Placenta Formation and Function

• The placenta is a temporary organ connecting the developing fetus to the uterine wall, facilitating nutrient uptake, waste elimination, and gas exchange via the mother's blood supply.

Placental Formation

• Placentation starts upon implantation of the blastocyst into the uterine endometrium.
• Trophoblast cells differentiate into cytotrophoblasts and syncytiotrophoblasts, invading the uterine wall.
• Syncytiotrophoblasts form the placenta's outer layer and directly contact maternal blood.
• Chorionic villi, which contain fetal blood vessels, extend into the intervillous space for exchange.
• Maternal blood fills the intervillous space, bathing the chorionic villi.

Placental Functions

• Oxygen and nutrients such as glucose, amino acids, fatty acids, and vitamins are provided to the fetus.
• Waste products like carbon dioxide, urea, and bilirubin are excreted from the fetus into the maternal circulation.
• Maternal antibodies (IgG) are transferred to the fetus, providing passive immunity.
• Hormones essential for maintaining pregnancy are produced.
• Acts as a selective barrier, preventing some harmful substances from crossing while allowing others.

Placental Hormones

• Human Chorionic Gonadotropin (hCG) is produced early to maintain the corpus luteum.
• Progesterone maintains the uterine lining and prevents uterine contractions.
• Estrogen (primarily estriol) stimulates uterine growth and prepares mammary glands for lactation.
• Human Placental Lactogen (hPL) affects maternal glucose and protein metabolism, supporting fetal growth.
• Placental growth factor (PlGF) and vascular endothelial growth factor (VEGF) promote angiogenesis.

Placental Transport Mechanisms

• Simple diffusion moves substances down their concentration gradient (e.g., oxygen, carbon dioxide, and some drugs).
• Facilitated diffusion uses carrier proteins to transport substances down their concentration gradient (e.g., glucose).
• Active transport requires energy to move substances against their concentration gradient (e.g., amino acids and some ions).
• Pinocytosis involves engulfment of maternal macromolecules by trophoblast cells.
• Bulk flow occurs when large amounts of water and solutes move together due to a pressure gradient.

Placental Structure

• Chorionic Villi are finger-like projections containing fetal blood vessels, increasing the surface area for exchange.
• Intervillous Space is the area surrounding the chorionic villi filled with maternal blood.
• Decidua Basalis is the maternal part of the placenta, derived from the endometrium, providing support.
• Umbilical Cord connects the fetus to the placenta, containing two umbilical arteries and one umbilical vein.
• Amniotic Membrane surrounds the fetus and contains amniotic fluid, providing protection and cushioning.

Placental Circulation

• Maternal blood enters the intervillous space through spiral arteries in the decidua basalis.
• Fetal blood circulates through the chorionic villi, allowing the exchange of gases, nutrients, and wastes.
• Deoxygenated fetal blood flows through the umbilical arteries to the placenta.
• Oxygenated fetal blood returns to the fetus through the umbilical vein.
• Placental blood flow increases throughout pregnancy to meet the fetus's growing demands.

Placental Barrier

• The placental barrier is composed of the syncytiotrophoblast, cytotrophoblast, connective tissue, and fetal capillary endothelium.
• It is selectively permeable, allowing essential nutrients and antibodies to pass while limiting harmful substances.
• The permeability of the placental barrier varies depending on the substance's molecular weight, charge, and lipid solubility.
• Some viruses (e.g., Zika, HIV) and drugs can cross the placental barrier and affect the fetus.

Placental Abnormalities

• Placenta Previa occurs when the placenta covers the cervix, possibly causing bleeding during pregnancy and delivery.
• Placental Abruption is the premature separation of the placenta from the uterine wall, leading to fetal distress and maternal hemorrhage.
• Placenta Accreta/Increta/Percreta refers to the abnormal attachment of the placenta to the uterine wall, with varying degrees of invasion.
• Gestational Trophoblastic Disease (GTD) is a group of tumors that develop from trophoblastic cells.
• Stillbirth is the death of a fetus at or after 20 weeks of gestation.

Factors Affecting Placental Function

• Maternal health conditions like hypertension, diabetes, and autoimmune diseases can impair placental function.
• Maternal smoking, alcohol consumption, and drug use can reduce placental blood flow and nutrient transfer.
• Multiple pregnancies can lead to placental insufficiency due to increased demands on placental function.
• Advanced maternal age is associated with an increased risk of placental abnormalities and complications.

Placental Development Timeline

• At 4-5 weeks gestation, the placenta begins to form as the blastocyst implants into the uterine lining.
• By 12 weeks gestation, the placenta is well-established and begins to produce significant amounts of hormones.
• Throughout the second and third trimesters, the placenta continues to grow and increase its surface area for exchange.
• At term (around 40 weeks gestation), the placenta weighs approximately 500-600 grams.

Placental Research and Diagnostics

• Doppler ultrasound assesses placental blood flow and identifies potential issues.
• Biochemical markers in maternal blood can indicate placental function and pregnancy complications.
• Placental biopsies examine the structure and function of placental tissue.
• Research on the placenta is ongoing to better understand its role in pregnancy and fetal development.

Placental Drug Transfer

• Most drugs cross the placenta to some extent.
• Factors influencing drug transfer include molecular weight, lipid solubility, protein binding, and ionization.
• Drugs with low molecular weight, high lipid solubility, and low protein binding cross more easily.
• The stage of pregnancy affects drug transfer due to changes in placental structure and function.

Maternal-Fetal Exchange

• Oxygen and carbon dioxide are exchanged via simple diffusion, driven by concentration gradients.
• Glucose is transported by facilitated diffusion via GLUT transporters.
• Amino acids are actively transported across the placenta.
• Lipids cross the placenta via simple and facilitated diffusion, with some synthesized by the placenta itself.
• Antibodies (IgG) are transported via receptor-mediated endocytosis, providing fetal immunity.

Genetic and Immunological Aspects

• The placenta expresses both maternal and paternal genes, playing a critical role in development.
• Immunological tolerance is necessary to prevent the maternal immune system from rejecting the fetus and placenta.
• Specialized immune cells at the maternal-fetal interface, such as uterine natural killer (uNK) cells, regulate immune responses.
• Human leukocyte antigen (HLA) molecules on placental cells influence immune interactions and pregnancy outcomes.

Placental Circulation Dynamics

• Uteroplacental blood flow increases significantly throughout pregnancy, reaching approximately 500-750 mL/min at term.
• The intervillous space is a complex network of channels and spaces where maternal blood bathes the chorionic villi.
• Fetal placental circulation involves the umbilical arteries carrying deoxygenated blood to the placenta and the umbilical vein returning oxygenated blood to the fetus.
• Placental vascular resistance decreases during pregnancy to facilitate increased blood flow.