Chapter 41. Placental Physiology - Review of Medical Embryology Book

DURING EARLY PREGNANCY, in particular, the placenta synthesizes glycogen, cholesterol, and fatty acids

Almost all materials are transported across placental membranes by simple diffusion, facilitated diffusion, active transport, and pinocytosis

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

Endotheliochorial type: seen in the cat and dog, consisting of the fetal vessels, connective tissue of villus, trophoblast, and the maternal vessel endothelium
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
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.

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

In most types of placentas, it reaches a maximum just before normal term
The decrease at the end of gestation can be attributed, in the hemochorial type, to the deposit of fibrinoid on the exchanging surface

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
AT THE END OF PREGNANCY, the placental membrane has a thickness of 2 to 6 mm

Exchange occurs both by passive diffusion and, especially, by selective and active transport resulting from activity of the membrane itself
The exchanging surface is further increased by the presence of microvill The "brush borders" seen in light microscopy correspond to these microvilli

In addition, one sees many mitochondria, ribosomes, pinocytotic vacuoles and lipid enclosures, indicating functional activity of synthesis and exchange

Exchange may involve not only physiologic necessities, but also elements or substances which could create a pathologic risk for the developing fetus

Near term, the uterus extracts 20-35 ml of oxygen per minute from maternal blood

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
Hormones: unconjugated steroid hormones pass freely; testosterone and synthetic progestins cross; protein hormones do not reach fetus in large amounts
Electrolytes: freely exchanged
Antibodies give the fetus some passive immunity; gamma globulin (7S, IgG) reaches the fetus readily
Wastes: CO2, urea, uric acid, bilirubin, et, clear the fetus
Drugs: most (and their metabolites) cross the placenta freely by diffusion
h. Infectious agents: rubella and Coxsackie viruses and those associated with variola, varicella, measles, encephalitis, and polio pass across the placenta