The placenta is an organ that develops during pregnancy in mammals to connect the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. It is a discoid, circular organ around 15-20 cm in diameter and 2.5 cm thick that weighs approximately 500g at term. The placenta has both fetal and maternal surfaces, with the fetal surface covered by the amnion and bearing the umbilical cord, and the maternal surface being rough and spongy where it attaches to the uterine wall. The placenta serves critical functions like nutrient, gas and waste exchange between mother and fetus, acting as an endocrine organ and barrier between

1. Shri Shankaracharya Mahavidyalaya,Junwani,Bhilai
Placentation in mammal
Dr. Sonia Bajaj
(Head of Department)

2. Placenta- The placenta is defined as an organ that develops during pregnancy in
mammals. The placenta provides oxygen and nutrients to the growing fetus in the
uterus of the mother.
Circular disc with a diameter of 15-20 cm and thickness of about 2.5 cm at its center.
• It thins off towards the edge.
• It feels spongy and weight about 500 gm.
• Proportion to the weight of the baby being roughly 1:6 at term and occupies about 30% of
the uterine wall.
• It presents two surfaces, fetal and maternal, and a peripheral margin.
Fetal surface
• Covered by the smooth and glistening amnion with the umbilical cord attached at or near its
center.
• Branches of the umbilical vessels are visible beneath the amnion.
• The amnion can be peeled off from the underlying chorion except at the insertion of the
cord.
• At term, about four-fifths of the placenta is of fetal origin.
Maternal surface-
Rough and spongy Dull red colour.
• A thin greyish, somewhat shaggy layer which is remnant of decidua basalis and has come
away wit the placenta, may be visible.
• 15-20 convex polygonal areas known as lobes cotyledons which are limited by fissures.

3. The decidua is the uterine lining (endometrium) during a pregnancy, which forms the
maternal part of the placenta. It is formed under the influence of progesterone and forms
highly characteristic cells.
Structure-
The part of the decidua that interacts with the trophoblast is the decidua basalis (also called
decidua placentalis), while the decidua capsularis grows over the embryo on the luminal
side, enclosing it into the endometrium. The remainder of the decidua is termed the decidua
parietalis or deciduavera, and it will fuse with the decidua capsularis by the fourth month of
gestation.
Three morphologically distinct layers of the decidua basalis can then be described:
Compact outer layer (stratum compactum)
Intermediate layer (stratum spongiosum)
Boundary layer adjacent to the myometrium (stratum basalis)

4. Within the decidua, occasional fibrinoid deposits form where the syncytiotrophoblast is
damaged. The region of fibrinoid deposition where trophoblasts meet the compact
portion of the decidua basalis is called Rohr's layer, while the fibrinoid deposits that
occur between the compact and spongy layer of the decidua basalis is termed
Nitabuch's layer. This layer is absent in placenta accreta.
The decidua has a histologically-
distinct appearance, displaying large polygonal decidual cells in the stroma. These are
enlarged endometrial stromal cells, which resemble epithelium.
Decidualization includes the process of differentiation of the spindle-shape stromal
fibroblasts into the plump secretory decidual cells, which create a pericellular
extracellular matrix rich in fibronectin and laminin (similar to epithelial cells).
Vascularity, as well as vascular permeability, is enhanced in the decidualizing
endometrium.
Its leukocyte population is distinct, with the presence of large endometrial granular
leukocytes being predominant, while polynuclear leukocytes and B cells are scant. The
large granular lymphocytes (CD56 bright) are called "uterine NK cells" or "UNK cell in
human.

5. Development-
After ovulation in placental mammals, the endometrial lining becomes hypertrophic
and vascular under the influence of Estrogens and progesterone.
In animals exhibiting hemochorial placentation, the endometrium undergoes
decidualization following implantation. If implantation does not occur, the secretory
lining will be absorbed estrous cycle or shed (menstrual cycle).
The decidua is shed with the placenta during parturition.

7. Types of Placenta
A. BEHAVIOR DURING PARTURITION-
1.Deciduate type Placenta: The allantochorianic villi penetrate into uterine villi. They are
intimately fused. Hence at the time of birth, the uterus is damaged. Bleeding occurs, the
utrine wall enters into formation of placenta is called deciduas.
2. Non deciduate type placenta: The chorianic villi are simple projections, they lie in
contact with uterus. They have a loose contact. There is no fusion. At the time of birth of
embryo uterus is not damaged.

