Microscopic Anatomy 2016

GI Glands (2 lectures)John Klingensmith, Ph.D.

Tuesday, October 25, 2016

The Accessory Organs of the Digestive System

While most of the mechanics of digestion are carried out in the alimentary tube proper, the overall success of the digestive processes is dependent on the so-called accessory organs: the salivary glands, pancreas, liver, and gall bladder. These organs all make and/or process exocrine secretory products that are conveyed by ducts to the lumen of the GI tube, and function in that lumen to aid in the breakdown of food. In addition, both the liver and pancreas have important endocrine functions and contribute hormones that act in distal locations of the body. The liver is also involved in the processing of breakdown products routed to it either by the hepatic portal system or via systemic circulation (both lymph and blood).

Salivary glands (Figs. 1,2) While the entire oral mucosa, including the tongue, is richly

supplied with such glands, it is customary to use the term salivary glands with specific reference to three large pairs of glands the parotid, the submandibular (or submaxillary), and the sublingual (Fig. 1). All three pairs of salivary glands lie outside the oral cavity proper and their secretory products are led via ducts into the mouth.

Fig. 1: Side view of face and oral cavity showing salivary glands and their ducts.

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Saliva The function of the salivary glands is to produce saliva, which

varies in consistency from viscous to watery. This is accomplished by the presence of two kinds of secretory cells, mucous and serous, and by both sympathetic (SNS) and parasympathetic (PNS) innervation. SNS stimulation, as when frightened, reduces the overall amount of saliva produced and favors secretion from mucous cells, while PNS stimulation, in keeping with its “feed and breed” mission, promotes secretion from serous cells, resulting in large amounts of hypotonic, protein-rich saliva (aids in eating). In the adult, about 1.5 L of saliva is secreted daily.

Saliva contains varying amounts of mucus, ptyalin (the salivary amylase), water, and ions. Saliva also contains some IgA (made by plasma cells in the connective tissue of the glands) and lactoperoxidase, along with desquamated cells from the stratified squamous oral epithelium. Saliva performs a number of important functions:

• digestion – ptylin from serous cells begins breakdown of starch. • solubilization – “flavor” molecules are dissolved in the watery secretions,

promoting the sensation of taste. • lubrication- mucous cells secrete a viscous solution rich in glycoproteins &/or

mucopolysaccarides that helps lubricate food. • cleaning- saliva continually cleans teeth and mucous membranes. • inhibition of bacterial growth -IgA and lactoperoxidase both play a role in

controlling microbial growth. • excretion- saliva helps excrete drugs, virus particles and metal ions, and facilitates

the expulsion of undesired material from the mouth.

Histology of the salivary glands The secretory cells of salivary glands are found in spherical

arrangements called acini, excretory secretion units composed of pyramidally shaped cells of similar type (Figure 2). An individual acinus may be composed of either serous or mucous cells, occassionally both (see below). Acinar cells have secretion granules located close to the luminal/apical surface. In serous cells, these granules stain darkly in H&E, presumably due the high concentration of protein present, while granules of mucous cells do not take up H&E stain and thus look very pale or “foamy”. Mucous cells are stained brightly, however with PAS staining. Nuclei of both kinds of acinar cells are located basally, but those of mucous cells are characteristically very flattened at the base of the cell.

Cells in an acinus secrete their products into the lumen of the acinus, which is drained by an intercalated duct, lined with cuboidal epithelium. These drain into larger striated ducts, which are composed of columnar epithelial cells. These cells have features

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common to ion-transporting cells, including aligned mitochondria and basal membrane invaginations; they are thought to transport sodium out of the lumen, making the saliva hypotonic. Both intercalated and striated ducts are referred to as intralobular. Interlobular ducts drain the intralobular ducts, and the ducts become larger until joining the main excretory duct, which exits in the mouth. Interlobular ducts have columnar (proximal) then pseudostratified epithelial linings. The excretory ducts are initially lined with a pseudostratified columnar epithelium which then becomes a stratified columnar epithelium.

Acini often have myoepithelial cells situated immediately on the basal surface, within the basal lamina that surrounds each acinus. These cells, which are rich in actin and myosin, contract to expel the contents from the acinus into the lumen, and from the intercalated ducts into the striated ducts. Myoepithelial cells associated with serous cells are stellate in shape and are also called basket cells, while those associated with mucous cells or the intercalated ducts are spindle-shaped.

