Structural Sources of Placental Hormones
The endocrine activity of the placenta is strictly tied to its histological structure. Hormones are produced by structures in both parts of the placenta:
- Fetal part: The primary source here is the chorionic epithelium. It is represented by two differentiated layers — 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 with its own target and specific function during pregnancy.
1. Pituitary Hormone Analogs Primarily synthesized in cytotrophoblast cells. Their role is to complement the action of both maternal and fetal pituitary hormones.
- Human Chorionic Gonadotropin (hCG): Production by trophoblast cells begins as early as the implantation stage. It serves as the primary marker for early pregnancy tests. Its main target is considered to be the embryonic organism (presumably stimulating gonad development).
- Placental Lactogen (Prolactin): Acts on the maternal organism, maintaining the function of the corpus luteum of pregnancy until the placenta takes over steroid synthesis.
- Other analogs: The placenta also produces analogs of growth hormone (somatotropin), thyroid-stimulating hormone (TSH), and melanocyte-stimulating hormone (MSH).
2. Female Sex Hormones (Steroids) Synthesized in the syncytiotrophoblast of chorionic villi. Placental production of these hormones is vital because maternal ovarian follicle growth eventually ceases (halting estrogen sources) and the corpus luteum regresses (progesterone levels drop).
- Estrogens: Active production starts by mid-pregnancy, with a sharp concentration peak observed right before delivery, preparing the body for labor.
- Progestins: Secreted throughout almost the entire pregnancy and cease only before childbirth. In the mother, they suppress myometrial contractility, maintaining pregnancy. Reaching the fetal organism, progestins become a crucial substrate for corticosteroid formation.
3. Relaxin Produced by decidual cells of the maternal placenta. Its main role is preparing the birth canal: it inhibits premature contractions, causes expansion and softening of the cervix, and provides relaxation of the pubic symphysis and pelvic ligaments.
Umbilical Cord (*Funiculus umbilicalis*)
The umbilical cord develops from the connecting stalk (amniotic 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 simple columnar epithelium and a thin lamina propria. The internal foundation (stroma) is formed by mucous connective tissue known as Wharton's jelly. The main cellular type of this tissue is mucocytes. It is important to emphasize that these are fibroblast-lineage cells (connective tissue) and should not be confused with epithelial mucocytes.
Vascular Component Within Wharton's jelly lie three major blood vessels:
- Two umbilical arteries: Transport deoxygenated blood from the fetus to the chorionic villi.
- One umbilical vein: Delivers oxygenated blood from the placenta to the fetus.
Rudimentary Structures Blood vessels initially develop alongside the allantois. Normally, the allantois and the yolk sac stalk regress, but their remnants can frequently be found as rudiments in histological cross-sections of the umbilical cord.