Chronological and cytological details of fertilization and early embryonic development in the domestic pig, Sus scrofa.

Abstract

AbstractThe sequence of cytological events from sperm penetration of the oocyte until emergence of the blastocyst from the zona pellucida was studied in 1441 eggs from 134 animals in which the time of ovulation had been controlled precisely by gonadotrophin injection. Observations were also made on the rate of egg passage through the Fallopian tubes, on the process of denudation, and on the increase in numbers of spermatozoa associated with the zona pellucida.Eggs may be penetrated and activated within three hours of mating or insemination close to the time of induced ovulation. A decondensation and swelling of the chromatin is seen very soon after incorporation of the sperm head into the vitellus, and central apposition of the pronuclei occurs three to five hours later. The male pronucleus is slightly larger than the female, and a portion of the flagellum is frequently closely associated with it until late syngamy. Cleaved embryos can be recovered within 14 to 16 hours of sperm penetration, but the two‐celled stage lasts only six to eight hours compared with 20 to 24 hours for the four‐celled stage. Embryos enter the uterus at the latter stage approximately 46 hours after ovulation. Morulae of 16 to 32 cells can occasionally be observed late on the third day of development, and blastocysts are present on the fifth day. However, the zona pellucida is not shed until the sixth day, after which the trophoblast commences the massive elongation characteristic of this ungulate blastocyst.