Structural sources of placental hormones
The endocrine activity of the placenta is strictly linked to its histological structure. Hormones are produced by structures of both parts of the placenta:
- Fetal part: the main source here is the chorionic epithelium. It is represented by two differentiated layers — the cytotrophoblast and syncytiotrophoblast.
- Maternal part: the synthesis of biologically active substances is carried out by decidual cells.
Classification of placental hormones
All hormones secreted by the placenta are generally divided into three main groups, each having its own target and specific function during pregnancy.
1. Pituitary hormone analogues Predominantly synthesized in cytotrophoblast cells. Their task is to complement the action of both maternal and fetal pituitary hormones.
- Human chorionic gonadotropin (hCG): begins to be produced by trophoblast cells as early as the implantation stage. It serves as the marker for early pregnancy tests. Its main target is considered to be the embryo (presumably, hCG stimulates gonad development).
- Placental lactogen (prolactin): acts on the maternal organism, supporting the function of the corpus luteum of pregnancy until the placenta takes over steroid synthesis.
- Other analogues: the placenta also produces analogues of growth hormone (GH), thyroid-stimulating hormone (TSH), and melanocyte-stimulating hormone (MSH).
2. Female sex hormones (steroids) Synthesized in the syncytiotrophoblast of chorionic villi. The production of these hormones by the placenta is vital, because over time the maternal ovaries cease follicular growth (the source of estrogens) and the corpus luteum regresses (progesterone levels drop).
- Estrogens: their active production starts from the middle of pregnancy, and a sharp peak in concentration is observed immediately before labor, which is necessary to prepare the body for childbirth.
- Progestins: secreted throughout almost the entire pregnancy and stop being produced only before labor. In the mother, they suppress myometrial contractility, maintaining the pregnancy. Entering the fetal body, progestins become a crucial substrate for the formation of corticosteroids.
3. Relaxin Produced by decidual cells of the maternal part of the placenta. Its main role is to prepare the birth canal: it inhibits premature contractions, causes dilation and softening of the cervix, and ensures relaxation of the pubic symphysis and pelvic ligaments.
Umbilical cord (Funiculus umbilicalis)
The umbilical cord is formed from the connecting stalk and serves as an anatomical bridge between the fetus and the placenta.
Membranes and stroma Externally, the organ is covered by the amniotic membrane, consisting of a single-layered columnar epithelium and a thin lamina propria. The internal foundation (stroma) is formed by mucous connective tissue known as "Wharton's jelly". The main cell type of this tissue is mucocytes. It is important to emphasize that these are cells of the fibroblastic lineage (connective tissue), which should not be confused with mucocytes of epithelial tissues.
Vascular component Within Wharton's jelly lie three large major vessels:
- Two umbilical arteries: carry deoxygenated (oxygen-poor) blood from the fetus to the chorionic villi.
- One umbilical vein: delivers oxygenated blood from the placenta to the fetus.
Rudimentary structures Initially, blood vessels develop along the allantois. Normally, the allantois and the yolk stalk regress; however, remnants of them can often be found as rudiments in histological sections of the umbilical cord.
Briefly: answers to topical questions
From which germ layer does the epithelium of the umbilical amniotic membrane develop?
The epithelium of the amniotic membrane of the umbilical cord develops from the extraembryonic ectoderm. The initial cell source for the formation of this epithelial layer is the amniotic ectoderm, which is classified as a derivative of the ectodermal germ layer. It forms the roof of the amniotic vesicle and subsequently lines the surface of the developing amnion as a single-layered columnar epithelium.
What is the mechanism of action of placental lactogen on maternal carbohydrate and lipid metabolism?
Placental lactogen participates in alterations of maternal carbohydrate and lipid metabolism in the second half of pregnancy. In carbohydrate metabolism, placental hormones decrease glucose utilization by maternal tissues, so the postprandial glucose level is higher than in non-pregnant women; physiological insulin resistance is associated with the influence of placental lactogen, estrogens, and progesterone. In lipid metabolism, placental lactogen enhances lipolysis, leading to increased levels of glycerol and free fatty acids in the plasma and enhanced ketogenesis. Ketone bodies freely cross the placenta and are used by the fetal liver and brain as energy sources.
What functions does Wharton's jelly of the umbilical cord perform besides structural support?
The mucous connective tissue of the umbilical cord performs protective, trophic, and metabolic functions. In addition to providing elasticity, it:
- anchors the umbilical vessels and protects them from compression and trauma;
- plays the role of vessels of the vessels (vasa vasorum), providing nutrition to the vascular wall;
- mediates metabolism between fetal blood and amniotic fluid.
The rule "Two are venous, one is arterial": the umbilical cord contains TWO arteries (but they carry VENOUS blood to the placenta) and ONE vein (carrying ARTERIAL blood to the fetus).
Class and exam questions
Why does the placenta begin synthesizing steroid hormones?
This occurs because during pregnancy follicles in the maternal ovaries cease to mature (a source of estrogens), and the corpus luteum eventually undergoes regression (progesterone levels drop).
What is the function of placental progestins?
In the mother, they suppress myometrial contractions to maintain pregnancy, while in the fetus they are used as a substrate for the synthesis of its own corticosteroids.
What are umbilical mucocytes and what tissue do they belong to?
Umbilical cord mucocytes are the main cells of the stromal mucous connective tissue. They belong to the fibroblastic cell lineage, not the epithelium.
What else to study on this topic
- Histological structure of the chorionic epithelium (differences between cyto- and syncytiotrophoblast)
- Morphological characteristics of decidual cells and their origin
- Mechanism of suppression of myometrial contractility by progestins
- Embryogenesis of the umbilical cord from the connecting stalk
- Structural features of mucous connective tissue ("Wharton's jelly")
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