Is the oocyte a non-professional phagocyte?

Although fertilization has been described as a series of events during whic h the spermatozoon penetrates the oocyte, introducing its nuclear contents, there is strong evidence that either gamete can be the active partner at d ifferent stages of this process. Indeed, while sperm motility is essential for its penetration of the egg vestments, immotile spermatozoa are capable of entering the ooplasm once they adhere to the oolemma. Entry of the sperm atozoon into the oocyte appears to require two distinct but perhaps related events, namely gamete cell membrane fusion, at the level of the equatorial segment of the sperm acrosome with the oolemma, and a quasi-phagocytic eve nt involving the incorporation by the oocyte of the rostral portion of the acrosome-reacted spermatozoon head within an oolemmal-derived vesicle. This review explores the biology of phagocytosis by macrophages and non-profess ional phagocytes, and in particular the roles played by phagocytosis-promot ing receptors (FcgR, complement receptors and integrins), in both signal tr ansduction and their linkage with the cytoskeleton. It asks whether the ooc yte might not utilize similar mechanisms during its incorporation of the sp ermatozoon.