Egg maturation and fertilization in marsupials.

This brief review summarizes our knowledge of the morphological events that are associated with oocyte maturation and fertilization in marsupials in which it has been suggested that there are marked differences from eutherians in both the developmental timetable of oocyte maturation and in some of the processes associated with sperm-egg interaction. Most of the data have been obtained from studies on four species: Monodelphis domestica, Sminthopsis crassicaudata, Sminthopsis macroura, and Trichosurus vulpecula. Differences between the species have been described for: (1) the arrangement of 'yolk' in the oocyte cytoplasm; (2) the time of formation of cortical granules: (3) the mode of sperm penetration through the zone pellucida: (4) the sperm membrane involved in sperm-egg fusion: (5) the fate of inner acrosomal and sperm plasma membranes: and (6) the rapidity of sperm chromatin decondensation in the ooplasm. Such differences suggest considerable variation in these processes between different marsupial species although some of the variation described may be due to technical differences in the obtaining of the data. Thus, whether there are fundamental differences between the two major extant infraclasses of mammals, marsupials and eutherians, in some of the processes associated with fertilization is conjectural at the present time. The interspecific variation in the results obtained cautions one in extrapolating from observations on one or two 'model' species to the infraclass as a whole: a conclusion that might not, on reflection, be too surprising bearing in mind the long and separate evolutionary history of the major extant marsupial lineages.