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 interspecific differences in the processes of sperm-egg int eraction. The observations described here are largely based on the das yurid Sminthopsis crassicaudata. They show that in oestrous females, a fter mating, a coagulum forms in the lateral vaginae and, within an ho ur 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. On ce oocytes are released, a few spermatozoa migrate to the higher reach es of the oviduct where sperm-zona binding occurs by way of the plasma lemma over the acrosomal region. The acrosome reaction takes place her e and, as the egg rotates, the tail of the spermatozoon becomes parall el 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. Ln some of these, electron-den se bridges between part of the inner and outer acrosomal membranes whi ch 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 filamento us. Once incorporated into the egg, the spermatozoon lacks a cell memb rane around the tail but vesicles close to the sperm head may, at leas t in part, be remnants of an inner acrosomal membrane. How generally a pplicable these observations are to other Australian marsupials remain s to be determined