Large glands of digestive system
1. General morphofunctional characteristic of large salivary glands and their classification.
2. Structural peculiarities of the parotid, submandibular and sublingual glands secretory portions (acini).
3. Main microscopic and ultrastructural signs of muco- and serocytes.
4. Large salivary glands excretory ducts.
5. Salivary glands excretory products and hormones.
6. Morphogenesis and regeneration of the salivary glands.
7. Salivary glands aging.
8. Description of the liver’s double blood supply.
9. Description of the complex structure of a hepatocyte and relation of its structure to its main functions.
10. Description of the classic hepatic lobule, the portal lobule, and the hepatic acinus (of Rappaport).
11. Principal components of a portal triad.
12. Characteristic of the major cell types that border the hepatic sinusoids and cells that border the space of Disse.
13. Description of the composition and production of bile.
14. Functions of the gall bladder.
15. Comparison of the wall of the gall bladders with that of the small intestine.
16. Description of the size, staining properties, and distribution of the islets of Langerhans in the pancreas.
17. Distinguishing of the A (alpha), B (beta), D (delta), and F (PP) cells of the islets of Langerhans in terms of the hormones they secrete, their location in the islets, their relative numbers, the appearance of their granules, and any special staining properties.
18. Role of the cells of the islets of Langerhans in regulating blood glucose levels.
19. Cytochemical pecularities of pancreas acinar cells.
20. Description of two types of pancreatic exocrine secretions in term of their composition, role in the digestion, cells primarily responsible for their secretion, and the enteroendocrinal hormone that stimulates their release.
21. Main enzymes secreted by the exocrine pancreas.
Salivary glands
Saliva is produces by three pairs of major salivary glands, the parotid, submandibular and sublingual glands, and numerous minor accessory glands scattered throughout the oral mucosa. The minor salivary glands secrete continuously and are in general under local control whereas the major glands mainly secrete in response to parasympathetic activity which is induced by physical, chemical and psychological stimuli.
Classification: Several criteria are used to classify salivary glands such as size, location, type of secretion (most commonly used), mode of secretion etc. Some of these classifications are listed below:
1.Size:
a) Major glands are the large extra-orally located pairs of glands, namely parotid, submandibular and sublingual.
b) Minor glands are small aggregates of salivary glands associated with oral mucosa and distributed in several regions of the oral cavity.
2. Type of secretion:
a) Serous acini produce a watery secretion containing most of the salivary proteins.
b) Mucous acini produce viscous secretion rich in carbohydrates. c) Mixed acini produce both types of secretion.
Development of salivary glands. Major salivary glands primordia appear in the embryo during the 6th -8th weeks. The primordium of the parotid gland appears at the beginning of the 6th week, that of the submandibular appears at the end of the 6th week and the sublingual primordium at the 7th -8th weeks. Primordia of the minor salivary glands appear after the third month in utero. Acini and ductal cells differentiate during the last 2 months of gestation and in general the glands continue to grow after birth up to 2 years.
All these primordia are derived from the ectodermal lining of the stomodeum and these epithelial strands develop into the parenchyma of the glands i.e. the acini and the ducts. The surrounding ectomesenchymal cells form the stroma, that is the connective tissue septa that divide the major glands into lobes and lobules. Also in the parotid and submandibular glands a connective tissue capsule surrounds the gland.
During the development of salivary glands, epithelial/mesenchymal interactions playa significant role in guiding the organization and the differentiation of different components of the gland. Fibroblast growth factors (FGFs) and transforming growth factor J3 (TGFJ3) are the major factors involved in sequential development of major salivary glands. Evidence indicates that ectomesenchyme is the determining factor in formation of the different parts of the epithelial parenchyma and deciding which will differentiate into acinar cells or various ductal cells. This role is similar to the one played by the dental papilla which determines the type of the developing tooth and whether it will be an incisor, canine, premolar or a molar.
Endocrine function of the salivary glands includes the secretion of such hormones as: parotine, nerve and epithelium growth factors, insulin like protein.
Saliva is a hypotonic watery (99 %) secretion containing variable amounts of mucus, enzymes (amylase, maltase, nuclease, pepsin and tripsin like proteins and the antibacterlal enzyme lysozyme), antibodies and inorganic ions (Na, K, Ca, I , Fe). Different cells may be observed in the saliva – epithelial cells, neutrophils and lymphocytes.
Two types of secretory cells are found in the salivary glands: serous cells and mucous cells.
The parotid glands consist almost exclusively of serous cells and produce a thin watery secretion rich in enzymes and antibodies. The subingual glands have predominantly mucous secretory cells and produce a viscid secretion. The submandibular glands contain both serous and mucous secretory cells and produce a secretion of intermediate consistensy. The overall composition of saliva varies according to the degree of activity of each of the major gland types.
Traditionally, the role of salivary amylase has been considered to be initiation of starch digestion in the oral cavlty: its primary role, however, is more likely, to be as a cleansing agent for starch debris retained around the teeth.
The glands are divided into numerous lobules each containing many secretory units (acini). Supporting tissue septa radiate between the lobules from an outer capsule and convey blood vessels, nerves and large excretory ducts.
Stroma: These are the connective tissue elements which surround and support the acini and ducts. In addition, the major salivary glands have connective tissue septa surrounding their lobes and lobules. Also the parotid and submandibular glands have connective tissue capsule around their outer margins. The stroma also contains the vascular, neural and lymphatic supply to the glands.
Parenchyma of salivary glands consists of parenchymal unites.The salivary secretory unit (acinus) consists of a terminal branched acinar structure composed exclusively of either serous or mucous secretory cells or a mixture of both types. Intercallated ducts and striated ones belong to the acini too.
