The placenta forms from the fusion of fetal and maternal tissues to nourish the developing embryo. It is composed of the chorion, which contains villi from the fetus, and the decidua, which are tissues from the maternal uterus. The placenta allows nutrients, waste, and gases to pass between mother and fetus. It is found in nearly all mammals and some other viviparous species, and plays a vital role in sustaining pregnancy.

1. Placenta
Tuhar Mukherjee

2. Definition
The organ by which embryos of viviparous species are nourished and waste products
removed, formed by the intimate apposition or fusion of extra embryonic membrane with
the uterine endometrium.
Occurrence
Almost exclusively found in mammals except Monotremes
Also in Peripatus, Salpa, Mutelus
Time
Implantation of the blastocyst in maternal uterine wall
Tissues participating
Maternal tissue: luminal epithelia or mucosa layer of maternal uterus/ uterine
endometrium
Fetal tissue: extra embryonic membranes (Chiefly chorion)

4. Types of Placenta basis
• Based on fate of uterine endometrium during
parturition (2 types)
• Shape of placenta (5 types)
• Involvement of chorionic regions (2 types)
• Histological arrangement of tissue layer (5
types)

5. Involvement of chorionic region
• Chorio-vitelline: Uterine wall and that part of
the chorion which is lined on the inside by the
yolk sac with its network of vitelline blood
vessels. (Macropus)
• Chorioallantoic: Uterine wall and that part of
the chorion that is lined by the allantois with
the allantoic blood vessels. (Eutherian
mammals, Parameles, Dasyurus)

6. Shedding of uterine endometrium
• Non-deciduous: The villi of chorion and maternal
uterine wall lie close together. The chorionic villi
are simply drawn out from the crypts in the
uterine wall at the time of parturition. No
damage to the uterine wall (Marsupials, Horses,
cattle)
• Deciduous: The union of maternal and fetal tissue
is so intimate that at the time of parturition the
fetal component along with a the part of
maternal tissue is torn away. Uterine wall is called
decidua. (Homo)

7. Structure and arrangement of villi
• Diffuse: villi scattered all over the chorion (Pig,
horse and camel)
• Cotyledonary: group or patches of villi called
cotyledon. (Cattle)
• Zonary: villi developed in the form of a belt or
band around the middle of the blastocyst which is
more or less elliptical (Dog, cat)
• Discoidal: villi are restricted to a small disc
shaped area on the blastocyst (Human, Bat, Bear,
Anthropoid apes)
• Bidiscoidal: two disc like villi (Monkey)

8. Histological arrangement
• Epitheliochorial
• Syndesmochorial
• Endotheliochorial
• Haemochorial
• Haemoendothelial

9. Formation of placenta
• Mammalian embryo do not depend on yolk for
development
• Maternal modifications are expansion of oviducts to form
uterus
• Fetus capable of deriving nutrients from the mother
• The fetal chorion is derived from the embryonic
trophoblast cells (FIG) and mesodermal cells derived from
the inner cell mass
• Chorion forms the fetal part of the placenta
• Chorion induces the uterine cells to form maternal portions
of placenta, decidua
• Decidua becomes vascularized

10. Blastocyst
Inner cell mass
Trophoblast
Epiblast
Hypoblast
Embryonic epiblast
Amnionic ectoderm
Extra embryonic
endoderm
Cytotrophoblast
Embryonic ectoderm
Primitive streak
Embryonic endoderm
Embryonic mesoderm
Yolk sac
Syncytiotrophoblast
Extra embryonic
mesoderm

11. • Inner cell mass cells expressing Nanog becomes
epiblast
• Inner cell mass cells expressing Gata6 becomes
hypoblast
• Epiblast cells is split by small clefts that eventually
coalesce to form the embryonic epiblast from
other epiblast cells that line the amniotic cavity
• Amnionic fluid acts as shock absorber and
prevents desiccation of the embryo

13. • Extra embryonic membrane joins the
trophoblastic extension and give rise to the
blood vessels that carries nutrition
• Narrow connective stalk of extra embryonic
mesoderm that gives rise to umbilical chord
connects the embryo to trophoblast

14. • Epiblast: outer layer of blastula; following gastrulation gives rise to ectoderm
• Hypoblast: The inner layer of the thickened margin of the epibolizing blastoderm
in the gastrulating zebra fish embryo or the lower layer of the bilaminar embryonic
blastoderm in birds and mammals. The hypoblast in fish(but not in birds and
mammals) contains the precursors of the endoderm and mesoderm.
• Trophoblast: The external cells of the early mammalian embryo (i.e., the morula
and the blastocyst) that bind to the uterus. Trophoblast cells form the chorion (the
embryonic portion of the placenta). Also called trophectoderm.
• Syncytiotrophoblast: A population of cells from the murine and primate
trophoblast that undergoes mitosis with-out cytokinesis resulting in multinucleate
cells. The syncytiotrophoblast tissue is thought to further the progression of the
embryo into the uterine wall by digesting uterine tissue.
• Cytotrophoblast: Mammalian extra-embryonic epithelium composed of the
original trophoblast cells, it adheres to the endometrium through adhesion
molecules and, in species with invasive placentation such as the mouse and
human, secretes proteolytic enzymes that enables the Cytotrophoblast to enter
the uterine wall

15. Why no antibody against fetus

16. Placenta bank
• Storage of Amnion and placental cells
• Rich source of stem cells
• Used for burn, diabetic ulcers
• Trials for brain injury, arthritis and
cardiovascular diseases.

17. Function
• Nutrition
• Anchorage
• Protection
• Respiration
• Excretion
• Human Chorionic Gonadotropic hormone