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 exa mined and the effect of maturation interval on the incidence of polysp ermic fertilization and embryonic development in vitro was investigate d. In Experiment 1, it was found that oocytes selected for in vitro ma turation varied considerably in terms of their meiotic progression. Ap proximately half of the oocytes were already undergoing germinal vesic le breakdown at the start of maturation while the remainder were at th e intact germinal vesicle stage. These two populations of oocytes deve loped to the metaphase-II stage by 24 h and 36 h of maturation respect ively. 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, redu cing the maturation interval from 44 h to 36 h did not affect the perc entage of oocytes penetrated but did decrease the rate of polyspermic fertilization (5% v. 34%; P < 0.05). In Experiment 3, development of t he embryos produced in vitro was assessed after culture for seven days . Reducing the maturation interval from 44 h to 36 h decreased the per centage of embryos developing to the 8-cell (9% v. 18%; P < 0.05), mor ula (3% v. 10%; P < 0.01), and blastocyst (1% v. 8%; P < 0.01) stages. These results suggest that maturation for 44 h gives rise to a popula tion of 'aged' oocytes which is susceptible to polyspermic fertilizati on.