Morphological events at the time of oocyte maturation and sperm–egg interactions during fertilisation in a dasyurid marsupial

In marsupials there have been several studies on oocyte maturation and sperm-egg interactions at the time of fertilisation, but controversy exists as to when and how some of the processes occur. Here we present a summary of relevant data from a species of dasyurid marsupial, the fat-tailed dunnart (Sminthopsis crassicaudata). The findings show that in oocytes of developing ovarian follicles cortical granules are produced with many of the granules sometimes initially congregating around the cytoplasmic vacuoles before migrating to the proximity of the oocyte cell membrane. During fertilisation the spermatozoon binds to the zona pellucida that surrounds the oocyte with its tail lying parallel to the long axis of the head. As the spermatozoon passes through the zona pellucida it is surrounded by the zona matrix and, when entering the egg cytoplasm, a localised area of elevated ooplasm occurs around the spermatozoon. Also, unlike a recent claim to the contrary, the head of the spermatozoon travels a considerable distance into the egg cytoplasm before chromatin decondensation occurs, an event that probably minimises the chances of chromatin disruption by the sperm tail at the time of its incorporation.