Serous secretory cells are organized into alveolar secretory unit. Serous acinar cells are pyramidal shaped cells with round nuclei located in the basal third of the cell, that is the part adjacent to the connective tissue surrounding the acinar cells but separated from them by a basement membrane. The cytoplasm appears basophilic in H&E sections reflecting the presence of abundant granular endoplasmic reticulum cisternae. The apical cytoplasm contains numerous secretory granules.
Mucous secretory cells are organized into tubular secretory unit. Mucocytes are rectangular shaped cells, have flattened nuclei, which are located at the basal cell membrane. The cytoplasm appears lightly stained in H&E stained sections.
Mixed acini: consist of mucous acinar cells that line the lumen. At their outer periphery i. e. at their basal ends crescent-shaped caps of serous acinar cells are present which are called serous demilunes. Intercellular canaliculi delivers the serous secretion of these demilunes to the lumen of the mixed acinus since the canaliculi are actually extensions of the lumen.
Ducts: Saliva from minor glands are transported to the oral cavity via short ducts but the major glands have a branched duct system which starting at the acinar lumina includes:
a) Intercalated ducts: These are located adjacent to and continuous with the acinar lumina. This is the first order of ducts through which saliva passes and they are lined by simple cuboidal cells.
b) Striated ducts: These ducts are larger and longer than intercalated ones and they are lined by simple columnar cells. The term striated is derived from the observation of striations in the basal cytoplasm.
c) Excretory ducts: These are lined by pseudo stratified columnar epithelium with few goblet cells. These ducts are mainly located in the connective tissue septa between lobules.
d) Main ducts: These are the ducts that open into the oral cavity and they are lined by stratified squamous epithelium.
In mixed secretory units where mucous cells predominate, serous cells often form semilunar caps called serous semilunes surrounding the terminal part of the mucous acini. Myoepithelial cells embrace the secretory units, their contraction helping to expel the secretory product. The terminal secretory units merge to form larger intercalated ducts which are also lined by secretory cells. The intercalated ducts drain into larger ducts called striated ducts which are named for their striated appearance in light microscopy. The striations result from the presence of numerous deep infoldings of the basal plasma membranes of the large cuboidal cells which line these ducts. In addition to the salivary proteins, the serous cells are involved in active secretion of a watery fluid containing a variety of inorganic ions. This secretion is isotonic with plasma.
In striated ducts, the ionic content of the secretion is modified by active reabsorption and further secretion of ions to produce saliva which is hypotonic with respect to plasma. The ionic composition of saliva varies at different flow rates but the concentrations of sodium and chloride ions are below that of plasma, and the concentrations of potassium and bicarbonate ions are above that of plasma. Transport processes in the striated ducts are facilitated by the large surface area offered by the basal infoldlngs of the lining cells and are fulled by their numerous associated rnitochondria.
Both serous and mucous acini are embraced by the processes of contractile cells called myoepithelial cells which, on contraction, force secretion from the acinar lumen into the duct system. Myoepithelial cells are located between the basal plasma membranes of secretory cells and the basement membrane. These cells are flattened and have long processes which extend around the secretory acinus but in secretion they can only be recognised by their large flattened nuclei lying within the basement membrane surrounding the acinus.
The striated ducts are lined by large cuboidal cells with large nuclei located between the centre and luminal surface of the cell, the basal cytoplasm appears striated, reflecting the presence of deep basal infoldings of the plasma membrane and associated columns of mitochondria: the apical cytoplasm is uniformly stained and not striated. In predominantly serous salivary glands, the striated ducts are larger than mucous ones, a feature associated with the role of the striated duct in modifying isotonic basic saliva to produce hypotonic saliva (ions are absorbed here).
Parotid salivary glands
They are the complex branched alveolar glands which secrete a serous product. They are situated below and in front of the pinna on each side of the face. They are flat and well encapsulated and the facial nerve runs through them, dividing them into superficial and deep portions. Parotid glands are purely serous in the adult but contain few mucous acini in the newborn. They have long and branched intercalated ducts, numerous well developed striated and excretory ducts. Main duct, Stensen’ s duct opens in the mouth opposite the second maxillary molar. Their stroma consists of well developed capsule and septa with numerous fat cells specially in old persons.
The parotids are composed entirely of the serous acini rich in zymogen granules, with a variable amount of adipose tissue in the interstitium between parotid lobules. Thus these glands have a well prominent lobular structure.
Photomicrograph of a parotid gland. Its secretory portion consists of serous amylase–producing cells that store this enzyme in secretory granules. Intralobular (intercalated and striated) ducts are also present. Pararosaniline–toluidine blue (PT) stain. Medium magnification.
Submandibular glands
They are the complex branched alveolar-tubular glands with mixed type of secretion. These roughly ovoid in shape glands are disposed on either side of the neck just below the mandible. Their ducts open into the floor of the mouth near the frenulum. Submandibular gland is mixed but predominantly serous as it contains mixed acini but the serous acini are more numerous than muCcous ones. The intercalated ducts are shorter than those in the parotid but the striated ducts are longer. Excretory ducts are well developed and the-main duct, Wharton’s opens in the floor of the mouth. Prominent septa and connective tissue capsule are present.
Photomicrograph of a submandibular gland. Note the presence of dense serous cells forming demilunes and pale-staining mucous cells grouped along the tubular portion of this tubuloacinar gland. Medium magnification.
