Pancreatic endocrine cells of Barbus conchonius (Teleostei, Cyprinidae), and their relation to the enteroendocrine cells

Abstract

The pancreatic endocrine cells of Barbus conchonius are concentrated in a large (principal) islet, located near the gall bladder, and in a number of smaller islets. Five types of endocrine cells can be distinguished in these pancreatic islets: B cells, A₁ (or D cells), 2 types of A₂ cells (A_2r cells with round granules; and A_2fl cells with flocculent granules) and a scarce 5th cell type. The hormones produced by B and A_2fl cells are probably insulin and glucagon respectively. The A_2r cell contains granules with the same diameter as the granules of the enteroendocrine type III cell of the gut. Both cell types may resemble the enteroglucagon-producing EG cell of mammals. The function of the A₁ cells, which are frequently found without secretory granules, and of the 5th cell type, will be discussed. The pancreatic islets of B. conchonius are strongly innervated, which suggests the presence of a direct nervous control system. Some intermediate or mixed cells containing exocrine and endocrine A_2r granules are found contiguous with the principal islet. The origin of pancreatic endocrine cells is also the subject of discussion.