Distribution of actin in Sertoli cell ectoplasmic specializations and associated spermatids in the ground squirrel testis.

We have investigated the possibility that the complex patterns of fluorescence associated with spermatids of the ground squirrel labeled with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin (NBD-phallacidin) are due to the presence of filamentous actin within the spermatids themselves rather than to actin in attached Sertoli cell ectoplasmic specializations, as previously reported (J. Cell Biol., 100:814-825). Enzymatic treatments (trypsin, DNAase 1) freed Sertoli cell ectoplasmic specializations from spermatids and resulted in a loss, from the spermatids, of the complex fluorescence patterns, suggesting that the latter were generated by labeled actin in ectoplasmic specializations. Moreover, ectoplasmic specializations that were detached enzymatically from spermatids demonstrated the same fluorescence patterns as those emitted from spermatids in the intact or mechanically fragmented seminiferous epithelium. Most spermatids, however, do display a weak and diffuse pattern of fluorescence that changes during spermatogenesis and that is localized between the acrosomal cap and nucleus. S-1 decoration confirmed this subacrosomal localization and further demonstrated that the actin in adjacent Sertoli cell ectoplasmic specializations is arranged in a unipolar fashion. We conclude that the complex patterns of actin fluorescence associated with mechanically isolated spermatids are a superimposition of both Sertoli cell and germ cell actin; however, the latter is either poorly detected or not detected at all when Sertoli cell ectoplasmic specializations overlie the germ cells.