Immunocytochemical characterization of the pancreatic islet cells of the nile tilapia (Oreochromis niloticus)

The cellular composition and topography of the pancreatic islet of Oreochromis niloticus, now known to be a donor source for islet xenotransplantation studies, were characterized. Whole tilapia islets were harvested using an enzymatic method and then further digested into single- cell preparations. Cell cytospin preparations of islet cells and paraffin sections of whole islets were stained using antisera against tilapia insulin, human glucagon, salmon somatostatin-25 (SST-25), human somatostatin-14 (SST- 14), and salmon peptide tyrosine-tyrosine (PYY) using the immunoperoxidase method. Cell counts, performed on cytospin preparations using a Quantimet 570 computerized image analysis system, revealed that O. niloticus islets contained 78% endocrine cells and 22% immunonegative cells (i.e., mainly nucleated erythrocytes and rare tissue eosinophils). The proportions of immunopositive endocrine cell types were: 42.3% insulin immunopositive cells, 11.5% glucagon immunopositive cells, 23.1% SST-25 immunopositive cells, 21.8% SST-14 immunopositive cells, and 1.3% PYY immunopositive cells. Islet cell topography was evaluated using histologic sections of whole endocrine pancreata including large, medium, and small islets. Round to polygonal insulin immunopositive cells with round central nuclei were distributed in clusters throughout both the principal and the smaller islets. Elongate SST- 14 immunopositive cells were closely associated with the clusters of insulin immunopositive cells; both were surrounded by SST-25 immunopositive cells, which were similar in shape to the insulin immunopositive cells. There were elongate glucagon immunopositive cells throughout the islets, whereas the PYY immunopositive cells were restricted to the periphery and to channels of fibrovascular connective tissue penetrating the islets.