as sea urchins. In sea urchins, fertilization occurs externally in the ocean, such that hundreds of sperm can encounter the egg within several seconds. Slow block of polyspermy In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the oviduct. This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg. Nevertheless, polyspermy preventing mechanisms are essential in mammals; a secretion reaction, the "cortical reaction" modifies the extracellular coat of the egg (the zona pellucida), and additional mechanisms that are not well understood modify the egg's plasma membrane. The zona pellucida is modified by serine proteases that are released from the cortical granules. The proteases destroy the protein link between the cell membrane and the vitelline envelope, remove any receptors that other sperm have bound to, and help to form the fertilization envelope from the cortical granules. The cortical reaction occurs due to calcium oscillations inside the oocyte. What triggers such oscillations is PLC-zeta, a phospholipase unique to sperm that is very sensitive to calcium concentrations. When the first spermatozoa get inside the oocyte, it brings in PLC-zeta, that is activated by oocyte's basal calcium concentrations, initiates the formation of IP3 and causes calcium release from endoplasmic reticulum stores, generating the oscillations in calcium concentration that will activate the oocyte and block polyspermy. Evolutionary advantage Female defenses select for ever more aggressive male sperm, however, leading to an evolutionary arms race. On the one hand, polyspermy creates inviable zygotes and lowers female fitness, but on the other, defenses may prevent fertilization altogether. This leads to a delicate compromise between the two, and has been suggested as one possible cause for the relatively high infertility rates seen in mammalian species. In some species, polyspermy is allowed to happen to result in more than one sperm entering the egg creating viable offspring without detrimental effects. See also Cortical reaction References Further reading Ginzberg, A. S. 1972. Fertilization in Fishes and the Problem of Polyspermy, Israel Program for Scientific Translations, Jerusalem. Jaffe, L. A. & M. Gould. 1985. Polyspermy-preventing mechanisms. In C. B. Metz & A. Monroy (editors) Biology of Fertilization. Academic, New York.Brendon magero; 223–250. External links Animation of polyspermy Reproduction