Placentation

Placentation is an apposition of fetal membranes to the endometrium to permit physiological exchange between the fetus and the mother.

Placenta is a unique organ that develops in mammalians for the development of the fetus.

The placenta is composed of two parts:

• The fetal placenta or allantois chorion
• The maternal placenta or endometrium

The yolk sac or amniotic chorion acts as primitive placenta for a few weeks in the early embryonic period.

Allantois develop as a diverticulum of hind gut and fuses with the chorion (trophoblastic capsule of the blastocyst) to form the chorioallantoic placenta.

The blastocyst gets attached to the endometrium and the fetal membranes including the allantois chorion develop during the first month or more of gestation.

At this time, the villiform projections of the chorion and the maternal crypts in the endometrium are rudimentary, small and friable, and the nutrition is from the uterine secretions.

Non-Rejection of Placenta in Pregnant Animals

In hemochorial placentas (man and rodents), greater the trophoblastic invasiveness, the greater the necrosis of both chorionic and endometrial tissue thus resulting in development and deposition of a mechanical acellular barrier of acid mucopolysaccharide.

In epitheliochorial placentas, (cow, sheep, mare and sow) where the interdigitation of microvilli of the chorion or trophoblast and endometrial epithelium are closely apposed, no extensive degeneration or deposition of fibrinoid is present.

Therefore, in the former (hemochorial placentas) an acellular mechanical barrier and in the latter (epitheliochorial placentas), the absence of trophoblastic antigenecity offer reasonable explanations for the retention of the placental homograft.

The sire contributes half of the genetic makeup of the fetus and placenta and hence there should be sufficient tissue incompatibility to induce an immune reaction in the dam and subsequent rejection of the conceptus.

The inability of the immunologically active maternal cells to penetrate in to fetal circulation may also be important.