Ultrastructural localization of serotonin and polypeptide YY (PYY) in endocrine cells of the human rectum

This study was performed with the aim of ultrastructurally localizing serotonin and polypeptide YY (PYY) in the endocrine cells of the human rectum. Existing basic methods for immunolocalization of antigenic sites in ultrathin sections were tested and modified to allow reproducible results with distinct localization of marker (colloidal gold probes coupled either to IgG or protein A). Probes signifying presence of serotonin were distinctly localized over all heteromorphous granules in argentaffin cells and, in addition, over some of the more monomorphous, rounded granules in a second cell type whose granules all were covered by probes showing localization of the PYY antigen. The results suggest that serotonin in endocrine cells of the gut is not confined to the enterochromaffin type but may also be present in trace amounts in non-enterochromaffin endocrine cells storing peptide hormones. Since probes marking sites of PYY were deposited over some heteromorphous granules in enterochromaffin cells, the evidence obtained also suggests that PYY may occur in low concentration in these cells. The distribution of probes in the sections indicated that antigenic sites were confined to granules in the cells.