Somatostatin-containing D cells exhibit immunoreactivity for rat somatostatin cryptic peptide in six mammalian species. An electron-microscopical study.

Antisera raised against rat somatostatin cryptic peptide (RSCP; corresponding to amino acids 63-77 of rat pro-somatostatin), somatostatin-28-(1-12) and somatostatin-28-(17-28) were used to compare the morphological distribution of these pro-somatostatin-derived sequences within the gastroenteropancreatic system of six mammalian species, including man. Using the immunogold staining procedure, RSCP, SS28-(1-12) and SS28-(17-28) immunoreactivity was found to be present in all the D cells of the tissues investigated. Extra-islet RSCP and SS28-(1-12) immunoreactive cells were also identified in some species. RSCP, SS28-(1-12) and SS-28-(17-28) immunoreactivities were also present in a single case of human duodenal somatostatinoma. Immunostaining of serial ultrathin sections from all specimens in this study revealed that RSCP and both somatostatin immunoreactivities were co-localised in a majority of the reactive cells. Corroborative evidence was obtained by double immunogold staining which further showed that RSCP, SS28-(1-12) and SS28-(17-28) immunoreactivities were co-localised to individual secretory granules in D type cells, both normal and tumour. RSCP and SS28-(17-28) immunoreactivities were invariably co-localised, whereas SS28-(1-12) immunoreactivity was restricted to a sub-population of secretory granules. Our findings suggest that RSCP immunoreactivity is conserved in a number of mammalian species and is stored in each secretory granule type. Consequently, detection of the RSCP sequence may serve as a useful marker for somatostatin-producing systems throughout the diffuse neuroendocrine system.