Ontogeny of Human Langerhans Islets. A Review of Some Light- and Electron-Microscopical, Immunohistochemical, and Functional Data on Fetal Development of the Endocrine Pancreas (Review)

Abstract

The parenchymal cells of the islets of Langerhans belong to the extensive human neuroendocrine system. Its messenger substances are biogenic amines and neurohormonal peptides. Like other neuroendocrine cells, the islet cells might have originated from the neural crest. However, in the fetal life, their stem cells are located in the epithelium of the pancreatic ductuli. As early as at the 8th gestational week, these stem cells have been found to contain secretory granules of the neuroendocrine type. Evidences for production of insulin, somatostatin, glucagon, and PP (the pancreatic polypeptide) have been obtained immunohistochemically in the samples from the 10–12th gestational weeks. In the samples from the 14th week, cell clusters have been observed, which are outgrowing from the ductular epithelium and forming primitive Langerhans islets. The insulin cells predominate markedly and are shown to respond functionally to glucose stimulation. By the 16th week, the islets become vascularized, with the primary innervation. The completely formed endocrine pancreas, as it is observed at birth, is revealed at the 26th gestational week. Based on some light-microscopical, ultrastructural, and immunohistochemical characteristics of the islet parenchymal cells and their supply with blood vessels and nerves, three phases of the gland embryonal/fetal development are identified.