Fig. 1. Position of ectoplasmic specializations in the seminiferous epithelium of the testis and immunolocalization of gelsolin to the actin component of the structures. (A) Ectoplasmic specializations are present only in Sertoli cells and occur apically at sites of adhesion to spermatids and basally at sites of adhesion to neighboring Sertoli cells. (B,C) Typical appearance of ectoplasmic specializations in transmission electron micrographs. The structures consist of the plasma membrane of the Sertoli cell, a layer of actin filaments and a cistern of endoplasmic reticulum. The junctions shown here are from sites of Sertoli cell attachment to spermatids in the ground squirrel testis. Bars, 100 nm. (D) Immunofluorescence localization of gelsolin to ectoplasmic specializations in frozen sections of perfusion fixed rat testis. Sections were treated with a primary monoclonal antibody to gelsolin and with a secondary antibody to conjugated to Texas Red. Actin filaments were labeled with fluorescent phalloidin. Within the seminiferous epithelium, gelsolin and actin are co-localized at ectoplasmic specializations. Sites of apical and basal ectoplasmic specializations are indicated by the `a' and `b', respectively, in the panel labeled for actin. Specific staining for gelsolin was not observed in any of the controls (not shown). Bar, 10 µm. (E) Immunoelectron microscopic localization of gelsolin to the actin zone of ectoplasmic specializations. Spermatids with attached ectoplasmic specializations were mechanically dissociated from perfusion-fixed testes and treated with a primary antibody to gelsolin and a secondary antibody conjugated to nanogold. The material was embedded and sectioned, and then the sections were silver enhanced and stained. Shown here is an ectoplasmic specialization attached to a spermatid head. Notice that silver grains (small arrows) are associated with the actin zone of the junction plaque. Bar, 500 nm.