A discrete population of cells containing immunoreactive somatostatin was shown by the peroxidase antibody bridge technique at both the light and electron microscopic level to be present in the pancreas of chick embryos. Based on the stage of development, somatostatin-positive cells, at the light microscopic level, were found in the alpha and beta islets, in isolated islets that did not correspond with any known alpha or beta islets, as well as in the acinar tissue. Quantitative determination indicated that, at all ages examined, there were a greater number of somotostatin cells associated with the alpha islets per square millimeter of tissue than with either the beta islets or acinar tissue of the exocrine pancreas.

Ultrastructural observations on day 15 of development confirm and reinforce the light microscopic observations pertaining to the localization of the somatostatin-positive cells. The results also show that the PAP reaction was localized only on the granules of the somatostatin positive cells. These granules were primarily found in the apical region of these cells adjacent to blood vessels.