THE PLACENTA

PRESENTED BY: MS. MUNEERA MAKRANI

 PLACENTA

• The human placenta is discoid, because of its shape;

haemochorial, because of direct contact of the chorion with
maternal blood and decidua, because some maternal tissue is
shed at parturition.

• The placenta is attached to uterine wall and establishes

connection between mother and fetus through the umbilical cord.
➢DEVELOPMENT:
• COMPONENT: It is developed from two components/sources-
oFetal component- Principal component which develops
from the chorion frondosum,

oMaternal component- Consists of decidua basalis.

• When the Interstitial implantation is completed on 11th day, the
blastocyst is surrounded on all sides by lacunar spaces around
cords of syncytial cells, called trabeculae.

• From the trabeculae develops the stem villi on 13th day which
connect the chorionic plate with the basal plate.

• Primary, secondary and tertiary villi are developed from the stem
villi.
• Arterio-capillary-venous system in the mesenchymal core of
each villus is completed on 21th day.
• Ultimately makes connection with intraembryonic vascular
system through the body stalk.
• Simultaneously, lacunar spaces become confluent with one
another and by 3rd-4th week , form a multilocular receptacle
lined by the syncytium and filled with maternal blood.
• This space becomes the future intervillous space.
• As the grow of embryo proceeds, decidua capsularis becomes
thinner beginning at 6th week and both the villi and lacunar
spaces in abembryonic area get obliterated, converting the
chorion into chorion leave.
• This is, however, compensated by-
a. Exuberant growth and proliferation of the decidua basalis and
b. Enormous and exuberant division and sub-division of chorionic
villi in embryonic pole (chorion frondosum).
• These two i.e. chorion frondosum and the decidua basalis form
the discrete placenta. It begins at 6th week and is completed by
12th week.

• Until the end of the 16th week, the placenta grows both in
thickness and circumference due to growth of chorionic villi with
expansion of intervillous space.
• Subsequently, there is little increase in thickness but it increases
circumferentially till term.
• The human haemochorial placenta derived its name from haemo
(blood) that is in contact with the syncytiotrophoblast of chorionic
tissue.

THE PLACENTA AT TERM

➢GROSS ANATOMY: The placenta, at term, is a most a circular
disc with a diameter of 15-20 cm and thickness of about 3 cm at
its centre.Its thins off towards the edge.
• It feels spongy and weighs about 500 gm, the proportion to the
weight of the baby being roughly 1:6 at term and occupies about 30%
of the uterine wall.

➢SURFACE: It presents two surface-

1. Fetal surface and
2. Maternal surface, and a peripheral margin.

1. Fetal surface: The fetal surface is covered by the smooth and

glistering amnion with the umbilical cord attached at or near its centre.
• Branches of umbilical vessels are visible beneath the amnion as they
radiate from the insertion of the cord.
• The amnion can be peeled off from underlying chorion except at
insertion of the cord.
• At term, about 4-5th of placenta is of fetal origin.

2. Maternal Surface: The maternal surface is rough and spongy.

• Maternal blood gives it a dull red colour.
• A thin greyish, somewhat shaggy layer which is the remnant of the
decidua basalis (compact and spongy layer) and has come away with
the placenta, may be visible.
• The maternal surface is mapped out into 15-20 somewhat convex
polygonal areas known as lobes or cotyledons which are limited by
fissures.
• Each fissure is occupied by decidual septum which is derived from
basal plate.
• Numerous small greyish spots are visible. These are due to deposition
of calcium in degenerated areas and are of no clinical significance.

• The maternal portion of placenta amounts to less than 1/5th of total

placenta.

• Only the decidua basalis and the blood in intervillous space are of
maternal origin.
➢MARGIN:
• Peripheral margin of placenta is limited by fused basal and chorionic
plates and is continuous with the chorion leave and amnion.
• Essentially, the chorion and the placenta are one structure but the
placenta is a specialized part of the chorion.

➢ATTACHMENT:
• The placenta is usually attached to the upper part of body of the
uterus encroaching to fundus adjacent to anterior or posterior wall
with equal frequency.
• The attachment to uterine wall is effective due to anchoring villi
connecting chorionic plate with the basal plate and also by fused
decidua capsularis and vera with chorion leave at margin.

