Development and Structure of the Placenta

As the placenta matures, the distribution of these villi becomes uneven. The chorion is divided into two distinct regions based on this pattern:

• Chorion frondosum: The "bushy" part of the chorion that contains numerous mature villi and adheres to the decidua basalis.
• Chorion laeve: The "smooth" part of the chorion that lacks villi and adheres to the decidua capsularis.

Maternal Structure: The Decidua

Once the endometrium undergoes the "decidual reaction" following implantation, it is referred to as the decidua. The decidua is rich in glycogen and lipids to support the developing embryo. The maternal portion of the placenta is specifically formed by the decidua basalis.

The decidua basalis is organized into functional units. Decidual septi arise from the decidual plate, dividing the placenta into approximately 15–20 compartments known as cotyledons.

Note on Fetal Membranes: While the decidua basalis forms the placenta, other parts of the uterine lining serve different roles: the decidua capsularis covers the embryo, and the decidua parietalis lines the remainder of the uterus.

Placental Circulation and the Barrier

A critical feature of the placenta is that the maternal and fetal blood supplies never mix directly. Instead, they circulate in separate spaces:

• Fetal Circulation: Blood moves from the embryo through the umbilical vessels into the capillaries located within the tertiary villi.
• Maternal Circulation: Maternal blood fills the intervillous spaces, surrounding the villi and allowing for exchange.

The placental barrier is the structure that regulates this exchange. Initially, the barrier is relatively thick, consisting of the syncytiotrophoblast, cytotrophoblast, mesoderm, and the fetal endothelium. As pregnancy progresses, the barrier gradually thins to optimize the transport of gases and nutrients, eventually consisting primarily of the syncytium and the endothelium.

Key Physiological Functions

The placenta is a multi-functional organ that acts as a respiratory, digestive, excretory, and endocrine system for the fetus:

1. Gas Exchange: Facilitates the simple diffusion of $\mathrm{O}_{2}$ to the fetus and the removal of $\mathrm{CO}_{2}$ and $\mathrm{CO}$ from fetal blood.
2. Nutrition and Metabolism: Transports essential nutrients and electrolytes to the fetus and performs metabolic tasks, such as the synthesis of glycogen.
3. Excretion: Acts similarly to a fetal kidney by excreting metabolic waste products from the fetal circulation.
4. Endocrine Synthesis: Produces and secretes vital hormones, including progesterone and human chorionic gonadotropin (hCG).
5. Immunity: Provides passive immunity by transferring maternal antibodies, specifically IgG, across the placental membrane to protect the fetus.

Exam Tip: When asked about the placental barrier, remember that it changes with age. It starts thick (multiple layers) and becomes thinner (fewer layers) to facilitate more efficient diffusion as the fetus grows.