Is the oocyte a non-professional phagocyte?

Although fertilization has been described as a series of events during which the spermatozoon penetrates the oocyte, introducing its nuclear contents, there is strong evidence that either gamete can be the active partner at different 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 spermatozoon 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 event 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-professional phagocytes, and in particular the roles played by phagocytosis-promoting receptors (FcgR, complement receptors and integrins), in both signal transduction and their linkage with the cytoskeleton. It asks whether the oocyte might not utilize similar mechanisms during its incorporation of the spermatozoon.