While secretory cell types are, in general, segregated into acini, acini are also segregated into separate glands. The parotid gland is almost entirely composed of serous acini, and the sublingual is mostly mucous. The submandibular is unique in that it contains both types of acini.

Fig. 2: Salivary acini and ducts: structure of the submandibular gland.

In the laboratory we will concentrate primarily on the submandibular

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gland, since it contains both types of acini on one slide. The submandibular gland is surrounded by a definite connective tissue capsule, projections of which extend inward between and into the lobules of parenchymal tissue. These connective tissue septa contain the blood and lymph vessels, nerves, and duct elements, but do not show the adipocytes seen in the parotid gland septa. While most of the acini found in the submandibular gland are serous, some of the acini are mucous-secreting. In samples fixed conventionally, some of the mucous acini have a cap of serous cells called serous demilunes. [Note: recent histological innovations suggest the serous cells in these mixed acini are actually intermingled with mucous secreting cells, rather than present as a “cap”]. Like the parotid, the submandibular gland contains both intercalated and striated ducts. Because the intercalated ducts are shorter, they are harder to find in cross sections of tissue.

The parotid gland is the largest of the three major salivary glands. It has a fibrous connective tissue capsule and prominent septa between lobules. The septa often have groups of adipocytes present. The gland is entirely composed of serous acini. These empty into long intercalated ducts and then into striated ducts, and ultimately into the very long parotid duct.

The sublingual glands are predominantly mucous secreting and contain many mucous acini (often capped with demilunes”) and few serous acini. The duct system ends in a series of ducts emptying directly into the mouth (Fig. 1). The sublingual glands are not discretely encapsulated as are the parotid and submandibular glands, but have somewhat more pronounced connective tissue septae.

Pancreas (Figs. 3,4) Like the salivary glands, the pancreas serves the digestive system in an exocrine

capacity, by virtue of the products of the pancreatic acinar cells. Exocrine secretions drain via a ductile system to the main pancreatic duct, which usually joins the common bile duct shortly before the point (duodenal papilla) where their contents empty into the duodenum (Fig. 3). Pancreatic secretions are very important in the digestive process and include:

• zymogens – e.g. trypsinogen, chymotrypsinogen, procarboxypeptidases A and B. • active enzymes-amylase, lipase, nucleases. • ions and anions – alkalinity (pH 7-8) is important in neutralizing acid chyme in

duodenum. As is the case with salivary glands, pancreatic exocrine secretion responds to cephalic stimuli (vagal branches; the pancreatic innervation is primarily parasympathetic). Secretion is also controlled hormonally.

In addition, the pancreas has important endocrine functions contributed by islets of Langerhans. The various cell types of the islets secrete hormones released into the bloodstream:

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• insulin – produced by beta cells, regulates glucose metabolism and glycogen synthesis.

• glucagon – produced by alpha cells, functions to raise blood glucose levels. • somatostatin- produced by delta cells, function unclear but inhibits glucocagon

and insulin secretion.

Fig. 3: Functional organization of the pancreas.

Fig. 4: Structure of the pancreatic acinus.

Histology of the pancreas Delicate connective tissue strands run between lobules of

pancreatic tissue (interlobular septae) and surround the various

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acini, carrying blood and lymph vessels, nerves, and elements of the duct system. The pancreatic acinar cells are serous, and secrete vesicles full of digestive enzymes, which are seen as “zymogen granules” at the apical ends of the cells (Fig. 4). The acinar/lobular architecture of the pancreas is very similar to that of the salivary glands, but with several differences. The acini of the pancreas are different from those of salivary glands in that the intercalated ducts start at the level of the single cells within the lumen of the acini. These long, squamous cells, the centroacinar cells, are characteristic of the pancreas (Fig. 4). They grade continuously into the low cuboidal intralobular ducts, thence into interlobular, and finally into the main pancreatic duct (Fig.3). The intralobular ducts of the pancreas are not nearly as prominent as those (striated ducts) of the salivary glands, and the cuboidal-to-columnar epithelial cells do not show any basal striations. The epithelium of the main duct (of Wirsung) consists of tall columnar cells with a few interspersed goblet cells, instead of the pseudostratified epithelium in salivary ducts. Nonetheless, in practice, observation of the islets of Langerhans is the easiest means of identifying pancreatic tissue in section.

