Prefertilization gamete maturation events in marsupials.

Despite many fundamental similarities between the gametes of marsupials and placental mammals, the regulation and timing of prefertilization gamete maturation are quite different. The marsupial acrosome is remarkably stable and an acrosome reaction (AR) is not induced by reagent effective for the sperm of placental mammals. The ultrastructure of the marsupial sperm AR is essentially similar to that of placental mammals, however, whether an equatorial segment (ES) persists to serve as the site of sperm-egg membrane fusion is unclear. Diacylglycerol induction of the AR suggests that the sperm of Australian species lack an ES, yet an ES-like region appears to be involved in fertilization in the opossum Monodelphis. The marsupial oocyte, unlike those of placentals, continues to grow throughout follicular life and major cytoplasmic maturation events occur late in oocyte development. Cortical granules only become evident shortly before ovulation and mature dark granules may only appear after ovulation. Further, the zona pellucida (ZP) changes in character and function during the peri-ovulatory period. In vitro fertilization has been achieved for an opossum but not for any Australian marsupial, owing to failure of sperm-ZP binding. Requirement for a sperm maturation process is likely, but capacitation treatments used for placental sperm in vitro have been ineffective. Since it is now feasible to experimentally manipulate marsupial gametes in vitro major advances in our understanding of their function can be expected.