EVIDENCE FOR A NONLYSOSOMAL ORIGIN OF THE ACROSOME

We studied the biogenesis of the acrosome in sperm cells in immunogold -labeled ultrathin cryosections of rat testis, using a variety of anti bodies against endosomal/lysosomal marker protein and acrosin, the maj or secretory protein of sperm cells. As expected, acrosomes and proacr osomal vesicles in the trans-Golgi region contained abundant acrosin. Rat lysosomal membrane glycoprotein (lgp) 120 and mouse lysosome-assoc iated membrane protein-1 were not detectable in the acrosomal membrane . Similarly, the late endosomal markers cation-dependent and -independ ent mannose 6-phosphate receptors were absent from the acrosome and pr oacrosomal vesicles. Therefore, acrosomes do not exhibit these endosom al/lysosomal features. Apart from (pro) acrosomal vesicles, both sperm atocytes and spermatids contained classical lysosomes (positive for ra t lgp 120, mouse lysosome-associated membrane protein-1, and cathepsin D) that were negative for acrosin. Quantitative analysis of the immun ogold labeling showed that spermatocytes express more mannose 6-phosph ate receptors and lgp 120 than spermatids, whereas the opposite situat ion existed for acrosin. These data indicate differential synthetic ac tivity of lysosomal and acrosomal constituents in different states of sperm differentiation. Together, our observations argue against a lyso somal/endosomal origin of the acrosome.