Given that the islets of Langerans perform an endocrine function, they are easily discernible in the masses of exocrine tissue as areas of highly vascularized tissue; these patches are surrounded by a mesh-work of fine reticular fibers, but you will not be able to discern any significant connective tissue capsule around them. There are several different cell types in the islets, usually distinguishable only by virtue of the behavior of their granules under different conditions of fixation, staining, etc. (Fig. 3).

• beta-cell (or Type B): most common about 65% of total islet cells. Usually found in the interior of islets, produce insulin.

• alpha-cell (or Type A), 25% of total islet cells. Found around the periphery, produce glucagon.

• delta-cell (or Type D), present in low numbers; produces somatostatin. • in some species a fourth, nongranular or C-cell is found.

You are not responsible for histologically distinguishing different types of islet cells for this course.

Table I. Comparison of salivary glands and pancreas

Gland Acini Features

parotid serous long intercalated ducts

striated ducts

fibrous capsule

adipocytes in septa

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main ducts pseudostratified

submandibular serious + mucous shorter intercalated ducts

+ serous demilunes some striated ducts

fibrous capsule

no adipose cells

sublingual mostly mucous no striated ducts

+ a few s. demilunes no fibrous capsule

loose CT septa

many ducts to mouth

pancreas serous long intercalated ducts

no striated ducts

centroacinar cells

islets of Langerhans

main ducts simple columnar

Liver (Figs. 5-9) The liver plays many critical roles in maintenance of the body, the

physiological bases of which are beyond the scope of this course. Here we will consider the structural basis of liver function, from the histological point of view, focusing on its role in the digestive processes. Your task is to understand the three-dimensional structure of the liver - the different cell types, how such cells are arranged, and how fluids, especially blood and bile, flow past them. Structural and functional organization of the liver

The lobes of the liver (except where it is in close contact with the diaphragm) are covered by a capsule consisting of mesothelium and underlying connective tissue. The connective tissue extends into the liver parenchyma (functional tissue), subdividing it into polygonal masses called lobules. (The connective tissue septae demarcating lobules are virtually absent in humans.) The lobular parenchyma is composed largely of hepatocytes, which are epithelial cells having both exocrine and endocrine functions. Their exocrine function is to secrete bile, which is conveyed by the bile duct system to the gall

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bladder and/or duodenum. (Bile is composed of water, bilirubin, cholesterol, bile salts (derivatives of cholesterol, lecithin, and electrolytes.) As endocrine cells, they secrete various plasma proteins directly into the blood. These functions involve the passage of compounds between the blood and the hepatocytes; we’ll return to them after considering blood flow through the liver.

Hepatocytes are arranged in sheets or plates—the muralia—which interconnect at various angles (Fig. 5), often leaving holes or gaps in the platework. When these sheets of liver cells are sectioned in histological preaprations, they are most often seen as rows of cells—either single or double—and such rows are referred to as liver cords. The muralia are separated by fairly large spaces - the blood sinusoids, or sinusoidal spaces, which are specialized capillaries.

Fig. 5: Blood and bile flow among the muralia of the liver.

Blood enters the liver via the hilum from two sources: the portal vein carries venous blood from the small intestine, rich in foodstuffs and toxins but relatively poor in oxygen. The hepatic artery carries in oxygenated blood. The connective tissue septae carry branches of both vessels inward along portal tracts (or portal spaces) weaving between the lobules. The vessels empty into the sinusoids where the venous (75%) and arterial (25%) blood supplies are mixed. Blood eventually percolates through the sinusoids to large thin-walled (little connective tissue) collecting vessels - the central veins, also called

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the terminal hepatic venules (Fig. 5). These anastomose and feed the blood into the hepatic veins; the hepatic veins drain into the inferior vena cava. In short, blood travels from the hepatic artery and portal vein, through the sinusoids, and thence to the central veins and hepatic veins.

