Placenta Structure and Function

Placenta Structure and Function

• The placenta serves as a crucial organ in pregnancy, facilitating the exchange of nutrients, gases, and waste products between the mother and the developing fetus. It connects to the fetus via the umbilical cord, a flexible structure that serves as a lifeline, providing all necessary substances required for fetal development.
• Fetal blood flows through the umbilical arteries into the placenta for material exchange. These arteries carry deoxygenated blood from the fetus to the placenta, where it releases carbon dioxide and metabolic waste while simultaneously taking up oxygen and nutrients from the maternal circulation.
• Oxygenated, nutrient-rich blood returns to the fetus through the umbilical veins. This venous blood is essential for supporting the growth and development of fetal tissues and organs, ensuring the fetus receives the oxygen and nourishment needed for proper development.
• The lecture focuses on the cross-section of the placenta to understand its structure and function in detail. This cross-sectional analysis enables a comprehensive view of the intricate organization of the cellular layers and vascular components essential for placental tasks, including its role as a barrier and mediator in fetal-maternal interactions.

Placental Villi and Lacunae

• The placenta is composed of numerous villi surrounded by syncytiotrophoblasts. These villi extend into the maternal tissue, maximizing the surface area for nutrient exchange. The syncytiotrophoblasts are the outer layer of the placenta and play a significant role in interfacing with maternal blood.
• Syncytiotrophoblasts have small spaces called lacunae, which form an interconnected network where maternal blood can fill and flow. These lacunae allow for efficient dynamic communication between maternal and fetal circulations.
• The structure is embedded in the maternal endometrium, specifically the decidua basalis, which is the modified inner lining of the uterus that provides a rich environment for placental implantation and growth. The decidua basalis is instrumental in nurturing the placenta during pregnancy.
• Invasive syncytiotrophoblasts invade maternal blood vessels, including spiral arteries. This invasive capacity is critical as it enables the remodeling of these vessels, allowing for increased blood flow which is essential for adequate fetal nourishment.
• Blood from spiral arteries flows into the lacunae, filling them with maternal blood. This flow creates a space where nutrient exchange can occur, highlighting the importance of the vascular architecture of the placenta.

Trophoblastic Shell and Anchoring Villi

• Cytotrophoblasts undergo proliferation upwards to form an outer cytotrophoblastic shell at the fetomaternal interface. This outer layer is important for maintaining the structural integrity of the placenta.
• Functions of the outer cytotrophoblastic shell:
  • Anchors the fetal part to the maternal part, ensuring that the placenta remains securely attached to the uterine wall throughout gestation.
  • Interconnects fetal and maternal parts of the placenta, allowing for a seamless exchange system that is vital for fetal health.
  • Prevents disruption of the structure of tertiary villi, which are necessary for efficient nutrient absorption and waste elimination.
• Anchoring villi connect to the maternal endometrium via the trophoblastic column, creating a firm attachment that sustains the weight and demands of the growing placenta.
• Free villi (absorbing villi) absorb nutrition from the maternal blood. They play a crucial role in ensuring that the developing fetus receives adequate nutrients necessary for its growth and metabolic needs.

Intervillous Spaces and Cotyledons

• Syncytiotrophoblasts degenerate between lacunae, widening the spaces. This degeneration is part of the normal developmental process of the placenta, creating a more effective environment for nutrient exchange.
• Lacunae are converted into intervillous spaces, which are essential for the effective circulation of maternal blood around the villi, facilitating the processes of gas and nutrient exchange.
• Maternal blood flows into intervillous spaces for material exchange, allowing for direct contact between the maternal blood supply and the chorionic villi, which are rich in fetal capillaries.
• Blood returns to the maternal circulation via endometrial veins. This return is crucial for maintaining maternal health and ensuring the removal of metabolic waste products from the maternal system.
• The decidua basalis forms finger-like processes that divide the placenta into incomplete divisions called cotyledons. These cotyledons are functional units of the placenta, which maximize the surface area available for maternal-fetal nutrient exchange.
• A typical placenta has 15-20 cotyledons, giving it a distinct cobblestone appearance. The structure not only provides stability but also organizes the placental architecture that supports efficient blood flow and nutrient transfer.
• Each cotyledon consists of two or more branched tertiary villi that enhance the absorptive capacity of the placenta by allowing multiple sites of exchange at a micro-level.

Material Exchange in Placenta

• Maternal blood enters the intervillous spaces, while fetal blood flows into the villous arteries. This dual flow creates a counter-current exchange system that maximizes the efficiency of nutrient and gas transfer.
• Oxygen, glucose, and other essential substances flow from maternal blood in intervillous spaces to fetal blood in villous arteries. This transfer is critical for fetal development, as these nutrients fuel cellular activities and growth processes.
• Waste materials move from fetal blood into the maternal blood. The ability of the placenta to effectively transport metabolic waste back to the mother helps to maintain a favorable environment for the fetus.

