FIGURE 18.23 • Photomicrograph of the pancreas. This H&E–stained specimen shows a number of pancreatic lobules separated by connective tissue septa that are continuous with the thin surrounding capsule of the gland. The pancreatic lobules consist largely of the exocrine acini and their intralobular duct system. Most of the lobules exhibit small, round, lighter-staining profiles, which are the islets of Langerhans (arrows). Adjacent to the lobules, at the lower left, is a large interlobular duct that serves the exocrine pancreas. 25.
unstained after this procedure. TEM allows identification of the principal cell types by the size and density of their secretory granules.
Islet cells, other than B cells, are counterparts of the enteroendocrine cells of the gastrointestinal mucosa.
In addition to the three principal islet cells, three minor islet cell types have also been identified by using a combination of the TEM and immunocytochemistry (Table 18.3). Each cell type can be correlated with a specific hormone, and each has a specific location in the islet.
B cells constitute about 70% of the total islet cells in humans and are generally located in its central portion. They secrete insulin (see Table 18.2). B cells contain numerous secretory granules about 300 nm in diameter with a dense polyhedral core and a pale matrix. The polyhedral core is believed to be crystallized insulin.
A cells constitute about 15% to 20% of the human islet population and are generally located peripherally in the islets. They secrete glucagon (see Table 18.2). A cells contain secretory granules about 250 nm in diameter that are more uniform in size and more densely packed in the cytoplasm than the granules of B cells. The granule is the site of stored glucagon (Fig. 18.26).
D cells constitute about 5% to 10% of the total pancreatic endocrine tissue and are also located peripherally in the islets. D cells secrete somatostatin, which is contained in secretory granules that are larger than those of the A and B cells (300 to 350 nm) and contain material of low to medium electron density (see Fig. 18.26).
The minor islet cells constitute about 5% of the islet tissue and may be equivalent to the pale cells seen after Mallory-Azan staining. Their characteristics and functions are summarized in Table 18.3.
Evidence suggests that some cells may secrete more than one hormone. Immunocytochemical staining has localized several hormones in addition to glucagon in the A-cell cytoplasm. These include gastric inhibitory peptide (GIP), CCK, and adrenocorticotropic hormone (ACTH)endorphin. Although there is no clear morphologic evidence for the presence of G cells (gastrin cells) in the islets, gastrin may also be secreted by one or more of the islet cells. Certain pancreatic islet cell tumors secrete large amounts of gastrin, thereby producing excessive acid  secretion in the stomach (Zollinger-Ellison syndrome).
Functions of Pancreatic Hormones
All of the hormones secreted by the endocrine pancreas regulate metabolic functions either systemically, regionally (in the gastrointestinal tract), or locally (in the islet itself).
Insulin, the major hormone secreted by the islet tissue, decreases blood glucose levels.