Sperm-egg interactions in an Australian dasyurid marsupial with official reference to changes in acrosomal morphology

The morphology of the acrosome of spermatozoa from an Australian dasyurid marsupial during in vivo fertilisation, as seen with the transmission electron microscope, is described. In some spermatozoa close to the outer zona surface with largely intact acrosomes, electron-dense struts between the inner and outer acrosomal membranes could be seen; in addition fusion between the outer acrosomal and overlying plasma membrane was sometimes evident. The zona matrix close to the acrosomal region of spermatozoa with heads partly embedded within the zona pellucida was less electron-dense and more filamentous than elsewhere; perhaps indicative of local lytic activity. Nevertheless some of these spermatozoa at least had partly intact acrosomes with localised regions of electron-dense bridges between the two acrosomal membranes. Spermatozoa with condensed chromatin were also observed within the egg cytoplasm some of which had membranes, vacuoles or cisternae close to the dorsal nuclear surface which could, in part, be acrosomal in origin. These observations lend some support to the recent view, based on findings from an American species, that acrosomal changes of spermatozoa of some marsupials at the time of fertilisation may be somewhat similar to those of eutherian mammals in spite of the considerable differences in gamete structure and design. How generally applicable these findings are to all marsupials remains, however, to be determined.