The hepatocytes of the muralia produce bile and lymph, and these substances are collected in a ductle system which drains into larger vessels in the portal spaces. Lymph and bile thus travel in the opposite direction from the blood (Fig. 5). Two major or main bile ducts leave the liver at the porta and join to form the hepatic duct. This courses toward the duodenum, joining with the cystic duct (from the gall bladder) to become the common bile duct. As the size of duct element increases there is an increase in the amount of connective tissue surrounding it and a progressive change in epithelial type from low cuboidal to tall columnar.

These considerations of fluid flow in the liver prompt three views of the functional organization of the liver parenchyma (Fig. 6). The classic lobule includes the muralia whose blood flow is drained by a single central vein, and is an actual morphological entity. The portal lobule emphasizes the exocrine function of the hepatocytes and includes the tissue whose bile and lymph products drain into each portal tract. The hepatic acinus considers as a unit the parenchyma irrigated by a single terminal branch of the portal vein. This emphasizes that the hepatocytes closest to the venule (zone I in Fig. 6) are exposed to the highest levels of oxygen, food stuffs and toxins from the sinusoidal blood, while those closer to the central vein (zone III) are exposed to the products of the more proximal hepatocytes. Such differences are reflected by selective patterns of disease and damage in the liver.

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Fig. 6: Functional units of the liver parenchyma.

Histology of muralia and sunsoids Hepatocytes do not confront the blood directly; the sinusoids are

lined by a meshwork of specialized endothelial cells (Fig. 7). This lining fits together very loosely to form blood vessels (sinusoidal capillaries) whose walls have fairly large holes (fenestrations) through which blood plasma, but not blood cells, can flow. The endothelial cells do not produce a typical basal lamina, but rather a discontinuous matrix of reticular and/or collagen-like fibers. The lining cells are separated from the hepatocytes by a narrow gap -the space of Disse. This gap is only a micron or so wide, but is often found to be grossly enlarged under some fixation conditions. Microvilli project from each hepatocyte into the space of Disse, increasing its surface area at least six-fold, and are involved in the exchange of molecules with the plasma. The hepatocyte contains various organelles (Fig. 7) which permit its many functions (discussed below and illustrated in Fig. 9).

Fig. 7: Schematic diagram of a hepatocyte and sinusoidal lining.

Sprinkled among the endothelial cells of the sinusoids are other cell types. Kupffer cells,are part of the mononuclear phagocytic system

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and act to engulf and remove debris. Importantly, Kupffer cells destroy faulty erythrocytes; this system takes over if the spleen is removed. Hepatic stellate cells (also called Ito cells) are present as well. The stellate cells are usually quiescent, but become activated in response to chronic liver injury (e.g. alcoholism). Activated stellate cells exhibit several characteristic features, the most important being the deposition of a fibrotic extracellular matrix rich in collagen. The resulting hepatic fibrosis has major functional consequences for liver function.

The membranes of adjoining hepatocytes are coupled with various junctions, including desmosomes and zonula occludens (Fig. 7). These act to seal off the sinusoids from other extracellular space, including a small opening between the cells into which bile is secreted – the bile canaliculus, which often contain microvilli from the hepatocytes. The bile canaliculi form short segments (“little canals”) into which bile is secreted from groups of cells. These are often discontinuous and are not lined with other cells, but empty into larger bile ductules (canals of Hering) (Fig. 8), which are lined with cuboidal epithelium. These in turn empty into bile ducts in the portal tract (Figs. 5,8).

Fig. 8: Bile canaliculi and bile ductules.

Functions of hepatocytes The liver performs many critical functions, all of which are intimately related to its

structure and involve the exchange of materials between the blood and the hepatocytes in the space of Disse. Major functions of the liver include:

• production of plasma proteins and lipoproteins (not immunoglobulins).

• production of lymph (50% of total body lymph).

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• production, recycling and secretion of bile.

• metabolism of various molecules such as steroids and drugs, including detoxification.

• storage of glucose (as glycogen) and management of blood glucose levels (responsive to insulin).

• storage of vitamins.

The cellular bases of some of these major functions are illustrated below (Fig. 9).

Fig. 9: Exocrine and endocrine functions of hepatocytes.

