Placenta of Animals
Placenta is the intimate apposition or fusion of extra embryonic membranes to the maternal tissue for the physiological processes of respiration, nutrition and excretion. In addition placenta protects the embryo.
Foetal membranes (or) Extra Embryonic membranes
Yolk Sac
The endoderm lines the inner border of the blastocyst, converting the blastocoele into the yolk sac cavity.
Amnion and Chorion
The newly formed chorionic sac in the early stages encloses a large cavity – the extra embryonic coelom (exocelom) that lies between chorion and amnion. In later stages the amniotic membrane fuses with chorion in the cow, ewe and sow to form an amniochorionic membrane.
Allantois
The allantois grows into the exocoelom and surrounds the amnion. During this period the yolk sac regresses. The allantois which is highly vascularised by foetal vessels, fuses with the chorion bringing foetal vessels to the periphery of chorionic sac and thus into closer apposition with the maternal tissue. This is the formation of the chorioallantoic membrane.
The compound allantochorionic membrane together with the uterine tissue is designated as chorioallantoic placenta and is classified on the basis of its form and also the number of layers of tissue separating foetal and maternal vascular system viz., degree of placental union.
The chorion which is beset with vascular villi that are in intimate association with the endometrium, constitutes the placenta which sub serves the functions of nutrition, respiration; excretion – and protection of the embryo.
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.