Further evidence for an argyrophilic fourth cell type in the pancreatic islet of the rat.

An argyrophilic fourth cell type in fetal and adult rat pancreatic islets can be identifed by using a modification of the Grimelius silver statin. This cell is much more abundant in the fetal pancreas than in the adult. By employing the modified silver technique followed by restaining with the indirect immunofluorescent procedure for somatostatin, the content of this argyrophilic fourth cell was studied further. Comparison of these histochemical studies demonstrated that somatostatin was not located in the fourth cell of either the adult or fetal rat pancreas. These results indicate that the D-cell and the fourth cell type are not the same cell. Thus far the only product associated with this argyrophilic cell is pancreatic polypeptide. As a result this cell probably represents the PP-cell of the Wiesbaden classification.