An important example of the compound functions of the liver is the production of bilirubin. This is a toxic breakdown product of hemoglobin released in the destruction of damaged red blood cells by the Kupffer cells and other mononuclear phagocytes. It is taken up from the space of Disse by the hepatocyte and is shuttled to the smooth endoplasmic reticulum. There it is conjugated to glucuronic acid, becoming water soluble, and is secreted into the bile canaliculi as a component of bile.Gallbladder (Fig. 10).

The gallbladder is a pear-shaped organ (about 10 cm long in humans) closely applied to the posterior surface of the liver and communicating via the cystic duct with the common bile duct. The gall bladder functions both to store and to concentrate bile, which is piped in from the liver when not actively needed in the duodenum.

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Structure of the gallbladder The gallbladder wall resembles that of the generalized alimentary

tube, but has distinctive features (Fig. 10). Unless fixed in a distended state, it is found to be extensively folded and convoluted. The gallbladder mucosa does not contain glandular elements. It consists of a simple tall columnar epithelium adjacent to which is a thick, richly vascularized lamina propria. The tall columnar epithelial cells are eosinophilic, with short microvilli on their apical surfaces and extensive interolateral surface membrane folds (the latter usually are detectable only with the EM). A muscularis externa resides just external to the lamina propria of the mucosa, but there are no muscularis mucosae or submucosa subdivisions. Surrounding the muscularis externa is a thick layer of dense connective tissue (CT) containing blood and lymph vessels as well as autonomic nerves. The free surfaces are covered by a serosa of mesothelium and a thin layer of loose CT. The connective tissue abutting the liver is called the adventitia.

The tapered end of the gall bladder is continuous with the cystic duct. Here there are occasional goblet cells within the columnar epithelium, as well as a few mucous glands. The muscularis is more prominent here and because the entire neck of the gall bladder is twisted, the muscle fibers run spirally - the so-called spiral valve. In fact, smooth muscle fibers, running circularly or obliquely, are also present in the hepatic and common bile ducts; in the latter, these muscle fibers are gathered to form the sphincter of Boyden, the closing of which diverts bile back up into the gall bladder.

Fig. 10: Schematic sectional view of the gallbladder wall.

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Function of the gallbladder The columnar epithelium cells of the mucosa act to pump ions and

water out of the bile, concentrating it. The water content drops from about 97% to approximately 84%, the color goes to a dark brown, the pH increases somewhat (maximum of about 8.6) and the various solids such as bile salts, etc., are retained. Of particular importance is the high concentration of cholesterol; crystalline precipitates of cholesterol may form, serving as nucleation sites for the deposition of gallstones. If such gallstones are expelled and become lodged in the common bile duct, bile flow is obstructed and jaundice results.

The muscularis functions to expel bile into the cystic duct and thus, via the common bile duct, into the duodenum. In intervals between meals the sphincter of Boyden is closed and bile is diverted into the distensible gall bladder where the rapid absorption of water helps to minimize the buildup of pressure. Both nervous (vagal branches; cephalic phase of digestion) and hormonal stimuli are involved in causing the sphincter to relax, the musclaris of the gall bladder to contract, and thus bring about flow of bile into the duodenum. The presence of foods, particularly fats, cause the duodenal mucosa to release the hormone cholecystokinin (CCK) into the circulation and it is this polypeptide which activates the gall bladder.

Terms to Know For GI Accessory Organs

Salivary glands

• sublingual

• submandibular (submaxillary)

• parotid

• saliva

• serous acinus

• mucous acinus

• demilunes

• myoepihelial cell

• basket cell

• intercalated duct

• striated duct

Pancreas

• acinar cell

• centroacinar cell

• intralobular duct

• zymogen granule

• main pancreatic duct

• islet of Langerhans

• alpha-cell

• beta-cell

• delta-cell

Liver

• portal tract (triad)

• portal vein

• hepatic artery

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• bile duct

• central vein/terminal hepatic venule

• hepatocyte

• muralia/cords

• sinus/sinusoid

• lobule (classic and portal)

• hepatic acinus

• fenestrated endothelium

• space of Dissse

• Kupffer cell

• stellate cell

• hepatic fibrosis

• plasma proteins

• bile

• bile canaliculis

• dile ductule/ canal of Hering

• bilirubin

Gall bladder

• muscularis

• mucosal folds

• common bile duct

• spiral valve

• sphincter of Boyden

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