Abstract
BackgroundOocyte cortical granules are important in the fertilization of numerous species including mammals. Relatively little is known about the composition, migration, and pre-fertilization release of mammalian oocyte cortical granules.ResultsResults obtained with confocal scanning laser microscopy indicated that mouse oocytes have at least two populations of cortical granules, one that bound both the lectin LCA and the antibody ABL2 and one that bound only LCA. Both types of granules were synthesized at the same time during oocyte maturation suggesting that the ABL2 antigen is targeted to specific granules by a sorting sequence. The distribution of both populations of cortical granules was then studied during the germinal vesicle to metaphase II transition. As the oocytes entered metaphase I, the first cortical granule free domain, which was devoid of both populations of cortical granules, formed over the spindle. During first polar body extrusion, a subpopulation of LCA-binding granules became concentrated in the cleavage furrow and underwent exocytosis prior to fertilization. Granules that bound ABL2 were not exocytosed at this time. Much of the LCA-binding exudate from the release at the cleavage furrow was retained in the perivitelline space near the region of exocytosis and was deduced to contain at least three polypeptides with approximate molecular weights of 90, 62, and 56 kDa. A second cortical granule free domain developed following pre-fertilization exocytosis and subsequently continued to increase in area as both, LCA and LCA/ ABL2-binding granules near the spindle became redistributed toward the equator of the oocyte. The pre-fertilization release of cortical granules did not affect binding of sperm to the overlying zona pellucida.ConclusionsOur data show that mouse oocytes contain at least two populations of cortical granules and that a subset of LCA-binding cortical granules is released at a specific time (during extrusion of the first polar body) and place (around the cleavage furrow) prior to fertilization. The observations indicate that the functions of the cortical granules are more complex than previously realized and include events occurring prior to gamete membrane fusion.
Highlights
Oocyte cortical granules are important in the fertilization of numerous species including mammals
Mouse oocytes contain two biochemically distinct populations of cortical granules To determine if mouse oocytes contain biochemically distinct populations of cortical granules, the lectin LCA and the polyclonal antibody ABL2 were used to double label fully-grown germinal vesicle intact oocytes
LCA and ABL2 both labeled the cortical granules in germinal vesicle intact oocytes (Figs. 1A and 1B)
Summary
Oocyte cortical granules are important in the fertilization of numerous species including mammals. Several proteins have been identified in mammalian cortical granules using cytochemical techniques These include p62 and p56 that appear immunologically related to sea urchin hyalin [11], ovoperoxidase [12], n-acetylglucosaminidase [13], calreticulin [14], p75, [15] and p32 [16]. The contents of granules are usually uniformly dense; light and dark granules have been reported based on differences in electron density in some species [3,19,20] It has not yet been determined if the difference in ultrastructural density in these granules represents a difference in biochemical composition, different stages in granule maturation, or different stages in exocytosis [3,19]
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