Sublingual glands
They are the complex branched tubular glands with mucous type of secretion. They are located in the floor of the mouth, one on either side of the frenulum of the tongue. Their short ducts open into the mouth near to, or with, the submandibular ducts. These glands are composed predominantly of mucous cells. Sublingual gland is mixed but predominantly mucous as pure serous acini are rare or absent but serous acinar cells are present as demilunes in mixed acini. The intercalated and striated ducts are poorly developed. Several small terminal ducts as well as the main duct, Bartholin’s open into the floor of the mouth. Prominent C. T. septa are present but only a poorly developed capsule exists.
Photomicrograph of a sublingual gland showing the predominance of mucous cells. H&E stain. Low magnification.
Myoepithelial cells: These are cells of epithelial origin that have contractile filaments in their cytoplasm and share some of the functional characteristics of smooth muscle cells. Myoepithelial cells have many processes that extend on the outer surface of acinar cells and intercalated ductal cells but they are located on the epithelial side of the basement membrane.
Nerve and vascular supply
Salivary glands are innervated by the autonomic nervous system, sympathetic and parasympathetic divisions. Neural elements form a network adjacent to ducts and acini and axons may form endings within this network. Also naked axons penetrate the basal lamina and end close to the acinar and ductal cell membranes. These axonal/epithelial relationships are referred to as subepithelial (extra- parenchymal) or intraepithelial (intra-parenchymal), respectively.
A rich network of blood vessels within the stroma supplies the avascular epithelial acini and ducts. The main arteries branch into arteioles which further form capillaries which are located in close proximity to acini and ducts, particularly striated ducts.
Fine Structural features (electron microscopic level)
Serous acinar cells: These are polarized cells with contents present in specific locations. The basal cytoplasm has around nucleus surrounded by numerous parallel cisternae of granular endoplasmic reticulum and a prominent Golgi complex as well as numerous mitochondria. The apical cytoplasm extending from the maturing (trans) face of the Golgi complex to the luminal membrane is occupied by diverse shaped secretory granules. These features characterize these cells as highly active in protein synthesis. A number of intercellular junctions are present between these acinar cells including tight junctions that separate acinar lumina from intercellular spaces. These junctions have selective permeability which regulates passages of ions, water and other molecules to the lumen.
Mucous acinar cells: These are also polarized cells similar to serous cells except that their nuclei are angular and are more basally located. Also their contents are quantitatively different from serous cells as they contain lesser number of granular endoplasmic reticulum cisternae and a more prominent Golgi complex which reflects their synthetic role in formation of carbohydrate rich components of saliva such as mucins. The apical cytoplasm is full of mucous secretory granules which often fuse with each other and seem to compress the organelles in the basal cytoplasm.
Striated ducts: Their main function is reabsorption and secretion of electrolytes and their fine structure reflects this. The striations seen by light microscopy are actually deep invaginations of the basal membranes of the cells that enclose radially arranged mitochondria. This specialization characterizes these cells as involved in active transport as the invaginations increase the surface area available for ion exchange and the mitochondria provide the required energy. The apical cytoplasm contains dense granules, possibly secretory ones and clear vesicles that may be involved in endocytosis.
Excretory ducts: The pseudo stratified epithelial cells contain diverse dense granules and electron lucent vesicles which reflect their role in modifying the composition of saliva. They also contain cells with long branching processes that seem to extend between the ductal epithelium. These cells may be dendritic cells i.e. antigen presenting cells and thus are part of the immune defense system.
Connective tissue stroma:
Loose connective tissue stroma forming the septa between lobes and lobules also extend as thin strands supporting individual acini and ducts. This connective tissue contains the following cells: fibroblasts, macrophages, dendritic cells, mast cells, plasma cells as well as fat cells. The extra- cellular compartment contains collgen and elastic fibers as well as gIycoproteins and proteogIycans. The stroma adjacent to excretory ducts or ducts of minor salivary glands often contains aggregates of lymphocytes. Plasma cells located adjacent to the acini and intralobular ducts produce secretory immunoglobulin A (sIgA) which is a dimer (2 19A molecules connected by a J chain). Salivary epithelial cells have receptors for this dimer at their basolateral membranes and they endocytose receptor-bound sIgA and release it with part of the receptor called the secretory piece. Small quantities of IgG and IgM are also present in saliva.
Secretion of Saliva: Newly formed saliva is released into the lumina from acinar cells when they receive a secretion stimulus e.g. sympathetic neurotransmitter epinephrine. This release is a well regulated active process called exocytosis. The secretory granule membrane fuse with the acinar cell luminal membrane and the site of fusion open into the lumen. Thus the contents within the secretory granule are released while the integrity of the acinar cell membrane is preserved.
The composition of primary saliva collected from the intercalated ducts is modified while it passes through the striated and excretory ducts, mainly by reabsorption and secretion of electrolytes and possibly by addition of other constituents. The primary saliva is isotonic, with sodium and chloride concentrations higher than potassium while the saliva reaching the oral cavity is hypotonic with low sodium and cloride but high potassium concentrations.
Functions of Saliva:
The composition of saliva is complex and varies according to the type of stimulus and the rate of flow. However several important functions are carried out by different components, the essential function is saliva coats the teeth and the mucosal lining of the oral cavity and as such it provides protection as well as it facilitates a number of physiological activities. Humans who suffer from a decreased salivary secretion due to diseases affecting acini or as a consequence of other systemic diseases or as side effect of certain medications have difficulty in eating, swallowing or speaking. These persons are susceptible to mucosal infections and high incidence of dental caries.
