Mating-induced cumulus-oocyte maturation in the shrew, Suncus murinus

The musk shrew, Suncus murinus, is an induced ovulator, the cumulus oophorus of which is unusual in several respects: it has no matrix, it always induces the acrosome reaction and it appears to be essential for fertilization. The present study documents distinctive features of the cumulus oophorus before and after ovulation, and of copulation-induced maturation of the ovulatory follicle, which has no antrum. In unmated females, potentially responsive ovarian follicles are distinguishable from large secondary follicles by differentiation of the granulosa into outer and inner cell layers, the latter being characterized particularly by intracellular glycogen deposits. The average number of responsive follicles equates with the number that ovulate. By about 10 h after mating, meiosis has reached metaphase II, with extrusion of the first polar body. Coincidentally, a cavity has developed between the inner and outer follicular layers, demarcating the smaller cells of the granulosa from the glycogen-rich cells of the cumulus oophorus. Subsequently, the glycogen becomes restricted primarily to the inner cumulus, and the corona cells began to retreat from the zona pellucida surface to form an unusual very distinct perizonal space that is clearly evident at the time of ovulation. The cumulus is stabilized by gap and tight junctions, and presents a smooth external surface that appears to initiate the acrosome reaction. After fertilization, at which time the zona pellucida becomes more resistant to both trypsin and dithiothreitol, the cumulus develops intercellular lacunae, and is eventually discarded about 15 h after ovulation.