Sperm Capacitation

Mammalian spermatozoa are composed of a head and a tail region. The head, surrounded by a plasma membrane, contains the apical acrosome (enlarged lysosome with outer and inner acrosomal membrane), nucleus (haploid complement of genomic DNA bound to sperm-specific histones), and postnuclear cap. The tail region includes the midpiece (densely packed, helical array of mitochondria that produce ATP), principal piece (self-powered flagel-lum with 9 + 2 array of microtubules), and terminal piece. Freshly ejaculated mammalian sperm are incapable of fertilizing ova but acquire functional competence through capacitation. Only capacitated sperm can penetrate the cumulus oophorus, bind to the ZP, and undergo the acrosome reaction.

Capacitation is a poorly defined maturational process that occurs in sperm during migration through the female uterus and into the oviducts, and includes a series of intracellular and membranal changes that remodel the plasma membrane surface and increase its fluidity. Capacitation is likely mediated by soluble factors secreted by the female reproductive tract, by the presence or removal of proteins in seminal plasma (e.g., human antifertility factor), and/or by removal of decapacitation factors on the sperm surface; identification of such proteins varies among species. The molecular process includes an initial cholesterol efflux and lowering of the cholesterol/phospholipid ratio in the plasma membrane followed by removal of glycoproteins from the sperm cell surface, tyrosine phosphorylation of plasma membrane proteins, an increase in intracellular pH, increase in bicarbonate and Ca2+ ion concentrations, and membrane hyperpolarization. Such intracellular and membrane changes facilitate binding of sperm to the ZP and induction of the acrosome reaction. Capacitated sperm detach from the epithelium of oviductal ampulla; express a transient hyperactive, frenzied, dancing motion; and either are attracted by substances from the ovulating follicle or ovum or are lost into the peritoneal cavity via the oviductal infundibulum.

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