Mammalian Placenta Structure & Function | UPSC Mains ZOOLOGY-PAPER-II 2022

Describe the structure and functions of mammalian placenta.

How to Approach

This question requires a detailed understanding of the mammalian placenta's structure and function. The answer should begin with a concise definition of the placenta and its role. Then, systematically describe its structural components (fetal and maternal parts) and their respective functions – including gas exchange, nutrient transport, waste removal, endocrine function, and protective role. A clear and organized presentation, potentially using bullet points or a table, will enhance clarity. Focus on the interplay between maternal and fetal tissues.

The placenta is a temporary physiological organ unique to eutherian mammals, connecting the developing fetus to the uterine wall of the mother. It facilitates the exchange of nutrients, gases, and waste products between the mother and the fetus, and also produces hormones crucial for maintaining pregnancy. Formed from both fetal (chorion) and maternal (endometrium) tissues, the placenta is essential for fetal development and survival. Its intricate structure is directly related to its multifaceted functions, ensuring a stable internal environment for the growing fetus. Understanding the placenta is fundamental to comprehending mammalian reproduction and potential complications during gestation.

Structural Components of the Mammalian Placenta

The mammalian placenta is composed of both fetal and maternal components, working in close coordination.

Fetal Component: Chorion

Chorionic Villi: These finger-like projections extend into the maternal blood spaces. They are the primary sites of nutrient and gas exchange.
Chorionic Plate: A flat disc from which the chorionic villi originate.
Umbilical Cord: Connects the fetus to the chorionic plate, containing umbilical arteries and veins for transporting blood.

Maternal Component: Decidua

Decidua Basalis: The part of the endometrium underlying the chorionic sac. It forms the maternal part of the placenta.
Intervillous Space: Spaces within the decidua basalis filled with maternal blood.
Spiral Arteries: Modified uterine arteries that supply blood to the intervillous space.

Functions of the Mammalian Placenta

The placenta performs a wide range of functions vital for fetal development.

1. Nutritional Support

The placenta transports nutrients like glucose, amino acids, fatty acids, vitamins, and minerals from the maternal blood to the fetal blood. This is achieved through facilitated diffusion, active transport, and endocytosis.

2. Respiratory Exchange

Oxygen diffuses from maternal blood to fetal blood across the placental barrier, while carbon dioxide diffuses in the opposite direction. This ensures the fetus receives oxygen and eliminates carbon dioxide.

3. Excretion of Waste Products

Fetal waste products, such as urea, uric acid, and creatinine, are transferred from the fetal blood to the maternal blood for excretion by the mother’s kidneys.

4. Endocrine Function

The placenta acts as a temporary endocrine organ, producing several hormones essential for maintaining pregnancy:

Human Chorionic Gonadotropin (hCG): Maintains the corpus luteum in early pregnancy.
Human Placental Lactogen (hPL): Promotes maternal glucose and lipid metabolism, providing nutrients to the fetus.
Estrogen & Progesterone: Maintain the uterine lining and prevent uterine contractions.

5. Immunological Protection

The placenta provides a partial immunological barrier, protecting the fetus from some maternal antibodies but allowing the transfer of IgG antibodies, providing passive immunity to the fetus. However, it doesn't block all harmful substances.

6. Barrier Function

While not absolute, the placenta acts as a selective barrier, preventing the passage of some harmful substances (like bacteria) while allowing essential nutrients and gases to pass through. However, viruses, certain drugs, and alcohol can cross the placental barrier.

Function	Mechanism
Nutrient Transport	Facilitated diffusion, active transport
Gas Exchange	Simple diffusion
Waste Removal	Diffusion
Hormone Production	Synthesis and secretion by placental cells

Additional Resources

Key Definitions

Placental Barrier

The placental barrier is the physiological barrier between the maternal and fetal circulations. It is formed by the layers of the chorionic villi and the syncytiotrophoblast, regulating the passage of substances between mother and fetus.

Syncytiotrophoblast

The outer layer of the chorionic villi, formed by the fusion of cytotrophoblast cells. It is responsible for hormone production and nutrient transport, and forms the primary interface between maternal and fetal circulations.

Key Statistics

Approximately 3-5% of pregnancies are affected by placental abnormalities, contributing to significant rates of preterm birth and stillbirth.

Source: WHO, 2023 (Knowledge Cutoff)

Placental insufficiency is estimated to contribute to approximately 25% of intrauterine growth restriction (IUGR) cases.

Source: National Institutes of Health (NIH), 2022 (Knowledge Cutoff)

Examples

Placenta Previa

Placenta previa is a condition where the placenta covers the cervix, potentially causing bleeding during pregnancy and requiring a Cesarean section for delivery. It exemplifies the importance of proper placental positioning for a safe pregnancy.

Frequently Asked Questions

▶What happens if the placental barrier is compromised?

If the placental barrier is compromised, harmful substances like viruses, drugs, and toxins can reach the fetus, potentially causing developmental abnormalities, infections, or even fetal death.