Placenta

Placental morphology and aging

The placenta has two surfaces: a maternal surface (remnants of the basal decidua) and a fetal surface (covered by the shiny amnion, beneath which vessels run). The organ has a lobular structure—it consists of cotyledons separated by partitions (septa). By term pregnancy, the diameter of the placenta reaches 15–18 cm, and its thickness is 2–3 cm.

Organ development continues up to the 32nd week, after which involutional and dystrophic processes (aging) are triggered. The mechanism of aging is as follows:

• Fibrin precipitates from the blood of the intervillous space.
• Settling on the villi, it triggers microthrombosis.
• The placental membrane thins, the endothelium thickens, and salts are deposited.
• Some of the villi are excluded from active exchange.

Although the placenta attempts to compensate for these losses by developing new villi, by the end of pregnancy its overall function steadily declines. The stages of maturation and aging are clearly visible via ultrasound placentography.

Placental barrier and metabolism

The placental barrier regulates the bidirectional transfer of substances and is comparable in its principle of action to the blood-brain barrier. It consists of four layers: syncytiotrophoblast, cytotrophoblast, villous stroma, and fetal capillary endothelium. Barrier permeability increases progressively up to the 32nd–35th week.

Main trophic functions:

• Carbohydrates: Glucose (the main nutritional substrate) passes via facilitated diffusion. Its concentration in the fetus can be higher than in the mother.
• Proteins: The placenta reorganizes amino acids from which the fetus synthesizes its own proteins, immunologically distinct from maternal ones.
• Lipids: Transported only after enzymatic cleavage within the placenta.
• Selective accumulation: The fetus regulates transport itself. For example, during intensive hemopoiesis, iron uptake increases, and during skeletal ossification, calcium and phosphorus uptake increases.

Protective function: The placenta prevents immunological conflict. It lets through protective immunoglobulins G (IgG) but blocks large IgM molecules. However, the barrier is vulnerable: viruses, bacteria, alcohol, nicotine, and many medications easily cross it.

Endocrine and enzymatic activity

The placenta is a potent endocrine organ. It selectively transmits maternal hormones (steroids pass easily, while large proteins and insulin do not) and synthesizes its own:

1. Placental lactogen: Produced exclusively in the placenta, it regulates maternal metabolism. Low levels are a marker of placental insufficiency.
2. Human chorionic gonadotropin (hCG): In early gestation, it controls the corpus luteum (corpus luteum), and later—estrogen synthesis.
3. Steroid hormones: The placenta takes over the synthesis of progesterone and produces estrogens (from fetal androgens), preparing the body for labor.
4. Prolactin: Participates in surfactant production in the fetal lungs.

Additionally, the placenta is rich in enzymes. For instance, oxytocinase breaks down maternal oxytocin, preventing it from crossing to the fetus, while physicians evaluate overall organ function using thermostable alkaline phosphatase levels.

Yolk sac as a provisional organ

Before the placenta assumes all functions, the yolk sac plays a crucial role (forming on days 15–16). Normally, its size is 4–6 mm.

• Hematopoiesis: From days 18–19, it supplies nucleated erythrocytes to the primary circulatory system.
• Source of germ cells: From days 28–29, primary gonocytes migrate from it.
• "Primitive liver": Until week 6, it synthesizes alpha-fetoprotein and transferrins.

By the end of the first trimester, the organ regresses. Premature regression (before the maturation of the fetal liver and spleen) is an extremely unfavorable sign, threatening miscarriage or missed abortion.

Umbilical cord and amniotic fluid

Together with the placenta and fetal membranes, the umbilical cord forms the afterbirth.

The umbilical cord develops from the allantois. Normally, it contains two arteries (carrying venous blood from the fetus to the placenta) and one vein (carrying arterial blood to the fetus). The vessels are surrounded by elastic Wharton's jelly, which protects them from compression and acts as vasa vasorum (supplying the vascular wall). Umbilical cord compression is dangerous due to the presence of nerve endings within it—this triggers neurogenic reactions in the fetus.

Amniotic fluid protects the fetus from trauma and ensures freedom of movement.

• Volume: Maximal at 37–38 weeks (1000–1500 ml); decreases sharply in post-term pregnancy.
• Exchange: Exchange rate reaches 500 ml/h. The fetus actively participates in the process: swallowing fluid and breathing it into the lungs.
• Diagnostics: Fetal lung maturity is assessed via the lecithin-to-sphingomyelin ratio (2:1), while a high concentration of alpha-fetoprotein in the fluid indicates malformations (e.g., neural tube defects). The fluid appears turbid due to lanugo (lanugo), epithelial scales, and fat droplets.

Class and exam questions

What else to study on this topic

• Mechanisms of transplacental transfer (pinocytosis, ultrafiltration, simple diffusion)
• Placental enzymatic activity and blood coagulation factors
• Dynamics of volume and chemical composition of amniotic fluid at different gestational ages
• Paracental pathway of metabolism (via amniotic fluid)
• Role of the fetus in the selective accumulation of trace elements (iron, calcium, phosphorus)

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.