Placental Structure, Circulation, and Hormonal Function
Vascular Villi Organization
The vascular villi are formed by the smooth chorion and become individualized. Along the axes of these primitive villi, 25 to 50 daughter villi (free or hooked) develop for each primitive villus. Most reach the basal plate and are inserted into it.
The primitive villus hypertrophies, and its trunk is called the first-order villous stem. This stem gives rise to second-order villous stems, which divide in turn into third-order villous stems that adhere to the basal plate. Placental growth is effected only by the hypertrophy and growth of each cotyledon.
Villous trunks spread away from each other, forming villous tree systems (or drum systems): a symmetrical arrangement of medial hooks around the central shaft.
Cytotrophoblastic Reorganization
The cytotrophoblast proliferates and gradually phases out within the villus, leaving only a few cells surviving in isolation under the syncytium. During this phase, cytotrophoblastic islets appear and coalesce around the fetal cotyledonary systems, forming a number of partitions known as incomplete intercotyledonary septa.
Placental Development and Circulation (Late Pregnancy)
Villous Capillary Network Maturity (4th Month Onwards)
The villous capillary network reaches maturity, featuring a very dense, strongly anastomosing capillary network developed near the basal plate and in the periphery of the villous tree systems.
Maternal Placental Circulation
The spiral arteries, now called uteroplacental arteries, have already opened into the intervillous chamber by the end of the 2nd week of gestation. Maternal blood is pumped intermittently into the intervillous chamber and is directed from the chorionic region toward the mouths of the uteroplacental veins, which are located throughout the basal plate.
Placental Physiology
Placental Boundary: Mother and Fetus Exchange
Placental Exchange Area (Transfer Mechanisms)
The villi draw necessary elements from maternal blood for the fetus and deposit fetal waste materials. Transfer mechanisms include:
- Blood Gases: Diffusion occurs easily.
- Water: Passes through osmosis, serving to renew the fetus and amniotic fluid.
- Proteins: Broken down into amino acids from the mother and reconstituted as proteins specific to the fetus.
- Lipids: Degraded and reconstituted in the same way.
- Carbohydrates: Spread easily. Fetal blood glucose is in equilibrium with maternal glucose levels.
- Vitamins: Cross the placenta, though the exact mechanism is often unknown.
The Placental Barrier Function
The placental barrier establishes contact between mother and child while allowing certain materials to pass and arresting others, thus respecting a genuinely independent environment. Key barrier functions:
- Proteins: Maternal and fetal proteins are different.
- Toxic Substances and Germs: The placental barrier is permeable to certain toxic substances and impermeable to others.
- Easily Passed: Iodine, bromine, lead.
- Small Amounts Passed: Arsenic.
- Detained: Bismuth, manganese.
- Medications: Penicillin, sulfa drugs, and antifungals pass with ease.
- Microbes: Microbes are generally slowed, but severe infections may cause placental injury.
The Placenta as an Endocrine Gland
The placenta produces hormones necessary for maintaining pregnancy and supporting fetal development:
- Steroid Hormones: Progesterone and Estrogen.
- Polypeptide Hormones:
- Human Chorionic Gonadotropin (HCG)
- Human Chorionic Somatomammotropin (HCS): Secreted by the syncytiotrophoblast of the villi. Its secretion follows a regular upward curve, peaking in late pregnancy. HCS has somatotrophic effects. The most important role of HCS is to ensure the fetus has a sufficient and constant energy intake in the form of glucose, often at the expense of a metabolic shift induced in the mother, who preferentially uses lipid substrates.