Specifically saliva performs the following functions:
1. Protection: The fluid nature and the components of saliva protects the oral cavity in several ways. Saliva provides a washing action that clears non-adherent potentially harmful substances in the oral cavity. Viscous components e.g. mucins lubricate oral tissues and form a barrier against microbial products. Bicarbonate, phosphate ions as well as basic proteins in saliva maintain the near neutral pH in the oral cavity which prevents demineralization of enamel that would otherwise occur due to acids produced by sugar metabolizing bacteria. Salivary proteins form a thin coating on tooth surfaces, the salivary pellicle which contributes to protecting these surfaces. Saliva is supersaturated with calcium and phosphate ions and this state is maintained by certain calcium binding proteins, notably acidic proline-rich proteins and statherin. This leads to posteruptive maturation of enamel surfaces which increases their hardness and resistance to demineralization. Further, such environment favors remineralization of beginning carious lesions (white spots) provided that cavitation did not occur.
2. Digestion: Saliva contains two digestive enzymes, namely amylase which breaks down complex carbohydrates such as starch into glucose and maltose and lipase (a product of lingual glands )which hydrolyzes triglycerides into mono- and diglycerides.
3. Defense: Saliva plays a major bacteriostatic role in the oral cavity. It interferes with microbial colonization and mucins form a physical anti-microbial barrier. Salivary IgA is an important factor in oral immune defense, together with salivary agglutinins (glycoproteins) sIgA causes clumping of certain microorganisms thus preventing them from adhering to oral and dental surfaces. Other components namely histatins, lysozyme, lactoferrin and peroxidase inhibit bacterial growth.
4. Taste: The solubilization of ingested material by saliva is essential for the functioning of taste buds which act as chemoreceptors when they are in contact with the dissolved material and initiate taste sensation.
Minor glands are located in the submucosa or within the muscular tissue around the oral cavity. The distinct types of ducts seen in major glands do not exist in minor ones as a rapid transition occurs in ducts connecting the acini and the oral cavity. The mucous minor salivary glands include labial, buccal, glosso-palatine, palatine and posterior lingual glands. The anterior lingual glands are mixed while the Von Ebner’s glands associated with lingual circumvallate papillae are serous.
liver
Liver is the largest gland in the human body. It lies in the abdominal cavity, in contact with diaphragm. Its mass is divided into several lobes, the number and size of which vary among species. In most mammals, a greenish sac – the gallbladder – is seen attached to the liver and careful examination will reveal the common bile duct, which delivers bile from the liver and gallbladder into the duodenum. The image below is of a liver from a dog, and illustrates these concepts. The panel on the left shows that aspect of the liver that faces the contents of the abdominal cavity. The right panel shows the flatter face of the liver which is in contact with the diaphragm.
Understanding function and dysfunction of the liver, more than most other organs, depends on understanding its structure. The major aspects of hepatic structure that require detailed attention include:
- The hepatic vascular system, which has several unique characteristics relative to other organs
- The biliary tree, which is a system of ducts that transports bile out of the liver into the small intestine
- The three dimensional arrangements of the liver cells, or hepatocytes and their association with the vascular and biliary systems.
The Hepatic Vascular System
The circulatory system of the liver is unlike that seen in any other organ. Of great importance is the fact that a majority of the liver’s blood supply is venous blood. The pattern of blood flow in the liver can be summarized as follows:
- Roughly 75% of the blood entering the liver is venous blood from the portal vein. Importantly, all of the venous blood returning from the small intestine, stomach, pancreas and spleen converges into the portal vein. One consequence of this is that the liver gets “first pickings” of everything absorbed in the small intestine, which, as we will see, is where virtually all nutrients are absorbed.
- The remaining 25% of the blood supply to the liver is arterial blood from the hepatic artery.
- Terminal branches of the hepatic portal vein and hepatic artery empty together and mix as they enter sinusoids in the liver. Sinusoids are distensible vascular channels lined with highly fenestrated or “holey” endothelial cells and bounded circumferentially by hepatocytes. As blood flows through the sinusoids, a considerable amount of plasma is filtered into the space between endothelium and hepatocytes (the “space of Disse”), providing a major fraction of the body’s lymph.
- Blood flows through the sinusoids and empties into the central vein of each lobule. The speed of blood flow is very slow.
- Central veins coalesce into sublobular veins and then hepatic veins, which leave the liver and empty into the vena cava.
The Biliary System
The biliary system is a series of channels and ducts that conveys bile – a secretory and excretory product of hepatocytes – from the liver into the lumen of the small intestine. Hepatocytes are arranged in “plates”(cords) with their apical surfaces facing and surrounding the sinusoids. The basal faces of adjoining hepatocytes are welded together by junctional complexes to form canaliculi, the first channel in the biliary system. A bile canaliculus is not a duct, but rather, the dilated intercellular space between adjacent hepatocytes. This structure is adequate to tubular secretory portion of exocrine gland.
Hepatocytes secrete bile into the canaliculi, and those secretions flow parallel to the sinusoids, but in the opposite direction that blood flows. At the ends of the canaliculi, bile flows into the cholangiols – short ducts which connect secretory portions and aroundlobular bile ducts, which are true ducts lined with epithelial cells. Bile ducts thus begin in very close proximity to the terminal branches of the portal vein and hepatic artery, and this group of structures is an easily recognized and important landmark seen in histologic sections of liver – the grouping of bile duct, hepatic arteriole and portal venule is called a portal triad.
The gall bladder is another important structure in the biliary system of many species. This is a sac-like structure adhering to the liver which has a duct (cystic duct) that leads directly into the common bile duct. During periods of time when bile is not flowing into the intestine, it is diverted into the gall bladder, where it is dehydrated and stored until needed.
