Primitive islets contain two populations of cells with differing glucagon immunoreactivity.

Immunofluorescence staining with insulin, glucagon, and somatostatin antisera revealed different immunoreactive cell types in the pancreatic islets of the ratfish. In particular, one cell type was present that reacted with C- and N-terminal anti-glucagon sera, while another type was only detected by N-terminal anti-glucagon sera. The latter cell, however, became reactive to C-terminal anti-glucagon after proteolytic treatment of the section. A similar cell type was found in the epithelium of the pancreatic duct and of the gastrointestinal mucosa. The data indicate that among glucagon immunoreactive cells, some cells situated exclusively in the pancreas possess both C- and N-terminal glucagon immunoreactants, while in other pancreatic as well as intestinal cells, the sequence that binds C-terminal anti-glucagon serum appears masked and becomes accessible to the antibody only after proteolytic digestion.