Polyclonal antisera raised against polypeptide components of antigenically related rat Sertoli cell secretory protein S70 and S45-S35 heterodimeric protein were used to identify antigenic sites in rat testis during spermatogenesis. Immunofluorescence, immunoperoxidase and immunogold electron-microscopic methods have shown that all antisera recognize immunoreactive sites in the acrosome of developing spermatids. Apical cytoplasmic regions of Sertoli cells in close association with bundles of developing spermatids displayed an immunoreactive product. The principal piece of the developing tail of maturing spermatids immunostained with antisera to Sertoli cell secretory S45-S35 heterodimeric protein, displayed characteristic apical-to-distal immunoreactive gradient patterns before acquiring uniform immunoreactivity at completion of maturation. Immunogold electron microscopy demonstrated that outer dense fibres were the predominant immunoreactive site. Results of this work, together with previous immunoblotting and immunofluorescent data, demonstrate that both Sertoli cell secretory proteins and components of the acrosome and tail of developing spermatids and epididymal sperm share antigenic homology
