Embryogenesis recapitulates oogenesis in swine.

Events during oogenesis can affect embryogenesis so dramatically that oocytes can be identified that are progenitors of embryos which would probably die if they remained in the host pig, but would live if appropriately transferred to another female. This review goes backward from embryonic to oocyte development, first discussing how subtle differences between littermate embryos can result in the death of some embryos and then relating the causes of those differences to events during follicular maturation. Embryonic development is not uniform in swine. The larger blastocysts within a litter synthesize estradiol sooner than their less developed contemporaries. Estradiol advances uterine secretions to the benefit of the more developed blastocysts, but results in an asynchronous and hostile environment for the less developed blastocysts. Through a series of experiments, the pattern of oocyte and follicular development was found to be one of the sources of subsequent disparity among blastocysts. In pigs mated before ovulation, the first oocytes released at ovulation were the first fertilized and became the more developed blastocysts 12 days later. Inversely, the later ovulated oocytes were the last to be fertilized and became the smaller blastocysts. These smaller blastocysts can develop normally, but because of estrogenic advancement of uterine secretions, they will preferentially die.