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)
