OTHER ENDOCRINE TISSUES
The other endocrine tissues are incorporated within organs of composite function. The most familiar example is provided by the endocrine component of the pancreas, the pancreatic islets, also known as the islets of Langerhans.
The general anatomy of the pancreas has already been described (p. 141). The endocrine component comprises many hundred (or thousand) islets of varying size unevenly distributed among the predominant exocrine tissue. The islets are not normally visible to the naked eye, but the larger ones—of pinhead size— can be made apparent by the use of intravital dyes. The islet tissue has the same origin as the exocrine pancreas and buds from the epithelial cords at an early stage; it remains solid when the remainder of the “tree” canalizes.The islet cells are of several types (the exact number is disputed); the two most numerous are the alpha and beta types, which produce glucagon and insulin, respectively. These hormones affect carbohydrate metabolism, and their role is best known from the diabetes that develops when insufficient insulin is produced by the islet tissue. The pancreas is also the source of certain other hormones, including somatostatin and pancreatic polypeptide. Other less numerous cells manufacture gastrin; the distinction and functions of yet other types are in dispute. The relative frequencies of the different types are not the same in all parts of the pancreas, and some evidence exists that different ratios occur in the parts that originate from the dorsal and ventral primordia.
The endocrine components and functions of the testes (p. 186), ovaries (p. 205), and placenta (p. 211) were sufficiently mentioned in Chapter 5.
The endocrine components of other organs are even more discrete and thus are not described as they make no gross representation. The most important examples are the renin-producing juxtaglomerular complexes within the kidney and the variety of enteroendocrine cells scattered within the gastric and intestinal epithelia (p. 131). The number, distinctions, and functions of the enteroendocrine cell types are inadequately known. Although mainly scattered singly, these cells are so numerous that they would constitute a considerable gland if massed together. They are considered to belong to the so-called APUD cell system (now shown to be of endodermal not neuroectodermal origin, as formerly supposed) and are believed to produce gastrin, secretin, glucagon, vasoactive intestinal peptide, gastric inhibitory peptide, and several other hormones.