stain, which was nphagocyzed by cells after injection into the human organism.
Illustrate and indicate: 1.Nucleus. 2. Vacuoles with methylene nblue
Specimen 3. Dense regular connective tissue (tendon).
Stained with haematoxylin and eosin.
This specimen of the ntendon shows the typical dense arrangement of collagen fibers, where mechanical nsupport is the primary function. Tendon is the densest form of connective ntissue proper, consisting of bundles of regullary arranged collagen fibers namong which are scattered rows of fibroblasts with elongated nuclei. Each ntendon is composed of small bundles of such dense tissue bound together by a nsmall amount of loose connective tissue, which contains the scanty blood supply nand tiny nerve fibers. The connective tissue of the tendon surface is smooth nand condensed with minimal connections with the surrounding tissue, so as to nallow relatively unimpeded movement of the tendon. In some sites, tendons are ninvested in a connective tissue sheath lined by synovium.
Illustrate and indicate: 1. Collagen fibers. 2. Nuclei of nfibrocytes. 3. Bundle of tendon fibers. 4.Interfascicular connective tissue. n5.Vessels.
Specimen 4. Dense irregular connective tissue.
Stained with haematoxylin and eosin.
This specimedemonstrate the reticular layer of the skin, where the collagen fibers of the ndense irregular connective tissue are arranged in coarse irregular interwovebundles which confer great tensile streight. Collagen is acidophilic n(pink-stained) due to its positively-charged side groups. The fibroblasts are ninactive with highly condensed nuclei and minimal cytoplasm.
Illustrate and indicate: 1.Collagen fibers. 2.Cells. 3. Ground nsubstance.
Specimen 5. White adipose tissue.
Stained with sudan III and haematoxylin.
The typical appearance of white adipose ntissue is illustrated in this specimen. Fat stored in adipocytes accumulates as nlipid droplets, which fuse to form a single large droplet which distends and noccupies most of the cytoplasm. The adipocyte nucleus is compressed and ndisplaced to one side of the stored lipid droplet and the cytoplasm is reduced to na small rim around the periphery. Note the minute dimensions of capillaries ncompared with the size of the surrounding adipocytes.
Illustrate and indicate: 1.Adipose cells. 2.Lipid droplet. n3.Nucleus.
References:
a) nmain
1. Practical nclasses materials:
3. nStevens A. Human Histology / A. nStevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 49-62.
4. nWheter’s Functional Histology : A nText and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier nLimited, 2006. – P. 65 – 82.
5. nInderbir Singh Textbook of HumaHistology with colour atlas / Inderbir Singh. – [fourth edition]. – nJaypee Brothers Medical Publishers (P) LTD, 2002. – P. n54-69.
6. nRoss M. Histology : A Text and Atlas n/ M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams nand Wilkins, 2011. – P. 158 – 198, 254 – 268.
b) nadditional
1. nEroschenko V.P. nAtlas of Histology with functional correlations / nEroschenko V.P. [tenth edition]. – Lippincott Williams nand Wilkins, 2008. – P. n73-85.
2. nJunqueira L. Basic nHistology / L. Junqueira, J. Carneiro, R. Kelley. n– [seventh edition]. – Norwalk,Connecticut : Appleton and Lange, n1992. – P. 89-120.
3. nCharts:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#inf3
4. Disk:
http://intranet.tdmu.edu.ua/data/teacher/video/hist/
5. nVolkov K. S. Ultrastructure of cells nand tissues / K. S. Volkov, N. V. Pasechko. – Ternopil n: Ukrmedknyha, 1997. – P.
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
Created nby Violetta Kulbitska
