Placenta - Structure and Function

Subhajit Chanda


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The placenta is a membranous part of the fetal body attached with the mother's uterus that provides mainly Oxygen and nutritional supplements.

Shape - Discoid

Diameter - 15 to 20 cm

Thickness - About 3 cm

Surface - Maternal and Fetal

What is Placenta?


The placenta is a fetomaternal organ that develops in the uterus during pregnancy especially help in transport system between the mother and the fetus.

a picture of a placenta

Fetal surface:

The fetal surface is 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 as they radiate from the insertion of the cord.

Maternal surface:

The maternal surface is rough and spongy. Maternal blood gives it a dull red colour. The maternal surface is mapped out into 15–20 somewhat convex polygonal areas known as lobes or cotyledons.


The placenta is usually attached to the upper part of the body of the uterus encroaching to the fundus adjacent to the anterior or posterior wall with equal frequency.


Placenta separates after the birth of the baby and the line of separation is through the decidua spongiosum.


The placenta consists of two plates. The chorionic plate lies internally. It is lined by the amniotic membrane. The umbilical cord is attached to this plate. The basal plate lies to the maternal aspect. Between the two plates lies the intervillous space containing the stem villi with their branches, the space being filled with maternal blood

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

CHORIONIC PLATE: From within outward, 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.

BASAL PLATE: It consists of the following structures from outside inwards.

(1) Part of the compact and spongy layer of the decidua basalis; 

(2) Nitabuch’s layer of fibrinoid degeneration of the outer syncytiotrophoblast at the junction of the cytotrophoblastic shell and decidua;

(3) Cytotrophoblastic shell;

(4) Syncytiotrophoblast.

The basal plate is perforated by the spiral branches of the uterine vessels through which the maternal blood flows into the intervillous space. At places, placental or decidual septa project from the basal plate into the intervillous space but fail to reach the chorionic 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.

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 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 the 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 villus. These major stem villi pass down through the 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 (total 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 square meters and 14 square meters. The fetal capillary 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 the 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 structures 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 placenta 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 sparse. 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 major histocompatibility complex (MHC) molecules.

What is the function of placenta?

Placental Function:

The main functions of the placenta are:

1. Transfer of nutrients and waste products between the mother and fetus. In this respect, it attributes to the following functions:

• Respiratory: The umbilical cord is attached to the placenta. The umbilical cord consists of one vein and two arteries. Through the umbilical vein, O2 added blood goes to the fetus and through the umbilical arteries CO2 rich blood passes to the mother.

• Excretory: Through umbilical arteries the fetal waste products are excrete out.

• Nutritive: Through umbilical vail all the nutrients are passed from mother to baby.

2. Endocrine function: Placenta is an endocrine gland. It produces both steroid and peptide hormones to maintain pregnancy.

3. Barrier function: The placenta acts as a barrier, it prevents all the micro-organisms from passing maternal body to the fetal body.

4. Immunological function: Placenta helps to pass maternal immunoglobulins from mother to baby that create passive immunity .

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