Paraformaldehyde-induced fluorescence as a histochemical test for 5-hydroxytryptamine in the epididymis of the opossum.

Epididymides of four opossums were tested histochemically for the presence of 5-hydroxytryptamine (5-HT). In addition, testes and epididymides of two opossums were analysed biochemically for the same substance. Biochemical analysis revealed a high concentration of 5-HT in the epididymis, while the amount in the testis proved to be insignificant. Fluorescence microscopy gave positive results in the apical portion of the epithelium. There was also fluorescence in the connective tissue in general, in mast cells, and in a thick subepithelial band in particular. Following treatment with 2% sodium borohydride in 1,2-propanediol for 15 minutes, the fluorescence of mast cells was removed, while there was a slight reduction in epithelial fluorescence and that in the subepithelial band and other connective tissue. The subepithelial band stained metachromatically with azure B, and this staining characteristic was removed by treatment with RNAse. Feulgen reaction revealed elongate nuclei in the band. It would seem that the 5-HT is located in both the mast cells and the epithelium. It seemed to be also present in the subepithelial band, but because of strong autofluorescence it is difficult to demonstrate. At the present time the function of 5-HT in the opossum epididymis is unknown, and is under further investigation.