Fine structure of Langerhans' islet cells in a marine teleost Conger japonicus Bleeker

Abstract

The Langerhans' islets of a marine fish, Conger japonicus, were studied by light and electron microscopy. The islets are scattered in the exocrine tissue, which is compact (tetrapodlike). The endocrine tissue does not form principal islets (Brockmann bodies). Four kinds of islet cells are found, and they are regularly arranged in the islets. In the periphery of the islets, there are in several layers of A cells; electron microscopically, their secretory granules are spherical (500–750 nm), with an oval core of moderate density, a smooth limiting membrane, and a clear narrow halo between the two. Centrally to the A cells, there are several B-cell layers, containing scattered argyrophile D cells. Electron microscopically the β-granules (500–750 nm) consist of a smooth limiting membrane and a dense round core with a narrow halo; the δ-granules are somewhat smaller (300–400 nm), with flocculent or fine granular material of low density within a smooth limiting membrane and an indistinct halo. The fourth type of cell occupies the central part of the islets, appearing as clear cell masses under the light microscope. Ultrastructurally, its secretory granules (500–750 nm) contain flocculent material of lower density within a somewhat irregular limiting membrane. Since immature secretory granules are frequently found within the Golgi area, it is considered that this fourth cell type also has secretory activity and that it may produce a hormone different from those of the other cell types.