The placenta develops from the chorion frondosum on the fetal side and the decidua basalis on the maternal side. During implantation, the trophoblast invades the endometrium and forms the decidua. Primary chorionic villi then develop from the trophoblast and invade the decidua basalis. Secondary villi develop when mesoderm invades the primary villi. Tertiary villi form when blood vessels develop in the mesoderm. By the end of the fourth week, the placenta has formed and a complex vascular network facilitates nutrient and gas exchange between the mother and fetus. The placenta continues to develop throughout pregnancy.

1. Formation and Structure of
Placenta
Presenter:
Dr. Anju B. Uppin
Assistant Professor
Department of Rachana Sharira
KAHER’s Shri BMK
Ayurveda Mahavidyalaya
Belagavi, Karnataka

2. Definition of Placenta
The placenta is the organ that facilitates
nutrient and exchange of gases between the
maternal and foetal compartments.

3. Implantation :
It is a process by which the embryo is embedded and fixed with
the endometrium of the uterus.
Certain Changes takes place:
 At the time of implantation, the embryo is in the form of
blastocyst. It usually occurs during 6–10 days after ovulation.
 The intimate contact of fetal and maternal tissues occurs by
implantation of the embryo.
 The blastocyst surrounded by zona pellucida enters the uterus
on the sixth day. The zona pellucida pre-vents it from sticking
to the wall of the uterus.
 As the blastocyst enlarges, the zona pellucida cover-ing it
becomes stretched and ultimately disappears. Consequently the
trophoblast is exposed.

4.  The trophoblast has the property of attaching itself to
any tissue with which it comes in contact with.
 The trophoblast sticks to the uterine endometrium.
The cells of trophoblast divide mitotically and form
new cells that lose their cell membranes and form a
mass of cells called syncytiotrophoblast.
 The syncy-tiotrophoblast invades the endometrium
with the help of proteolytic enzymes secreted by its
cells.
 The blastocyst goes deeper and deeper until it com-
pletely lies within the endometrium (interstitial
implantation).

5. Normal Site of Implantation
Normally the blastocyst is implanted in the upper
part of the posterior wall of the uterine cavity.

6. 8-12 DAYS
6TH DAY

7. Decidua basalis
Amnion
Chorion
Decidua parietalis
Extra embryonic coelom
(Chorionic cavity)
Uterine cavity
Myometrium
Decidua capsularis
Amniotic cavity INTERSTITIAL
IMPLANTATION
11th day

8. Decidua
 After implantation of the blastocyst, the functional layer
of the endometrium of the uterus is termed Decidua.
 The endometrium of the uterus is in the secretory phase
of menstruation at the time of implantation.
 As the embryo is implanted the syncytiotrophoblast
starts secreting human chorionic gonadotrophin (HCG)
hormone, which intensifies the changes that occur in the
secretary phase, viz., stromal cells enlarge, vacuo-late,
and get filled to the brim with glycogen and lipids.
 This change in the stromal cells is called Decidual
Reaction and changed character of the endometrium
after implantation of the ovum is now called Decidua.

9. Parts of Decidua
1. Decidua Basalis: It is that part of decidua which lies
deep to the embryo (developing blastocyst).This
contributes to the development of the placenta.
2. Decidua Capsularis: It is that part of decidua which
forms a capsule around the embryo and separates it
from the uterine cavity.
3. Decidua Parietalis: Rest of the decidua (excluding
decidua basalis and decidua capsularis) is termed
decidua parietalis.

10. Formation of Placenta

11. The placenta develops from two entirely different
sources.
 The fetal portion develops from chorion frondosum
and the maternal portion from the decidua basalis.
 By the end of third week, the anatomical
arrangements necessary for physiological exchange
between the mother and embryo are established.
 By the end of fourth week, a complex vascular
network is formed within the placenta, which
facilitates the exchange of gases, nutrients, and
metabolic waste products between the mother and
embryo.

