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 glycocalyx 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. 18.22). 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.
FIGURE 18.21 • Electron micrograph of the pancreatic acinus and intercalated duct. Note, that pancreatic acinus is formed from a pyramidal in shape acinar cells. Their basal area contains nucleus surrounded by extensive rER and Golgi apparatus. The apical portion exhibit well preserved zymogen granules. The origin of the intercalated duct lined by the centroacinar cells is visible on this electron micrograph. 5,800. (Courtesy of Dr. Holger Jastrow.)
Duct System of the Exocrine Pancreas
The centroacinar cells (see Figs. 18.20a and 18.21) are the beginning of the duct system of the exocrine pancreas. They have a centrally placed, flattened nucleus and attenuated cytoplasm, which is typical of a squamous cell.
Centroacinar cells are intercalated duct cells located in the acinus.
Centroacinar cells are continuous with the cells of the short intercalated duct that lies outside the acinus. The structural unit of the acinus and centroacinar cells resembles a small balloon (the acinus) into which a drinking straw (the intercalated duct) has been pushed. The intercalated ducts are short and drain into intralobular collecting ducts. There are no striated (secretory) ducts in the pancreas.
The complex, branching network of intralobular ducts drains into the larger interlobular ducts, which are lined with a low columnar epithelium in which enteroendocrine cells and occasional goblet cells may be found. The interlobular ducts, in turn, drain directly into the main pancreatic duct, which runs the length of the gland parallel to its long axis, giving this portion of the duct system a herringbone-like appearance (see Fig. 18.19). A second large duct, the accessory pancreatic duct, arises in the head of the pancreas.
The intercalated ducts add bicarbonate and water to the exocrine secretion.
The pancreas secretes about 1 L of fluid per day, about equal to the initial volume of the hepatic bile secretion. Whereas bile is concentrated in the gallbladder, the entire volume of the pancreatic secretion is delivered to the duodenum. Although the acini secrete a small volume of protein-rich fluid, the intercalated duct cells secrete a large volume of fluid rich in sodium and bicarbonate. The bicarbonate serves to neutralize the acidity of the chyme that enters the duodenum from the stomach and to establish the optimal pH for the activity of the major pancreatic enzymes.
Pancreatic exocrine secretion is under hormonal and neural control.