Placental Membrane Layers

• Substances must cross multiple layers to move between maternal and fetal blood:
  • Syncytiotrophoblast layer, which serves as the first barrier and actively facilitates transport processes.
  • Cytotrophoblast layer, which offers additional structural support and plays a role in cell signaling during the exchange process.
  • Mesenchymal core, which aids in immune modulation and protects the developing fetus from maternal immune responses.
  • Endothelium of fetal blood vessel that line the capillaries within the villi, facilitating the final steps of nutrient uptake.
• These layers form the placental membrane, which is critical both structurally and functionally in mediating the complex interactions between maternal and fetal blood supplies.

Changes for Efficiency in Placental Membrane

• The placental membrane thins out for efficient exchange as gestation progresses:
  • Thick syncytiotrophoblast becomes a thin film, optimizing the exchange process by reducing the distance for diffusion.
  • Syncytial knots (aggregates of nuclei) may burn off and enter maternal circulation, enabling dynamic adjustments to placental efficiency.
  • Cytotrophoblasts are lost in many places, further facilitating direct contact between maternal blood and syncytiotrophoblasts.
  • Mesenchymal core is attenuated, which reduces barriers to diffusion and enhances exchange efficiency.
  • Endothelium of fetal blood vessels directly contacts the syncytiotrophoblast layer to maximize nutrient transfer.
• Vascular syncytial membranes are parts of the placental membrane where endothelium of fetal blood vessels directly contacts syncytiotrophoblasts, serving as primary sites for material exchange. These specialized sites enhance the placenta's ability to function as an efficient gas and nutrient exchange organ.

Endovascular Invasion by Cytotrophoblasts

• Cytotrophoblast cells proliferate and invade maternal blood vessels, specifically spiral arteries, as part of the implantation process. This invasion is crucial for establishing an adequate blood supply for the growing fetus.
• They undergo an epithelium-to-endothelial transition, meaning they change their cellular properties to integrate with the maternal vessel endothelium.
• Remodeling results in larger caliber vessels with less resistance to blood flow, optimizing blood perfusion and ensuring an adequate supply of maternal blood to the placenta.
• This process improves the blood flow into the intervillous spaces, mitigating fetal hypoxia, which can occur if blood flow is insufficient.
• Improper remodeling can lead to insufficient blood flow to the intervillous spaces, potentially causing fetal hypoxia, which can have serious consequences for fetal development and health.

Preeclampsia and Eclampsia

• Fetal hypoxia can cause the fetus to release chemicals (e.g., sFlt-1) that increase maternal blood pressure as a compensatory mechanism in response to perceived nutrient deficits.
• Increased maternal blood pressure and damage to the glomeruli by the released chemicals leads to proteinuria, an indicator of kidney stress and dysfunction.
• Preeclampsia is characterized by fetal hypoxia along with increased maternal blood pressure and proteinuria. It poses significant risks to both maternal and fetal health and can necessitate immediate medical intervention.
• Eclampsia refers to the progression of preeclampsia with seizures or central nervous system involvement, representing a severe complication that can result in life-threatening conditions for both the mother and the fetus.

Macroscopic Placental Structure

• The maternal surface of the placenta (maternal plate) is formed by the decidua basalis and exhibits a cobblestone appearance due to the presence of cotyledons. This structure reflects the organization necessary for effective blood supply and nutrient distribution.
• The fetal part of the placenta (chorionic plate) is smooth and exhibits a pinkish-blue color due to the overlying amnion, formed by the amniochorion or chorionic frondosum. This part is vital for containing the fetal blood supply.
• Umbilical arteries carry deoxygenated blood from the fetus to the placenta, where it undergoes crucial gas and nutrient exchanges necessary for maintaining fetal vitality.
• Chorionic blood vessels branch from the umbilical vessels and enter the villi, supporting the overall vascular architecture of the placenta and ensuring efficient function.

Placental Cross Section

• Chorionic blood vessels divide into villous branches that enter tertiary branch villi, facilitating the distribution of blood across the entire placenta.
• Placental septa incompletely divide the placenta into 15-25 cotyledons, which serve as distinct functional units that support independent vascular and nutrient exchange modules.
• Each cotyledon has two or more branched villi, which maximize the contact area between fetal and maternal blood supplies and ensure effective exchange.
• Maternal blood in intervillous spaces directly contacts the chorionic villi, creating an environment where nutrient transfer is optimized for fetal nutrition.
• A hemochorial placenta is a type of placenta in which maternal blood directly contacts the villi (as seen in humans). This unique feature of the hemochorial placenta enhances the efficiency of material transfer between mother and fetus.
• The outer trophoblastic or outer cytotrophoblastic shell covers the placental septa, providing structural integrity and protecting the internal vascular structures, which are essential for maintaining the placenta's overall functionality.