17. 1. Formation of chorion frondosum and chorion laeve
(a) During the development of placenta small finger-like
projections arise from chorion (tro-phoblast and underlying
mesoderm) into the decidua.
(b) Initially chorionic villi are formed all around the chorionic sac.
(c) As the chorionic sac enlarges the chorionic villi in relation to
decidua capsularis get com-pressed and degenerate. As a result,
this part of chorion becomes smooth and is now called chorion
laeve.
(d) The chorionic villi in relation to decidua basa-lis grow
extensively into the decidua basalis. This part of the chorion is
called chorion frondosum

18. 2. Formation of trabeculae and intervillous spaces
(a) The cells of trophoblast (made up of single layer of
cells) proliferate, move on the surface, and losetheir
cell membranes to form a continuous sheetof
cytoplasm containing many nuclei. This sheet/layer
is called syncytiotrophoblast. Thus, trophoblast is
differentiated into two layers:
 A deep layer called cytotrophoblast and
 Asuperficial layer called syncytiotrophoblast.
(b) The syncytiotrophoblast grows rapidly and erodes
the decidua basalis, and becomes thick. The small
cavities (lacunae) appear in thickened
syncytiotrophoblast.

19. The syncytio-trophoblast continues to erode the decidua
basalis. The lacunae increase in size. They lie radially
around the developing embryo (blas-tocyst). They are
separated from each other by portions of
syncytiotrophoblast termed trabeculae (villous
primordia).
(c) As the syncytiotrophoblast further grows it erodes the
blood vessels of the endometrium and the blood enters
into the lacunae along with the secretions of the
endometrial glands. Later the lacunae communicate
with each other around the trabeculae and form the
intervillous spaces. Thus, each trabeculus gets
surrounded all around by the blood-filled space in the
endometrium.

23. Development of Villi
1. Primary villi: The cytotrophoblast forms finger-like projections
that invade the trabeculae in its center. This finger-like
projection of cytotropho-blast surrounded by a layer of
syncytiotrophoblast is called the primary villus.
2. Secondary villi: The extraembryonic somatopleu-ric mesoderm
lying deep to the cytotrophoblast now invades the center of each
villus.As a result, now each villus consists of three layers. From
inside to outside these are mesoderm, cytotrophoblast, and
syncytiotrophoblast. This villus is now termed secondary villus.
3. Tertiary villi: The blood vessels develop in the mesoderm of the
secondary villi. The secondary villus with blood vessels in its
mesoderm is called tertiary villus.

24. Decidua basalis
Trophoblast
Extraembryonic mesoderm
STAGES IN FORMATION OF CHORIONIC VILLI
Cytotrophoblast
Syncytiotrophoblast
Decidua basalis
Lacunae in
Syncytiotrophoblast
8th Day
9th Day

25. Trabeculae
Decidua basalis
Lacunar space
Lacunar space
Y
X
Transverse section along XY

27. Trabeculae Maternal blood
in Lacunar space
Uterine blood vessel
Transverse section along XY
X Y

28. Decidua basalis
Syncytiotrophoblast
Maternal blood in Lacunar space
Cytotrophoblast
X Y
Cytotrophoblast
Syncytiotrophoblast
PRIMA RY VILLUS
Maternal blood in Lacunar space
Transverse section along XY
13th Day

29. Decidua basalis
SECONDARY VILLUS
Syncytiotrophoblast
Cytotrophoblast
Extraembryonic mesoderm
Maternal blood in Lacunar space
X Y
Cytotrophoblast
Syncytiotrophoblast
Extraembryonic mesoderm
Transverse section along XY
16th Day

30. Decidua basalis
TERTIARY VILLUS
Maternal blood in Lacunar space
Capillary in extraembryonic mesoderm
Cytotrophoblast
Syncytiotrophoblast
Cytotrophoblast
Intervillous space
Syncytiotrophoblast
Capillary in extraembryonic mesoderm
X Y
Transverse section along XY
16th Day

31. Uterine blood vessel
Cytotrophoblastic shell
Syncytiotrophoblast
Maternal blood in Lacunar space
Capillary in villus
Fetal blood vessels in
extraembryonic mesoderm

32. Decidua basalis
Chorionic villi
Decidua capsularis
Uterine lumen
2nd month

33. Chorion frondosum
Chorion laevae

34. (Decidual plate)
2nd MONTH

35. Definitive Placenta
4th month
3rd MONTH

39. PLACENTA
MATERNALASPECT
PLACENTA FETAL
ASPECT

40. Extraembryonic
mesoderm
PLACENTAL MEMBRANE

41. ANOMALIES IN FORM OF PLACENTA
Bidiscoidal Placenta Lobed placenta
Diffuse placenta Placenta succenturiata
Fenestrated Placenta Circumvallate placenta

42. Thank you