Localization of immunoreactive testibumin in the testis and epididymis of adult rats.

Testibumin is a glycoprotein previously isolated from the spent media of primary Sertoli cell-enriched cultures prepared from 20-day-old rats. Immunoassayable testibumin is found in the highest concentrations in testis, epididymis and fluids of the male reproductive tract in adult rats. In the present study, light microscopy was used to show that immunostainable testibumin in paraffin sections of rat testis was localized along the base of the seminiferous epithelium and in finger-like projections from the base of the epithelium, corresponding to the position of the Sertoli cells. The immunostaining of Sertoli cells was shown to be specific since either purified testibumin or crude Sertoli cell-enriched culture medium could compete with antibody for binding sites in tissue sections. The observations using light microscopy were confirmed when Sertoli cells were examined by electron microscopy using a pre-embedding immunostaining technique. The epithelium of the epididymis also contained immunoreactive testibumin which was localized in the caput, corpus and cauda. Immunostainable testibumin was also localized in the corpora lutea of the rat ovary and in the epithelium of the uterine endometrium. These observations are consistent with previous reports that immunoreactive testibumin is present in these organs as demonstrated by radio-immunoassays. We conclude that (i) the immunolocalization of testibumin in Sertoli cells adds to a growing list of observations suggesting that it is made in this cell type; (ii) the other sites of testibumin synthesis in the male are uncertain but the ovary and uterus are possible sites in the female; (iii) electron microscopy following pre-embedding immunostaining and epitope selection can be used as an adjunct to conventional immunocytochemistry to localize proteins in Sertoli cells.