Responses of gametes to the oviductal environment

Spermatozoa and oocytes are separately formed in highly specialized biological compartments (testes and epididymis, ovarian follicle), then deposited communally in the oviduct in an environment designed to facilitate their final maturation, promote their union, and nurture the resultant zygotes and early embryos. The mammalian oviduct undergoes hormonally-mediated cyclical modifications that climax during the periovulatory period thus ensuring production of the specialized environment required for the gametes. While the number of potential bioeffectors present in the oviduct is immense, there are four classes of modulators that are of particular note for the 'capacitation' or maturation of both the spermatozoa and the eggs or early embryos: beta-amino acids, bicarbonate ion, progesterone, and oviductins. Only the oviductins are unique to the oviduct, while the other three are present either at higher concentrations than in other tissues or arrive within the oviductal milieu coordinately with the gametes. For spermatozoa, beta-amino acids, bicarbonate ion, and progesterone work interactively to promote motility, capacitation, and the acrosome reaction, while oviductins facilitate capacitation and species-specific zona pellucida recognition and adhesion. For embryonic development, progesterone works indirectly by promoting a permissive oviductal environment, bicarbonate ion is required for cleavage, and beta-amino acids, acting as organic osmolytes, membrane stabilizers, and/or antioxidants, are facilitatory. Oviductins adhere to the zona pellucida of the ovulated egg thereby increasing sperm adhesion and speed of sperm penetration. Oviductins in the perivitelline space or endocytosed by the pre-implantation embryo may regulate differentiation during the morula to blastocyst transition. The roles of these mediators and their mechanisms of action for the gametes and early embryos are reviewed and discussed.