Carcinoid tumors of the rectum. Silver reactions, fluorescence, and serotonin content of the cytoplasmic granules.

The argyrophil and argentaffin staining reactions of the intracytoplasmic granules of 23 carcinoid tumors of the rectum were studied. The granules were evaluated for the presence of amines by means of formalin-induced fluorescence and an immunoperoxidase method for serotonin. The results indicate that most rectal carcinoids are argyrophil when the more sensitive Grimelius method is employed. Only occasionally are rectal carcinoids argentaffin, a feature that correlates with positive fluorescent reactions and suggests amine content. Of three argentaffin tumors, one stained positively for serotonin, and a second contained clusters of pleomorphic electron-dense granules of the type more typically seen in known amine-containing carcinoid tumors. For practical purposes, the Grimelius argyrophil stain is most useful in confirming a light-microscopic diagnosis of a rectal carcinoid.