Differences in Starfish Oocyte Susceptibility to Polyspermy During the Course of Maturation

During the course of starfish oocyte maturation, the optimum period for insemination is between germinal vesicle breakdown and the first polar body formation. The rate of polyspermy increases with increasing time intervals between the first polar body formation and sperm addition, although the fertilization membrane is formed normally in all oocytes. As long as polyspermy is prevented, however, the oocytes remain viable even after the formation of second polar bodies. Thus, it is postulated that there is some mechanism (s) of protection against polyspermy between the first sperm entry and the fertilization membrane formation. The mechanism(s) may be gradually lost after the first polar body formation and disappear completely after the second polar body formation.