Architecture of Hepatic Tissue
The liver is covered with a connective tissue capsule that branches and extends throughout the substance of the liver as septae. This connective tissue tree provides a scaffolding of support and the highway which along which afferent blood vessels, lymphatic vessels and bile ducts traverse the liver. Additionally, the sheets of connective tissue divide the parenchyma of the liver into very small units called lobules.
The hepatic lobule is the structural unit of the liver. It consists of a roughly hexagonal arrangement of plates of hepatocytes radiating outward from a central vein in the center. At the vertices of the lobule are regularly distributed portal triads, containing a bile duct and a terminal branch of the hepatic artery and portal vein. Lobules are particularly easy to see in pig liver because in that species they are well deliniated by connective tissue septae that invaginate from the capsule.
Bile flows out of the liver through hepatic ducts, which join and extend as the common bile duct (also known simply as the bile duct) to traverse the wall of the duodenum and deliver bile into its lumen. In species with a gallbladder, the hepatic ducts join with the cystic duct, which conveys bile to and from the gall bladder.
Hepatocytes are the chief functional cells of the liver and perform an astonishing number of metabolic, endocrine and secretory functions. Roughly 80% of the mass of the liver is contributed by hepatocytes.
In three dimensions, hepatocytes are arranged in plates that anastomose with one another. The cells are polygonal in shape and their sides can be in contact either with sinusoids (sinusoidal face) or neighboring hepatocytes (lateral faces). A portion of the lateral faces of hepatocytes is modified to form bile canaliculi. Microvilli are present abundantly on the sinusoidal face and project sparsely into bile canaliculi.
Hepatocyte nuclei are distinctly round, with one or two prominent nucleoli. A majority of cells have a single nucleus, but binucleate cells are common. The micrographs below (H&E stain) demonstrate these features in sections of liver from a pig (left) and raccoon (right).
Hepatocytes are exceptionally active in synthesis of protein and lipids for export. As a consequence of these activities, ultrastructural examination of hepatocytes reveals bountiful quantities of both rough and smooth endoplasmic reticulum. In contrast to most glandular epithelial cells that contain a single Golgi organelle, hepatocytes typically contain many stacks of Golgi membranes. Golgi vesicles are particularly numerous in the vicinity of the bile canaliculi, reflecting transport of bile constituents into those channels.
Another important function of hepatocytes is to synthesize and secrete very low density lipoproteins. These complexes are seen in electron micrographs as electron-dense particles within smooth endoplasmic reticulum. Another type of particle observed in copious quantities in liver is glycogen. Glycogen is a polymer of glucose and the density of glycogen aggregates in hepatocytes varies dramatically depending on whether the liver is examined shortly after a meal (abundant glycogen) or following a prolonged fast (minimal quantities of glycogen). When viewed with an electron microscope, glycogen particles are typically arranged in chrysanthemum-like clusters of electron-dense particles, as seen in the following image:
In paraffin sections of liver stained with hematoxylin and eosin, accumulations of glycogen in hepatocytes do not stand out. However, when stained with using the periodic acid-Schiff (PAS) technique, glycogen stains bright pink in color. The images below represent PAS-stained sections of liver from two mice:
- Left panel: from a mouse that fasted overnight and thus had very low levels glycogen in liver.
- Right panel: from mouse that stuffed himself on mouse chow two hours prior to fixing the liver, and thus had high levels of hepatic glycogen. These accumulations are seen as pink areas of PAS-positive material throughout the section.
Sinusoids are low pressure vascular channels that receive blood from terminal branches of the hepatic artery and portal vein at the periphery of lobules and deliver it into central veins. Sinusoids are lined with endothelial cells and flanked by plates of hepatocytes.
The space between sinusoidal endothelium and hepatocytes is called the space of Disse. Sinusoidal endothelial cells are highly fenestrated, which allows virtually unimpeded flow of plasma from sinusoidal blood into the space of Disse. This arrangement has at least two important consequences:
Hepatocytes are bathed in plasma derived in large part from venous blood returning from the small intestine. Following meals, that plasma is nutrient-rich.
Plasma which collects in the space of Disse flows back toward the portal tracts, collecting in lymphatic vessels and forming a large fraction of the body’s lymph.
Another important feature of hepatic sinusoids is that they house an important part of the phagocytic system. Sinusoids are populated by numerous Kupffer cells, a type of fixed macrophage. Identifying Kupffer cells in conventionally-stained sections of liver is not easy. However, they stand out sharply when full of phagocytosed ink particles.
The images below are of a mouse liver fixed two hours after an intravenous injection of a small quantity of India ink, which provides a clear view of these tiny warriors. All of the black masses are ink-laden Kupffer cells lying in sinusoids.
As a final point of interest, examine the following micrograph of ovine liver (H&E stain). The sinusoids are packed with clusters of basophilic cells that have not been seen in previous micrographs of liver – what are they?
This section of liver was taken from a 100 day sheep fetus. The clusters of cells in sinusoids are various types of immature blood cells. In the fetus, the liver is a major site of hematopoiesis, or formation of blood cells. Hepatic hematopoiesis is not normally seen after birth, although it can occur under certain pathologic conditions.
The hepatic acinus is the other functional unit of the liver
The acinus is more difficult to visualize than the lobule, but represents a unit that is of more relevance to hepatic function because it is oriented around the afferent vascular system.
As seen overlayed onto lobules in the diagram below, the acinus consists of an irregular shaped, roughly ellipsoidal mass of hepatocytes aligned around the hepatic arterioles and portal venules just as they anastomose into sinusoids.
