Two hormones secreted by the enteroendocrine cells of the duodenum, secretin and cholecystokinin (CCK), are the principal regulators of the exocrine pancreas (see Tables 17.1 page 583). The entry of the acidic chyme into the duodenum stimulates the release of these hormones into the blood:  Secretin is a polypeptide hormone (27 amino acid residues) that stimulates the duct cells to secrete a large vol ume of fluid with a high HCO3  concentration but little or no enzyme content. CCK is a polypeptide hormone (33 amino acid residues) that causes the acinar cells to secrete their proenzymes.
The coordinated action of the two hormones results in the secretion of a large volume of enzyme-rich, alkaline fluid into the duodenum. In addition to hormonal influences, the pancreas also receives autonomic innervation. Sympathetic nerve fibers are involved in regulation of pancreatic blood flow. Parasympathetic fibers stimulate activity of acinar as well as centroacinar cells. Cell bodies of neurons occasionally seen in the pancreas belong to parasympathetic postganglionic neurons.
The endocrine pancreas is a diffuse organ that secretes hormones that regulate blood glucose levels.
The islets of Langerhans, the endocrine component of the pancreas, are scattered throughout the organ in cell groupings of varying size (Fig. 18.23). It is estimated that 1 million to 3 million islets constitute about 1% to 2% of the volume of the pancreas but are most numerous in the tail. Individual islets may contain only a few cells or many hundreds of cells (Plate 68, page 662). Their polygonal cells are arranged in short, chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas PANCR EAS 649
FIGURE 18.22 • Electron micrograph of the apical cytoplasm of several pancreatic acinar cells. One pancreatic acinar cell is outlined by the dashed line. Nuclei (N) of adjoining cells are evident at the bottom left and right of the electron micrograph. The apical cytoplasm contains extensive rough endoplasmic reticulum (rER), mitochondria (M), zymogen-containing secretory granules (Z), and Golgi profiles (G). At the apices of these cells, a lumen (L) is present, into which the zymogen granules are discharged. A junctional complex (JC) is indicated near the lumen. 20,000.
irregular cords that are profusely invested with a network of fenestrated capillaries. The definitive endocrine cells of the islets develop between 9 and 12 weeks of gestation.
In H&E–stained sections, the islets of Langerhans appear as clusters of pale-staining cells surrounded by more intensely staining pancreatic acini. It is not practical to attempt to identify the several cell types found in the islets in routinely prepared specimens (Fig. 18.24). After Zenkerformol fixation and staining by the Mallory-Azan method, however, it is possible to identify three principal cell types designated A (alpha), B (beta), and D (delta) cells (Table 18.2 and Fig. 18.25). With this method, the A cells stain red, the B cells stain brownish orange, and the D cells stain blue. About 5% of the cells appear to be