Diagram of an islet of Langerhans stained by the Mallory-Azan method. A cells display red cytoplasmic staining, B cells (comprising most of the islet cells) display brownish-orange staining, and D cells show a blue cytoplasm.
islets. They secrete glucagon (see Table 17.2). 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. 17.25).
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. 17.25).
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 17.3.
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).
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