8. B. ON THE BASIS OF DISTRIBUTION OF VILLI:- According to the distribution of villi five
kinds of placenta are seen.
1.Diffused type placenta: The villi are uniformly distributed on the surface of blastocyst,
except at the extreme ends. Ex: Horse, pig.
2.Cotyledonary placenta: The villi are arranged in groups. Each group is called cotyledon.
Each cotyledon fits into caruncles for uterus. Ex: Sheep, Cow, Deer.
3. Intermediate type Placenta: It is a rare type, it shows free villi on cotyledons. Hence it
is called intermediate type placenta. Ex: Giraffe
In these three types of placenta during perturition the foetus will not damage uterus.
4. Zonary placenta: The placenta takes the form of a complete or incomplete band of
tissue surrounding the fetus.. Ex: Cat, Dog, Carnivores.
5. Discoid type placenta : A single placenta is formed and is discoid in shape. When the
embryo Is growing It move saway from uterus hence the with look like a disc. Ex: Rabbit

9. C. BASING ON HISTOLOGY CONNECTION: According to number of layers of cells present
between foetus and uterus blood supply the placenta Is classified into five types.
a) Epithelio chorion placenta: The foetal chorion Is In contact with epithelium of the
uterus hence it is called epithello chorion placenta. In between foetal, maternal parts six
layers are present. Ex: Pig, Horse
• Endothelium of mother blood vessel.
• Maternal syndesmose connective tissue.
• Epitheliurn of mother
• Chorion of foetus.
• Foetus connective tissue (syndesmose)
• Endothellum of foetal blood vessel.
If all the six layers are present the placenta is called epithellochorlal placenta.

10. b) Syndeumose chorial placenta:
The allanto-chorianic with will pierce into the uterus of the mother, the chorion will come
in contact with syndesmose of mother’s uterus. Hence it is called syndesmose chorial.
Ex: Sheep, Cow.
c) Endotheliochorial placenta: The chàrion of the foetus will come in contact with the
endotheli of mother ‘s uterus, hence it is called endothelio-chorial placenta. Ex: Dog,
Carnivores.
d) Hemo-chorial placenta: The placental connections are more intimate. The chorion of
foetus will float In the blood pools of mother’s uterus. Hence It Is called haemochorIal
placenta. Ex: Bat, Man, Primates
e) Hemo-endothelial placenta: Hence guinea-pig will float In mother’s blood. Hence it
called hemo endothelial placenta. Ex: Rat, Rabbit,

11. D. LAYER THAT FROM YOLK SAC
a) Yolk sac placenta- consisting of the vascular embryonic yolk sac wall intimately
associated with the vascular maternal uterine or oviducal wall, and serving to nourish
the embryo. Ex. Metatheria( Marsupials)
b) Allantois Placenta -the allantois is intimately associated with the chorion,
contributing blood vessels to that structure as it forms—in conjunction with the
endometrium. Ex. Eutharian (Apis)
c) Chorionic Placenta- The fetal part of the placenta is known as the chorion. The
maternal component of the placenta is known as the decidua basalis. Oxygen and
nutrients in the maternal blood in the intervillous spaces diffuse through the walls of
the villi and enter the fetal capillaries. Ex. Primates

12. FUNCTIONS OF PLACENTA:
1.Placenta will form a physiological barrier between mother and foetus. It will possess
foetal and maternal blood mixing.
2.Placenta allows the diffusion of monosaccharaides, amino adds, hormones, vitamins,
oxygen, .carbon dioxide, water and other waste materials, because of this it supplies food,
oxygen to foetus.
3.It works as an excretory organ of foetus. It releases the nitrogenous waste materials Into
mother blood.
4.It works as an endocrine gland. It will secretes lactogen ,progesterone,etc. hormones.
5.The placenta will manufacture fructose from glucose.

13. References-
Modern text book – R.L.Kotpal
Jantu Vigyan- S.M. Sexsena
Jantu Vigyan- Dr. H.N. Baijal