Asynchronous meiotic progression in porcine oocytes matured in vitro: a cause of polyspermic fertilization?

The progression of meiosis in porcine oocytes matured in vitro was examined and the effect of maturation interval on the incidence of polyspermic fertilization and embryonic development in vitro was investigated. In Experiment 1, it was found that oocytes selected for in vitro maturation varied considerably in terms of their meiotic progression. Approximately half of the oocytes were already undergoing germinal vesicle breakdown at the start of maturation while the remainder were at the intact germinal vesicle stage. These two populations of oocytes developed to the metaphase-II stage by 24 h and 36 h of maturation respectively. Maturation for 40 h and 44 h did not increase the percentage of oocytes at metaphase-II stage observed at 36 h. In Experiment 2, reducing the maturation interval from 44 h to 36 h did not affect the percentage of oocytes penetrated but did decrease the rate of polyspermic fertilization (5% v. 34%; P < 0.05). In Experiment 3, development of the embryos produced in vitro was assessed after culture for seven days. Reducing the maturation interval from 44 h to 36 h decreased the percentage of embryos developing to the 8-cell (9% v. 18%; P < 0.05), morula (3% v. 10%; P < 0.01), and blastocyst (1% v. 8%; P < 0.001) stages. These results suggest that maturation for 44 h gives rise to a population of 'aged' oocytes which is susceptible to polyspermic fertilization.