PLACENTA

PLACENTA

·         The term placenta in its broadest sense refers to any region in viviparous organisms where maternal and foetal tissues of any kind are closely apposed, and serves as a site for physiological exchange between mother and foetus (Kent, 1987)

·         Placenta is a spongy mass of flesh that absorbs nutrients and oxygen from the mother’s body.

·         These nutrients and oxygen is conveyed to the developing foetus by means of umbilical cord.

·         During labour the placenta is released from the uterus, because of chemical changes that occur during the delivery, and is removed from the mother by pulling the umbilical cord.

·         The placenta is where the nutrients for the baby is located, while the umbilical cord serves as link between the bay and placenta.

Ø  The placenta from the point of view of origin consists of two parts – Foetal placenta and Maternal placenta.

Ø  The maternal placenta is furnished by single endometrium

Ø  The foetal placenta is furnished by four elements – amnion, chorion, yolk sac and allantois.

Ø  The amnion makes no direct contribution to the placenta, and hence can be discarded here.

Ø  But from the other three membrances, the chorion makes immediate contact with the endometrium.

Ø  For the formation of placenta, the chorion needs to be vascularized.

Ø  This vascularization takes place by two possible ways – via yolk sac (Chorio-vitellie placenta) and via allantois (Chorio-allantoic placenta).

Ø  Thus there are two basic types of placenta on the basis of origin

1.       Chorio-vitelline placenta (Yolk-sac placenta): found in most marsupials, the allantois remains relatively small and never makes contact with the chorion. Yolk sac becomes large and fused broadly with chorion.

2.       Chorio-allantoic or Allantoic placenta: in some marsupials (Parameles) and all eutherians, the yolk sac remains rudimentary and allantois becomes well developed and vascularized to fuse with chorion to furnish the chorion, the blood supply.

TYPES OF CHORIO-ALLANTOIC PLACENTA

On the basis of morphological characters the chorio-allantoic placenta is of following types:

1.       NON-DECIDUOUS PLACENTA (OR SEMI PLACENTA):

·         Implantation is superficial

·         The blascocyst lies in a cavity of the uterus in contact with uterine wall.

·         The intimacy of contact may be increased by formation of villi. Which penetrate into uterine wall.

·         At the time of birth when parturition (separation of foetus and its membranes from mother’s body) occurs, the chorionic villi are just drawn out from the depression in the uterus.

·         Found in pigs, cattle and other mammals.

2.       DECIDUOUS PLACENTA ( OR PLACENTA VERA): ALSO REFFERED TO AS TRUE PLACENTA

·         In this type of placenta, the intimacy between the maternal and foetal tissue is further increased.

·         The wall of uterus gets eroded to various degrees and the embryonic tissues penetrate into uterine wall.

·         During parturition, more or less extensive haemorrhage from the uterine wall takes place due to the injury to the endometrium which gets teared away during parturition.

·         The maternal tissues are expelled out of uterus and is called as DECIDUAE.

3.       CONTRA-DECIDUAE PLACENTA:

·         In parameles and talpa loss of both the maternal as well as foetal tissues take place, this type of placenta is called contra deciduate placenta. The damaged maternal and foetal tissues are absorbed in-situ by the maternal leucocytes.

CLASSIFICATION OF THE BASIS OF DISTRIBUTION OF VILLI ON CHORIONIC SURFACE

1.       DIFFUSE PLACENTA: found in pigs, horse, lemurs: villi remains scattered all over the surface of chorion.

2.       COTYLEDONARY PLACENTA:  found in cattle, deer, sheep: villi are developed in groups or patches while the rest of surface remains smooth.

3.       ZONARY PLACENTA: found in carnivores – cat, dog etc: villi developed in from of belt or band around middle of chorionic sac.

4.       DISCOIDAL PLACENTA: found in insectivores, bats, rabbit, dear etc: villi are restricted to circular disc or plate.

5.       META DISCOIDAL PLACENTA: found in man, monkeys and apes: villi are first scattered, but later form on or two discs – monodiscoidal (in man), bidiscoidal (in monkeys and apes)