➢SEPARATION:
• Placenta separates after the birth of the baby and the line of
separation is through the decidua spongiosum.
 STRUCTURE
➢PLATE: The placenta consists of two plate-
• The chorionic plate
• The basal plate

1 The chorionic plate:

• lies internally. It is lined by amniotic membrane. The umbilical cord is
attached to this plate.
• From within outwards, it consists of-
i. Primitive mesenchymal tissue containing branches of umbilical
vessels.
ii. A layer of cytotrophoblast and
iii. Syncytiotrophoblast.
• The stem villi arise from the plate.
• It forms the inner boundary of the choriodecidual space.
2 The Basal plate:
• Lies to maternal aspect.
• Between the two plates lies intervillous space containing the stem
villi with their branches, the space being filled with maternal blood.
• It consists of the following structure from outside inwards.
i. Part of the compact and spongy layer of the decidual basalis.
ii. Nitabuch’s layer of fibrinoid degeneration of the outer
syncytiotrophoblast at the junction of cytotrophoblast shell and
decidua.
iii. Cytotrophoblast shell.
iv. Syncytiotrophoblast

• The basal plate is perforated by spiral branches of uterine vessels

through which the maternal blood flows into intervillous space.
• At places, placental or decidua septa project from the basal plate into
the intervillous space but fail to reach the chorion plate.
• The septum consists of decidual elements covered by trophoblastic
cells.
• The areas between the septa are known as cotyledons (lobes), which
are observed from the maternal surface, numbering 15-20.

➢AMNIOTIC MEMBRANE:
• It consists of single layer of cubical epithelium loosely attached to the
adjacent chorionic plate. It takes no part in formation of placenta.

• The basal plate is perforated by spiral branches of uterine vessels

through which the maternal blood flows into intervillous space.
➢INTERVILLOUS SPACE:
• It is bounded on the inner side by the chorionic plate and the outer
side by the basal plate, limited on the periphery by the fusion of the
two plates.
• It is lined internally on the all sides by the syncytiotrophoblast and is
filled with slow flowing maternal blood.
• Numerous branching villi which arise from the stem villi project into
the space and constitute chief content of intervillous space.
➢STEM VILLI:
➢These arise from the chorionic plate and extends to the basal plate.
• With the progressive development-Primary, secondary and tertiary
villi are formed.
• Functional unit of the placenta is called a fetal cotyledon or
placentome, which is derived from a major primary stem villi.
• These major stem villi pass down through intervillous space to
anchor onto the basal plate.
• Functional subunit is called a lobule which is derived from a tertiary
stem villi.
• About 60 stem villi persist in human placenta.
• Thus, each cotyledon (totaling 15-29) contains 3-4 major stem villi.
• The villi are the functional unit of the placenta.
• The total villi surface, for exchange, approximately varies between
10-14 square meters.
• The fetal capillaries system within the villi is almost 50 km long.
• Thus, while some of the villi are anchoring the placenta to the
decidua, the majority are free within intervillous space and are called
nutritive villi.
• Blood vessels within the branching villi do not anastomose with the
neighboring one.
➢STRUCTURE OF A TERMINAL VILLUS:
• In the early placenta, each terminal villus has got the following
structure from outside inward:
1 Outer syncytiotrophoblast
2 Cytotrophoblast
3 Basement membrane
4 Central stroma containing fetal capillaries, primitive mesenchymal
cells, connective tissue and a few phagocytic (Hofbauer) cells.
• In placental at term, syncytiotrophoblast becomes relatively thin at
places overlying the fetal capillaries and thicker at other areas
containing extensive endoplasmic reticulum.

• The former is probably the site for transfer and the latter, the site for
synthesis.
• The cytotrophoblast is relatively spare.
• Basement membrane becomes thicker.
• Stroma contains dilated vessels along with all the constituents and
few Hofbauer cells.
• Hofbauer cells are round cells that are capable of phagocytosis and
can trap maternal antibodies crossing through the placenta (immune
suppressive).
• These cells can express class II MHC molecules.