What is Placenta and placentation ?
Placenta (Greek- a flat cake) is a temporary structure formed by the
association / fusion between the extraembryonic membranes of the foetus
and the endometrium of mother for the purpose of physiological exchange
of materials.
Placentation: The method of formation and fusion of the foetal placenta
to the uterine wall is called placentation.
Types of placenta:
On the basis of the types embryonic tissues involved in placentation:
1. Choriovitelline or Yolk-sac placenta:
In such placenta, the allantois remains small and never makes contact with
the chorion. Whereas the yolk-sac becomes very large and fuses with the
chorion.
It is also termed as chorio-vitelline placenta because the chorion
receives its blood supply from the yolk-sac (vitelline circulation).
For e.g in most marsupials (Macropus, Didelphys) and some carnivores,
rodents and insectivores develop such placenta temporarily or permanently.
2. Chorio-allantoic placenta:
Here, the yolk-sac remains ill developed but the allantois becomes very
well developed and fuses with the chorion and provides the chorionic
circulation.
For e.g. in most eutherian mammals and some marsupials (Parameles,
Dasyurus).The chorio-allantoic placenta can be classified as follows:
A.
Based on the degree of intimacy of foetal and maternal intimacy
1. Non-deciduous/semi placenta:
Implantation is superficial at the point of contact with uterine wall,
the blastocyst surface gives out finger-like projection called chorionic
villi that penetrate into depressions of uterine wall and are loosely
united.
Therefore, less bleeding occurs during birth. Seen in pigs, cattle, horse
etc.
2.Deciduous placenta /placenta vera (True placent):
The degree of intimacy between chorionic villi and the uterine wall
increases eroding the endometrial tissue.
Therefore, extensive bleeding occurs during parturition in such
placenta. Found in cats, dogs, rodents and primates etc.
3.Contra-deciduate placenta:
It is a modified type of deciduate placenta seen in prototherian,
Parameles and Talpa (Mole) where both the maternal tissue as well as
foetal placenta are lost at the time of parturition.
B. Based on the distribution of Villi
Diffused placenta: Villi are numerous and distributed uniformly
over the whole of chorion. Found in Ungulates (Pig, mare Horse etc) and
Cetaceans.
i. Cotyledonary: Here the villi are aggregated in special patches
forming small tufts. The rest areas of the chorion remains smooth. Found
in ruminants (Cattles, sheep, Deer etc.). in Camel and Giraffe, it is of
intermediate type with scattered villi arranged in cotyledons.
ii. Zonary placenta: The villi are confined to an annular or
girdle-like zone on the chorion which is elliptical in shape. Found in
carnivores. It may be complete zonary as in dogs, cats and seals; or
incomplete as in racoon.
iii. Discoidal placenta: Here the villi are restricted to a
circular disc-like area on the dorsal surface of blastocyst. Found in
insectivores, bats, rodents (rat, mouse),rabbit and bear.
Metadiscoidal placenta: The villi are first spread all over the chorion
but later become restricted to one or two discs. It may be thus
monodiscoidal as in man, and bidiscoidal as in monkey.
Histologically, there are generally 6 tissue barriers in between the
foetal and maternal blood streams (in order of their sequence from
mother to foetus) which are- endothelium of maternal blood vessel,
endometrial connective tissue, uterine epithelium, chorionic
epithelium, chorionic connective tissue and endothelium of foetal
blood vessel. On the basis of presence or absence of these tissues
they may be:
i. Epitheliochorial type: here all the 6 tissue barriers are
present. Therefore the chorionic epithelium and uterine epithelium lie
side by side. Found in pig, mare, horse etc.
ii. Syndesmochorial: Here, varying amount of uterine epithelium
may be absent. As a result there is direct contact between chorionic
epithelium and uterine epithelium. So, only 5 barriers exist in this
type. Found in ungulates.
iii. Endotheliochorial: here the uterine mucosa is reduce and
the chorionic epithelium comes in contact with the endothelial wall of
maternal blood vessels so that only 4 barriers exist in this type of
placenta. Found in carnivores.
iv. Haemochorial: Here, as the endothelial wall of maternal
blood vessel disappears and the chorionic epithelium is directly bathed
in maternal blood only 3 barriers are found. Found in insectivores,
primates and chiropterans.
v. Haemoendothelial placenta: Due to reduction ofchorionic
epithelium and mesenchymal layer, the endothelial wall of the foetal
blood vessels remain in contact with the maternalblood. Therefore,
only one barrier exists here. Found in mouse, rat,rabbit, guinea pig
etc.
Functions of placenta
i. Anchorage: Anchorage of developing embryo with the uterine
wall.
ii. Nutritional role: The foetus get its nutrition from maternal
blood. Monosaccharides, lipids, amino acids, vitamins and hormones
pass by diffusion or active transport.
Macromolecules of polysaccharides, lipids, and proteins are
absorbed by the trophoblast cells by pinocytosis. Water and
electrolytes such as phosphates of sodium, potassium and magnesium
pass by diffusion.
iii. Respiratory function: exchange of oxygen and carbon dioxide
takes place by diffusion through the foetal membranes.
iv. Excretory role: The nitrogenous wastes- urea uric acid and
creatinine are eliminated via placenta from embryonic blood to
maternal blood. They are later excreted by the kidneys of the mother.
v. Storage function: Glycogen, fats and some inorganic salts are
stored in the placenta for foetus to be utilized when diet in the
maternal blood is inadequate.
vi. Enzymatic function: Enzymes such as diamine oxidase, oxytocin's
and phospholipase A2 produced by placenta are protective.
vii. Endocrine function: Secretes hormones like estradiol,
progesterone, chorionic gonadotropin in most mammals and placental
lactogen in human.
viii. Immunological role: Antibodies developed in maternal blood
against diphtheria, scarlet fever, small pox, and measles are passed
on to the foetus immunizing it passively in the first period after
birth.
ix. Barrier function: Acts as barrier against transportation of
microbes from maternal blood to the foetus.
Destructive function
1. Failure of barrier function leads to passage of pathogenic organism
from mother to foetus and infect the foetus.
2. Any drug used by mother may cross the placenta and may cause adverse
effects.
E.g., There are evidences that sedative, Thalidomide used by mother
during pregnancy is teratogenic (causes deformities in limb, anus and
heart).
More Topics