The acinus is roughly divided into zones that correspond to distance from the arterial blood supply – those hepatocytes closest to the arterioles (zone 1 below) are the best oxygenated, while those farthest from the arterioles have the poorest supply of oxygen. This arrangement also means that cells in the center of the acinus (again, zone 1) are the first to “see” and potentially absorb blood-borne toxins absorbed into portal blood from the small intestine.
The net result is that a variety of pathologic processes lead to lesions that reflect acinar structure; for example, necrosis of hepatocytes at the periphery of the acinus. In the image below, a depiction of the arterial and venous blood supply emanating from one triad has been painted onto a micrograph of porcine liver.
The liver is bounded by a connective tissue capsule which extends into its substance as highly branched septae. The afferent blood vessels and lymphatics follow this connective tissue highway throughout the liver. Efferent vessels traverse a route separate from connective tissue scaffolding.
In the section of equine liver below (Masson’s trichrome stain), the capsule and septae are stained blue , while hepatocytes are magenta . Notice how the capsule extends as a septum into the liver about one-third of the way from left, immediately below a large capsular blood vessel.
The connective tissue septae invaginating from the capsule delineate hepatic lobules, the structural unit of the liver. Relative to other common species, the connective tissue surrounding lobules is particularly abundant and easy to identify in pig livers, as shown below in an H&E-stained section
As you can observe above, a lobule is a roughly hexagonal arrangement of plates of hepatocytes radiating outward from a central vein (CV) in the center. Central veins are quite prominent and provide an easy means of orientation in sections of liver.
At the vertices of the lobule are regularly distributed portal triads (also known as portal tracts). Examination of a triad in cross section should reveal a bile duct and branches of the hepatic artery and hepatic portal vein. Due to plane of section, one can often observe more than one of each of these structures in a given portal tract or absence of one or more structures. Lymphatic vessels are also present, but are tough to see in standard paraffin sections, which is probably why it is not called a portal tetrad (portal tract which includes 4 compounds – interlobular artery. Interlobular vein, interlobular bile duct and lymphatic capillary).
Portal tract in equine liver (trichrome stain)
Portal tract in porcine liver (H&E stain) without visible bile duct
Lobules are almost impossible to miss in porcine liver, but one should also be able to recognize them in other species. Although the precise boundaries of lobules are sometimes difficult to discern, orienting on central veins and portal tracts allow “easy” identification. It goes without saying, of course, that the majority of lobules seen in a tissue section are not as “typical” as seen here and in other histology texts.
THE GALL BLADDER
The gallbladder is a distensible sac and, wheot distended, its mucosa is thrown into many folds. The lumen of the gallbladder is lined with a high columnar epithelium. The connective tissue wall contains abundant elastic fibers and layers of smooth muscle which predominantly run obliquely. The mucosa, connective tissue and muscularis of a canine gall bladder are seen below (H&E stain).
As seen below at a higher magnification, the epithelium of the gall bladder is quite uniform in appearance throughout the organ, and rests on a highly vascularized lamina propria. This is simple columnar brushed epithelium. Epithelial cells have microvilli over the apical surface and are devoted to absorption of inorganic salts and water, and provide the mechanism for the gallbladder’s ability to concentrate bile. Between the smooth muscle layers and serosa is a thick subserosal layer of connective tissue. One face of the gallbladder is attached to the liver, and in that area, the connective tissue of the two organs is shared.
PANCREAS
The pancreas is a elongated organ, light tan or pinkish in color, that lies in close proximity to the duodenum. It is covered with a very thin connective tissue capsule which extends inward as septa, partitioning the gland into lobules. The image below shows a canine pancreas in relation to the stomach and duodenum.
The bulk of the pancreas is composed of exocrine and endocrine portions. The pancreas is surrounded by a very thin connective tissue capsule that invaginates into the gland to form septae, which serve as scaffolding for large blood vessels. Further, these septae divide the pancreas into distinctive lobules, as can clearly be seen in the image of mouse pancreas below (H&E). The large spaces between lobules seen in this image are a commonly-observed artifact of fixation.
Serous acini are structural units of exocrine pancreas. Each acinus consists of 10-12 pancreatic exocrine cells and their intercalated ducts. Embedded within this exocrine tissue are roughly one million small clusters of cells called the Islets of Langerhans, which are the endocrine cells of the pancreas and secrete insulin, glucagon and several other hormones. In the histologic image of an equine pancreas seen below, a single islet is seen in the middle as a large, pale-staining cluster of cells. All of the surrounding tissue is exocrine.
The Acinus
The exocrine pancreas is classified as a compound tubuloacinous gland. The cells that synthesize and secrete digestive enzymes are arranged in grape-like clusters called acini, very similar to what is seen in salivary glands. In standard histologic sections, most acini are cut obliquely, making it difficult to discern their characteristic shape. In the image of equine pancreas below, one fairly-good cross section through an acinus is circled; note the wedge-shapped cells arranged around a small lumen:
Pancreatic Ducts
Digestive enzymes from acinar cells ultimately are delivered into the duodenum. Secretions from acini flow out of the pancreas through a tree-like series of ducts. Duct cells secrete a watery, bicarbonate-rich fluid which flush the enzymes through the ducts and play a pivotal role in neutralizing acid within the small intestine. Pancreatic ducts are classified into four types which are discussed here beginning with the terminal branches which extend into acini.
Intercalated ducts receive secretions from acini. They have flattened cuboidal epithelium that extends up into the lumen of the acinus to form what are called centroacinar cells.
Intralobular ducts have a classical cuboidal epithelium and, as the name implies, are seen within lobules. They receive secretions from intercalated ducts.
