Figure 1719

Diagram of pancreas, duodenum, and associated excretory ducts.

The main pancreatic duct (of Wirsung) traverses the length of the pancreas and enters the duodenum after joining with the common bile duct. An accessory pancreatic duct (of Santorini) is commonly present, as shown, and empties into the duodenum at a separate minor duodenal papilla. The site of entry of the common bile duct and main pancreatic duct into the duodenum is typically marked by a major duodenal papilla visible on the inner surface of the duodenum.

denum at the hepatopancreatic ampulla (of Vater) through which the common bile duct from the liver and gallbladder also enters the duodenum. The bepatopancreatic sphincter (of Oddi) surrounds the ampulla and not only regulates the flow of bile and pancreatic juice into the duodenum but also prevents reflux of intestinal contents into the pancreatic duct. In some individuals, an accessory pancreatic duct (of Santorini) is present, a vestige of the pancreas's origin from two embryonic endodermal primor-dia that evaginate from the foregut.

A thin layer of loose connective tissue forms a capsule around the gland. From this capsule, septa extend into the gland, dividing it into ill-defined lobules. Within the lobules, a stroma of loose connective tissue surrounds the parenchymal units. Between the lobules, larger amounts of connective tissue surround the larger ducts, blood vessels, and nerves. Moreover, in the connective tissue surrounding the pancreatic duct, there are small mucous glands that empty into the duct.

The pancreas is an exocrine and endocrine gland

Unlike the liver, in which the exocrine and secretory (endocrine) functions reside in the same cell, the dual functions of the pancreas are relegated to two structurally distinct components.

• The exocrine component synthesizes and secretes enzymes into the duodenum that are essential for digestion in the intestine.

• The endocrine component synthesizes and secretes the hormones insulin and glucagon into the blood. These hormones regulate glucose, lipid, and protein metabolism in the body.

The exocrine pancreas is found throughout the organ; within the exocrine pancreas, distinct cell masses called islets of Langerhans are dispersed and constitute the endocrine pancreas.

Exocrine Pancreas

The exocrine pancreas is a serous gland

The exocrine pancreas closely resembles the parotid gland, with which it can be confused. The secretory units are acinar or tubuloacinar in shape and are formed by a simple epithelium of pyramidal serous cells (Fig. 17.20a). The cells have a narrow free (luminal) surface and a broad basal surface. Periacinar connective tissue is minimal.

The serous secretory cells of the acinus produce the digestive enzyme precursors secreted by the pancreas. Pancreatic acini are unique among glandular acini; the initial duct that leads from the acinus, the intercalated duct, actually begins within the acinus (Fig. 17.20b). The duct cells located inside the acinus are referred to as centroacinar cells.

The acinar cells are characterized by distinct basophilia in the basal cytoplasm and by acidophilic zymogen granules in the apical cytoplasm (see Fig. 17.20a). Zymogen granules are most numerous in the pancreas of fasting individuals. The squamous centroacinar cells lack both er-gastoplasm and secretory granules; thus, they stain very lightly with eosin. This weak staining helps identify them in routine histologic sections.

Zymogen granules contain a variety of digestive enzymes in an inactive form

Pancreatic enzymes can digest most food substances. The inactive enzymes, or proenzymes, contained in pancreatic zymogen granules are listed below along with the specific substances they digest when activated.

• Proteolytic endopeptidases (trypsinogen, chymotrypsin-ogen) and proteolytic exopeptidases (procarboxypeptidase, proaminopeptidase) digest proteins by cleaving their internal peptide bonds (endopeptidases) or by cleaving amino acids from the carboxyl or amino end of the peptide.

• Amylolytic enzymes (a-amylase) digest carbohydrates by cleaving the glycosidic linkages of glucose polymers

• Lipases digest lipids by cleaving ester bonds of triglycerides, producing free fatty acids.

• Nucleolytic enzymes (deoxyribonuclease and ribonu-clease) digest nucleic acids, producing mononucleotides.

The pancreatic digestive enzymes are activated only after they reach the lumen of the small intestine. Initially, the proteolytic activity of enzymes (enterokinases) in the glyco-calyx of the microvilli of the intestinal absorptive cells converts trypsinogen to trypsin, a potent proteolytic enzyme. Trypsin then catalyzes the conversion of the other inactive enzymes as well as the digestion of proteins in the chyme.

The cytoplasmic basophilia of the pancreatic acinar cells when observed with the TEM appears as an extensive array of rER and free ribosomes. The presence of these numerous organelles correlates with the high level of protein synthetic activity of the acinar cells (Fig. 17.21). A well-developed Golgi apparatus is present in the apical cytoplasm and is involved in concentration and packaging of the secretory products. Mitochondria are small and, although found throughout the cell, are concentrated among the rER cisternae. Acinar cells are joined to one another by junctional complexes at their apical poles, thus forming an isolated lumen into which small microvilli extend from the apical surfaces of the acinar cells and into which the zymogen granules are released by exocytosis.

Duct System of the Exocrine Pancreas

The centroacinar cells (see Fig. 17.20a) are the beginning of the duct system of the exocrine pancreas. They have a

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