Localization of bindin expression during sea urchin spermatogenesis

Expression of the bindin gene was examined in testicular cells of the sea urchin Strongylocentrotus purpuratus. In situ hybridization studies, using an 35S-labeled antisense RNA probe transcribed from a bindin cDNA, reveal that bindin mRNAs are localized in spermatogenic cells displaced towards the lumens of maturing testicular acini. Little or no hybridization is observed in spermatogenic cells displaced towards the perivisceral epithelium or in somatic cells of the testis. A similar localization of the bindin protein itself is observed using a rhodamine-conjugated polyclonal antibody against bindin, which shows a punctate immunofluorescence pattern in late spermatogenic cells. Immunogold labeling of ultrathin sections and electron microscopy reveal that this punctate immunofluorescence is an apparent result of localized deposits of bindin in intracellular vesicles. Through the terminal stages of spermatogenesis, these bindin-containing vesicles apparently fuse to form the single acrosomal vesicle of the mature spermatozoon. These results indicate 1) that bindin mRNAs are transcribed relatively late in spermatogenesis, 2) that bindin is translated soon after production of its mRNA, 3) that bindin quickly associates with intracellular vesicles during or soon after its synthesis, and 4) that these vesicles fuse to form the single acrosomal vesicle during the terminal stage of spermatogenesis.