Interlobular ducts are found between lobules, within the connective tissue septae. They vary considerably in size. The smaller forms have a cuboidal epithelium, while a columnar epithelium lines the larger ducts. Intralobular ducts transmit secretions from intralobular ducts to the major pancreatic duct.
The main pancreatic duct received secretion from interlobular ducts and penetrates through the wall of the duodenum. In some species, including man, the pancreatic duct joins the bile duct prior to entering the intestine.
Histologic features of the pancreatic duct system are illustrated in the following images:
A longitudinal section through an intercalated duct (Cynomologous monkey pancreas, H&E stain). The duct is running from upper left to lower right. Note the low cuboidal, almost squamous epithelium.
Section of equine pancreas (H&E stain) showing a longitudinal section through an intercalated duct emptying into an intralobular duct. Note the cuboidal epithelium in the intralobular duct.
Small interlobular ducts (equine pancreas; H&E stain): note the columnar epithelium. A thin interlobular septum is seen running horizontally immediately above the duct.
A low magnification image of equine pancreas (H&E stain) showing a large interlobular duct in association with a pancreatic artery (A) and vein (V). An intralobular duct (D) is seen on the right side.
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Pancreatic exocrine cells are arranged in grape-like clusters called acini (a single one is an acinus). Basal portions of cells (homogenous zonez) are basophilic because of well developed rough endoplasmic reticulum producing enzymes. The exocrine cells themselves are packed with membrane-bound secretory granules which contain digestive enzymes that are exocytosed into the lumen of the acinus. These granules which contain a mixture of inactive enzymes lie in the apical part of cells (zymogenous zone). From there these secretions flow into larger and larger, intralobular ducts, which eventually coalesce into the main pancreatic duct which drains directly into the duodenum.
The lumen of an acinus communicates directly with intralobular ducts, which coalesce into interlobular ducts and then into the major pancreatic duct. Epithelial cells of the the intralobular ducts actually project “back” into the lumen of the acinus, where they are called centroacinar cells. The anatomy of the main pancreatic duct varies among species. In some animals, two ducts enter the duodenum rather than a single duct. In some species, the main pancreatic duct fuses with the common bile duct just before its entry into the duodenum.
If you examine a section of pancreas at low magnification, you will undoubtedly observe distinctive areas of pale staining embedded within the exocrine tissue of lobules. These are the Islets of Langerhans, the endocrine component of the pancreas. Islets contain several different endocrine cell types:A,B,D,D1 and PP cells. A cells, B cells and D cells are the major endocrine cells; D1 cells, and PP cells are minor endocrine cells. A cells usually are located along the periphery of islets. A cells have an irregularly shaped nucleus and secretory granules that contain glucagon. B cells comprise about 70% of the islet endocrine cell population and are centrally located in islets. B cells have large, round nuclei. D-cells located at the islet periphery, close to A cells.
The most abundant are beta cells (basophilic cells), which produce insulin, and alpha cells (acidophilic cells), which secrete glucagon. D – dendritic cells are producing somatomedin, D1 – vasoactive intestinal polipeptyde and PP cells – pancreatic polipeptyde. In sections stained with H&E, the different endocrine cell types cannot be differentiated from one another. Special stains, or better yet, immunostaining, is required to identify specific cell types. The endocrine cells within islets are arranged as irregular cords around abundant capillaries, which receive the secreted hormones for delivery into the systemic circulation. Examine the images of islets below and note their rich vascularity, evidenced by clusters and streams of red blood cells next to endothelial cells that can be identified by their flattened nuclei.
Student’s Practical Activities:
Students must know and illustrate such histologic specimens:
Specimen 1. Parotid gland
Stained with haematoxylin and eosin
At a low magnification there seen a lobular structure of the gland. There are vessels, interlobular excretory ducts and nerves in the connective tissue layers. At a high magnification thin intercalated ducts consisting of dark small cells are well observed in the lobules. Striated ducts are larger and consist of tall epithelial cells with basal striations (cell membrane folds). They continue into interlobular ones, whose wall consists of bilayer epithelium and common duct – of stratified epithelium. Secretory portions are round-shaped and consist of serocyte and myoepithelial (basket) cells, which surround the first ones.
Illustrate and indicate: 1. Lobules. 2. Interlobular connective tissue septa. 3. Vessels. 4. Excretory ducts: a) interlobular; b) striated; c) intercalative. 5. Secretory portion: a) serocytes; b) myoepithelial cells.
Specimen 2. Submundibular gland
Stained with haematoxylin and eosin.
At a high magnification two types of secretory portions are seen: serous and mixed. The serous ones are round-shaped and consist of 12-18 serocytes inside and peripherally disposed myoepithelial cells and then – basement membrane. Mixed acini are larger and have two types of cells: serocytes and mucocytes. Mucous cells are conical with wide basal part and light cytoplasm. Serocytes lie near the basal portion of mucocytes thus producing so called serous semilunes or caps of Djianutsi.
Serocytes have basophilic cytoplasm and centrally disposed nuclei. Outside they are surrounded with myoepithelial cells layer. Excretory ducts are similar to that ones in the parotid glands.
Illustrate and indicate: 1. Mucous acini. 2. Mixed acini: a) mucocytes; b) serocytes; c) myoepithelial cells.
Specimen 3. Liver
Stained with haematoxylin and eosin.
The principal cells of the liver, the hepatocytes, are arranged into lobules. The boundaries of the human liver lobules are not defined by distinct connective tissue. Portal tracts define the angles of the lobule margins and a central vein defines the centre of each lobule.
