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

Abstract

The progression of meiosis in porcine oocytesmaturedin vitro was examined and the effect ofmaturation interval on the incidence of polyspermic fertilization andembryonic development in vitro was investigated. InExperiment 1, it was found that oocytes selected forin vitro maturation varied considerably in terms oftheir meiotic progression. Approximately half of the oocytes were alreadyundergoing germinal vesicle breakdown at the start of maturation while theremainder were at the intact germinal vesicle stage. These two populations ofoocytes developed to the metaphase-II stage by 24 h and 36 h of maturationrespectively. Maturation for 40 h and 44 h did not increase the percentage ofoocytes at metaphase-II stage observed at 36 h. In Experiment 2, reducing thematuration interval from 44 h to 36 h did not affect the percentage of oocytespenetrated but did decrease the rate of polyspermic fertilization (5%v. 34%; P < 0·05). In Experiment 3, development of the embryos producedin vitro was assessed after culture for seven days.Reducing the maturation interval from 44 h to 36 h decreased the percentage ofembryos 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 thatmaturation for 44 h gives rise to a population of ‘aged’ oocyteswhich is susceptible to polyspermic fertilization.