VAN 211: Veterinary Histology and Embryology (2+2)

PLACENTATION IN HIGHER MAMMALS

• Placentation includes the events of implantation and development of placenta.
• Implantation: At the 8 to 16-cell stage, the embryo has reached the uterus in about 2 to 51/2 days in most species of mammals. However, the pig embryo enters the uterus relatively in the 4-celled stage, while in the bitch; the time taken is 6 to 8 days.
• The blastocyst – enlarges and fills up the uterine lumen. Zona pellucida is shed and the free-living embryo is nourished by uterine secretion. The embryo is said to be implanted when it is fixed in position and does not float freely in the uterine lumen. In farm mammals the attachment to the uterine wall of the blastocyst is loose prior to formation of placenta. Implantation takes place about 11 to 40 days in cow and 10 to 22 days in sheep, after coitus.