ISLETS OF LANGERHANS

The pancreas is a mixed gland: the endocrine part of the pancreas consists of more than a million spherical islets of Langerhans. They can be easily seen in the pancreatic parenchyma at the lowest magnification, as they measure about 100-200 μm, are lighter than the serous acini (which represent the exocrine component of the gland), from which they are separated by a fine connective tissue sheath. The islets themselves consist of polygonal epithelial cells arranged in cords; the space between the cords is filled by a small amount of reticular connective tissue containing a network of sinusoidal capillaries. Transmission electron microscopy of the glandular cells of the islets of Langerhans confirms the characteristic of polypeptide-producing cells: their cytoplasm contains numerous cisterns of rough endoplasmic reticulum and secretory granules accumulating in the cytoplasm attached to the sinusoids. According to the nature of the secretory granules, different cell types with different endocrine activity can be distinguished. The most numerous (60-80%) are B cells, which are mainly found in the central parts of the islets. These cells produce insulin, which increases the transfer of glucose into the cells and therefore lowers blood sugar levels. A cells representing nearly 20% of insular cells are found mainly in the peripheral parts of the islets. They are slightly larger than B cells and produce glucagon, which acts as an insulin antagonist, i.e. it raises blood glucose levels. D cells represent about 5% of insular cells; their product is somatostatin, which inhibits the release of other hormones from the islets of Langerhans and also reduces exocrine pancreatic secretion. D cells are mainly scattered in the superficial parts of the islets. F cells (PP cells) are found at the ventral areas of the islets and produce pancreatic polypeptide, which inhibits the secretion of pancreatic juice and bile. The least numerous endocrine cells (less than 1% of all cells) are D1 cells, which produce vasoactive intestinal peptide, EC cells (releasing substance P and serotonin), and G1 cells, which perinatally produce gastrin.

The function and structure are explained in more detail in the e-course Endocrine System II.

The pancreas, including its exocrine and endocrine portions, is of endodermal origin (arises from fusion of the dorsal and ventral buds of the embryonic duodenum).