Placenta

Overview of the Placenta

The human placenta is discoid, because of its shape; hemochorial, because of direct contact of the chorion with the maternal blood and deciduate, because some maternal tissue is shed at parturition.

The placenta attached to the uterine wall and establishes a connection between the mother and the fetus through the umbilical cord.

Development of Placenta

Formation and Components

The placenta is developed from two sources:

• Fetal component which develops from the Trophoblast.
• Maternal component consists of decidua basalis.

As fertilization of the ovum occurs and a zygote is formed, this cleavage continues till it is 16 cell staged and is called as Morula. The cells of the outer layer give rise to a structure called as the trophoblast. The trophoblast differentiates 7-9 days after fertilization into cytotrophoblast and syncytiotrophoblast. The trophoblast gives rise to the Amnion, Chorion, and the fetal side of the placenta.

The syncytiotrophoblast grows into the endometrium (Decidua). The maternal blood vessels are opened up and blood from them fills the lacunar space. So, lacunae have maternal blood. Each trabecula is, initially made up entirely of syncytiotrophoblast. Later, cells of cytotrophoblast begin to multiply and grow into each trabecula.

The Placenta at Term

The placenta, at term, is almost a circular disk with a diameter of 15–20 cm and a thickness of 3 cm at its center.

It weighs 500 gm.

It has two surfaces:

1. Maternal surface: is rough and spongy.
2. Fetal surface: is covered by the smooth and glistening amnion with the umbilical cord attached at or near its center. At term, about four-fifths of the placenta is of fetal origin.

• A cotyledon or lobule is the functional unit of the placenta.
• Only the decidua basalis and the blood in the intervillous space are of maternal origin.

Placental Circulation

Types of Circulation

• Uteroplacental circulation: It is concerned with the circulation of the maternal blood through the intervillous space.
• Fetoplacental circulation: The two umbilical arteries carry the blood from the fetus to the placenta. Fetal blood circulation is around 400ml per min.

Types of Placenta

Umbilical Cord

The range is between 30-100 cm.

It has 2 arteries and one vein. The two arteries are smaller in diameter than the veins.

The extracellular matrix, which is specialized connective tissue, consists of Wharton’s Jelly.

Fetal Membrane

• Chorion (outer layer)
• Amnion (inner layer)

Function of Placenta

• Transfer of nutrient and waste products.
• Enzymatic function.
• Barrier function.
• Immunological function.
• Storage.
• Endocrine function.