The plates of hepatocytes are usually only one cell. You can focuse on a portal triads between three adjacent liver lobules. Each portal triad contains vessels of three main types. Firstly, the largest diameter vessels are branches of the hepatic portal vein which have the typical, thin-walled structure and irregular outline of all veins. Secondly, the smaller diameter, thick-walled vessels, with the typical structure of arterioles and arteries, are branches of the hepatic artery which supplies oxygenated blood to the liver. Thirdly, ducts of variable size lined by simple cuboidal or columnar epithelium are the bile-collecting ducts which ultimately drain into the common bile duct. A fourth type of vessel, lymphatics, are also present in the portal tracts, but since their walls are delicate and often collapsed they are not readily seen.
Illustrate and indicate:
Specimen 4. Gall bladder.
Stained with haematoxylin and eosin.
The gall bladder is a muscular sac lined by a simple columnar epithelium. In speciment you can see the wall of a gall bladder in the non-distended state in which the lining mucosa is thrown up into many folds. The relatively loose submucosal connective tissue is rich in elastic fibers and contains many blood and lymphatic vessels, which drain water reabsorbed from bile during the concentration process. The muscle layer is seen to separate the submucosa from the outer adventitial connective tissue. In the neck of the gall bladder, mucous glands are often found in the submucosa; this mucus may provide a protective surface film for the biliary tract.
The simple epithelial lining of the gall bladder consists of very tall columnar cells with basally located nuclei. Although not usually evident with light microscopy, the luminal surface of the cells is formed into very numerous, short irregular microvilli. Bile is concentrated 5- to 10-fold by an active process, mediated by the lining cells, which involves absorption of water into the vessels of the lamina propria.
Illustrate and indicate: 1.Mucosa: a) simple columnar epithelium; b) lamina propria of the mucosa; 2.Muscle layer; 3.Adventitia.
Specimen 5. Pancreas.
Stained with haematoxylin and eosin.
The pancreas is a highly lobulated gland invested by a loose connective tissue capsule which extends as delicate septa between the lobules. The exocrine component of the pancreas consists of closely packed, secretory acini which drain into a highly branched duct system.
The endocrine tissues of the pancreas form islets of various sizes, the islets of Langerhans, which are scattered throughout the exocrine tissue.
At higher magnification, details of the pancreatic acini and duct system can be seen. Each acinus is made up of an irregular cluster of secretory cells which drain into a minute, central duct. These minute ducts then drain into the system of ducts of progressively increasing size. The small ducts are lined by simple cuboidal epithelium which becomes stratified cuboidal in the larger ducts. With increasing size, the ducts are supported by a progressively thicker layer of dense connective tissue and the wall of the main pancreatic duct contains smooth muscle.
The pancreatic acini are seen to consist of roughly pyramidal-shaped cells with their apices projecting towards the lumen of a minute duct. The acinar cells are typical protein-secreting (zymogenic) cells. The nuclei are basally located and surrounded by basophilic cytoplasm rich in rough endoplasmic reticulum; the apices of the cells are packed with eosinophylic, zymogen secretory granules. The smallest excretory ducts merge with the lumen of the acini, and duct lining cells are often seen in the centre of secretory acini; these duct lining cells are thus described as centroacinar cells and are recognized by their pale-stained nuclei and sparse, pale-stained cytoplasm. Cells of similar appearance can be seen between the acini. The cells lining the small ducts are responsible for elaborating the bicarbonate component of pancreatic secretion.
Illustrate and indicate:
Task No 3. Students should be able to indicate elements in the electron micrographs:
1. Hepatocytes in hepatic lobule.
2. Biliary poles of hepatocytes.
3. Binucleated hepatocyte.
4. Hemocapillary in liver.
5. Interlobular bile duct.
6. Pancreatocytes (apical portion).
7. Pancreatocytes (basal portion).
8. Insulocytes.
References:
1. Douglas F. Paulsen. Basic Histology. – Prentice – Hall International Inc. – 1990.
2. Paul R., Wheater H., George Burkitt, Victor G. Daniels. Functional Histology: a text and color atlas. – Churchill Livingstone Inc. – 1987.
3. L. Carlos Junqueira, Jose Carneiro, Robert O. Kelley. Basic Histology. – 7th ed. Appleton and Lange.
4. Inderbir Singh. Textbook of Human Histology with colour atlas. – 4th ed. Jaypee Brothers Medical Publishers (P) LTD, 2002, pp. 248-258.
5. Wheater P.R., Burkitt H.G., Daniels V.G. Functional Histology: a text and colour atlas. – 2nd ed. Longman Group UK Limited, 1987. – pp. 224-236.
6. Victor P. Eroschenko. Atlas of Histology with functional correlations. – 9th ed. Lippincott Williams and Wilkins, 2000. – pp. 221-237.
7. Ham, A. W.: Histology, eg.7.
8. Webster’s Medical Desk Dictionary. –
9. Tables:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#n15
10. Disk:
http://intranet.tdmu.edu.ua/index.php?dir_name=cd&file_name=index.php#3
11. Волков К.С. – Ультраструктура клiтин i тканиню Навчальний посiбник-атлас. 1997 р., 95 с.
http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
12. О.Д.Луцик і співавт. – Гiстологiя людини. – Київ: Книга плюс, 2003 р. –592 с.
http://intranet.tdmu.edu.ua/data/books/gistologia_lucyk.pdf
http://www.morphology.dp.ua/hist.php?lang=uk
http://en.wikipedia.org/wiki/Histology
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.kumc.edu/instruction/medicine/anatomy/histoweb/
http://www.udel.edu/biology/Wags/histopage/histopage.htmhttp://intranet.tdmu.edu.ua/data/books/gistologia_lucyk.pdf
