How does sperm meet egg?--in a marsupial

Australian marsupials exhibit a wide range of variation in sperm head morphology, and in thickness of the zona pellucida around the oocyte, suggesting interspecfic differences in the processes of sperm-egg interaction. The observations described here are largely based on the dasyurid Sminthopsis crassicaudata. They show that in oestrous females, after mating, a coagulum forms in the lateral vaginae and, within an hour of insemination, numerous spermatozoa congregate in the isthmus of the oviduct in which the vanguard population undergoes transformation with the head rotating on its axis with the tail to form a T-shape. Once oocytes are released, a few spermatozoa migrate to the higher reaches of the oviduct where sperm-zona binding occurs by way of the plasmalemma over the acrosomal region. The acrosome reaction takes place here and, as the egg rotates, the tail of the spermatozoon becomes parallel to the head. A small region of acrosome sometimes appears to remain intact at this time because spermatozoa with partly intact acrosomes have been found within the zona matrix. In some of these, electron-dense bridges between part of the inner and outer acrosomal membranes which may act as stabilizing structures, were also seen. The zona matrix is tightly packed around the penetrating spermatozoon, but that close to the acrosomal region becomes less electron-dense and more filamentous. Once incorporated into the egg, the spermatozoon lacks a cell membrane around the tail but vesicles close to the sperm head may, at least in part, be remnants of an inner acrosomal membrane. How generally applicable these observations are to other Australian marsupials remains to be determined.