41. Placental Physiology

  1. Three activities of the placenta: metabolism, transfer, and endocrine secretion
    1. DURING EARLY PREGNANCY, in particular, the placenta synthesizes glycogen, cholesterol, and fatty acids
      1. Almost all materials are transported across placental membranes by simple diffusion, facilitated diffusion, active transport, and pinocytosis
  2. Types of placentas: the human placenta is of the hemochorial type: the fetal tissue or chorion is directly in contact with the maternal blood. The membrane consists of only 3 layers: the syncytiotrophoblast, connective tissue, and the vascular fetal endothelium
      1. Endotheliochorial type: seen in the cat and dog, consisting of the fetal vessels, connective tissue of villus, trophoblast, and the maternal vessel endothelium
      2. Syndesmochorial type: seen in the sheep and ruminants, consisting of the same layers as in (1) above, plus the connective tissue of the maternal mucosa
      3. Epitheliochorial type: seen in the pig and horse, consisting of the same 5 layers as in the syndesmochorial type plus the epithelium of the maternal mucosa which persists.
  3. Permeability according to placental type
    1. STUDIES OF THE PASSAGE OF CERTAIN SUBSTANCES ACROSS THE PLACENTA have shown that the intensity of exchange is inversely proportional to the thickness of the placental membranes and increases regularly during gestation
      1. In most types of placentas, it reaches a maximum just before normal term
      2. The decrease at the end of gestation can be attributed, in the hemochorial type, to the deposit of fibrinoid on the exchanging surface
  4. Placental membrane and fetal-maternal exchange
    1. DURING PREGNANCY, the placental membrane becomes progressively thinner, and by month 4, exchange is favorable. It consists of 3 layers: the syncytiotrophoblast, the fetal vascular endothelium, and a thin sheet of connective tissue between the two
    2. AT THE END OF PREGNANCY, the placental membrane has a thickness of 2 to 6 mm
      1. Exchange occurs both by passive diffusion and, especially, by selective and active transport resulting from activity of the membrane itself
      2. The exchanging surface is further increased by the presence of microvill The "brush borders" seen in light microscopy correspond to these microvilli
        1. In addition, one sees many mitochondria, ribosomes, pinocytotic vacuoles and lipid enclosures, indicating functional activity of synthesis and exchange
      3. Exchange may involve not only physiologic necessities, but also elements or substances which could create a pathologic risk for the developing fetus
        1. Gases: oxygen, carbon dioxide, carbon monoxide cross by simple diffusion
          1. Near term, the uterus extracts 20-35 ml of oxygen per minute from maternal blood
        2. Nutrients: water is freely exchanged; vitamins (water soluble cross faster than fat soluble); glucose; small amounts of free fatty acids; little to no transfer of maternal cholesterol, triglycerides, or phospholipids
        3. Hormones: unconjugated steroid hormones pass freely; testosterone and synthetic progestins cross; protein hormones do not reach fetus in large amounts
        4. Electrolytes: freely exchanged
        5. Antibodies give the fetus some passive immunity; gamma globulin (7S, IgG) reaches the fetus readily
        6. Wastes: CO2, urea, uric acid, bilirubin, et, clear the fetus
        7. Drugs: most (and their metabolites) cross the placenta freely by diffusion
        8. h. Infectious agents: rubella and Coxsackie viruses and those associated with variola, varicella, measles, encephalitis, and polio pass across the placenta
placental physiology: image #1