Fate of Acrosomal Glycoproteins During the Acrosomal Reaction and Fertilization: A Light and Electron Microscope Autoradiographic Study

Abstract

The acrosomes of rabbit and guinea pig spermatozoa were labeled by L-fucose-[³H]-6 or by glucosamine-[³H]-1 during spermiogenesis. It was anticipated that this label would be incorporated in acrosomal glycoproteins which in their turn may represent, to a considerable extent, the acrosomal enzymes.

The subcellular localization of the label was checked by light and electron microscope autoradiography, and its fate was further studied during acrosome disruption or in vitro-induced acrosomal reaction and during zona penetration (rabbit). By quantitative evaluation of the light autoradiograms, the relative amount of label was compared in rabbit and guinea pig spermatozoa after the different treatments and in rabbit spermatozoa during fertilization in vivo.

After acrosome disruption or induction of acrosome reaction in vitro, label was detected on the expanded acrosomal material and on the acrosomal vesicles but not on the inner acrosomal membrane. During fertilization in vivo label was easily detectable in the sloughed-off acrosomal cap remaining on the surface of the penetrated zona pellucida. The deposition of labeled material within the penetration slit in the zona, if there is such a process, could not be demonstrated. Only single silver grains, not exceeding the background values, were found over the zona material. It was possible, on the other hand, to detect residual labeling on sperm heads in the perivitelline space. This labeling constituted one-fifth or less of the labeling in uterine spermatozoa and closely agreed quantitatively with the amount of label that remained on the sperm heads after acrosome cap removal in vitro. In the latter experiments, the residual radioactivity was situated in the region of the equatorial segment; it was not sensitive to a treatment that extracts acrosin, but was eliminated by removal of all remaining membranes.