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Placenta
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The placenta is a temporary organ of pregnancy situated in the uterus. It is formed from fetal and maternal components. The fetal portion is formed by the chorion frondosum, while the maternal portion is formed by the decidua basalis. Moreover, the placenta is the meeting point of two circulatory systems: fetal circulation and maternal circulation.
The main function of the placenta is the interchange between the mother and the fetus. More specifically, it provides nutrition and oxygen to the fetus and removes waste material and carbon dioxide.
In this article, we will explore the anatomy and function of the placenta.
| Definition | Circular, discoid-shaped organ that develops in the uterus during pregnancy and allows metabolic exchange between mother and fetus |
| Portions |
Fetal portion: chorion frondosum Maternal portion: basal decidua |
| Surfaces |
Fetal surface (chorionic plate) with umbilical cord Maternal surface (basal plate) |
| Functions |
Fetal respiration, nutrition and excretion Fetal protection and immunity Endocrine (hormone production) |
Gross anatomy
The placenta is a discoid-shaped organ weighing about 450-500g at full term. The placental thickness is usually proportional to the gestational age. The placenta is normally located along the anterior or posterior wall of the uterus and may expand to the lateral wall with the course of the pregnancy.
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Structure
The placenta is composed of two different surfaces, the fetal surface (or chorionic plate) and the maternal surface (or basal plate).
Fetal surface of the placenta
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The fetal surface of the placenta (or chorionic plate) is covered by the amnion, or amniotic membrane, which gives this surface a shiny appearance. The amniotic membrane secretes amniotic fluid which serves as a protection and cushion for the fetus, while also facilitating exchanges between the mother and fetus.
Underlying the amnion is the chorion, a thicker membrane continuous with the lining of the uterine wall. The chorion contains the chorionic vessels which are continuous with the vessels of the umbilical cord. Originally, early in the development of the placenta, the entire chorionic plate is covered with chorionic villi. The villi located adjacent to the decidua capsularis (portion of the decidua that overlies the embryo) degenerate to produce the smooth (nonvillous) chorion laeve. The villi adjacent to the decidua basalis persist, increase in size and produce the chorion frondosum or fetal portion of the placenta. The chorionic villi of the fully developed placenta contain a network of fetal capillaries, allowing a maximal contact area with the maternal blood. The exchanges between the fetal and maternal circulation occurs in the intervillous space.
The umbilical cord, which is the connection between the placenta and the fetus, inserts in a slightly eccentric position into the chorionic plate. The umbilical cord contains one vein (the umbilical vein) that carries nutrients and oxygen from the placenta to the fetus and two arteries (the umbilical arteries) that carry waste products from the fetus back to the placenta.
Maternal surface of the placenta
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The maternal surface of the placenta, or basal plate, is an artificial surface, which emerges from the separation of the placenta from the uterine wall during delivery. This surface is composed of the decidua, the modified or specialized endometrium (or mucosal lining of the uterus) that forms in preparation for pregnancy. The decidua has several parts:
- Decidua basalis - forms the maternal portion of placenta;
- Decidua capsularis - overlies the embryo;
- Parietal decidua - the rest of the decidual tissue.
Also visible on the maternal surface of the placenta are slightly elevated regions called lobes or cotyledons (approximately 10 to 40), which are separated by grooves or sulci. Inside the placenta, the grooves correspond to the placental septa. Each lobe visible on the maternal surface corresponds to the position of the vilous trees arising from the chorionic plate.
This quiz helps you identify the structures of the placenta.
Functions
The placenta is a highly-specialized organ that plays an essential role during pregnancy. It is responsible for providing nutrition and oxygen to the fetus as well as removing waste material and carbon dioxide. It is also responsible for creating a separation between the maternal and fetal circulation (known as placental barrier). Besides that, the placenta protects the fetus from infections and other maternal disorders, while also helping in the development of the fetal immune system. Additionally, this organ has an endocrine function as it secretes hormones (such as human chorionic gonadotropin) that affect the pregnancy, metabolism, fetal growth, and parturition.
Clinical notes
Anatomical variations
Variant morphologies of the placenta are frequently encountered, such as is the case of a bilobed placenta where this organ is separated into two near equal-sized lobes. The estimated incidence of this specific variation is at up to ~4% of pregnancies. It can be associated with some complications such as first-trimester bleeding.
Placenta disorders
Besides the developmental abnormalities referred above, the placenta may also be affected by a number of medical conditions. An example of these disorders is a condition known as placenta previa, which is the implantation of the placenta over the cervical os. This condition usually presents as painless vaginal bleeding in the third trimester. In these cases, mother and fetus need careful monitoring and delivery is often by cesarean section.
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