Placentation is the mechanism by which the foetal and maternal blood circulations are brought very close together for conducting various metabolic functions such as respiration, excretion and nutrition of foetus. In all histological types of placentae, there exists a placental barrier which may include two to six kinds of tissues. Due to the placental barrier, the blood of foetus and mother is never mixed. Physiologically, the placental barrier is like a ultra-filter or semipermeable membrane.
Accordingly, there is a relationship between placental transmission and molecular size, smaller molecules passing more readily through placental barrier than the larger ones. This means that water, oxygen, carbon dioxide, soluble inorganic materials as chloride and phosphates of sodium, potassium and magnesium and soluble organic substances as monosaccharides, hormones, vitamins and urea pass by diffusion.
Macromolecules of polysaccharides, lipids and proteins may be absorbed by trophoblast cells by pinacocytosis. Exchange of substances also occurs by active transport and leakage, i.e, by breaking the placental membrane.
1. Nutritive Function of Placenta:
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The foetus obtains its nutrients from the maternal blood and when the diet is inadequate, then by depletion of maternal tissue storage occurs. Thus, to avoid the depletion of maternal tissue storage a rich diet is essential for the pregnant mother. Glucose which is the principal source of energy of foetus is transported to the foetus by facilitated diffusion.
Lipids needed for foetal growth are transported from the mother to the foetus or synthesised in the foetus (e.g., triglycerides and fatty acids). Amino acids are transferred to the foetus by active transport and then proteins are synthesised in the foetal body. Water, sodium, potassium and chloride enter the foetal blood by simple diffusion.
Whereas calcium, phosphorus (phosphate), iron and iodine enter the foetal blood by active transport. Water soluble vitamins are transported by active transport but the fat soluble vitamins enter the maternal blood by simple diffusion. Hormones like insulin, steroids from the adrenals, thyroid hormones and chorionic gonadotropin or placental lactogen cross the placenta at a very slow rate.
2. Respiratory Function of Placenta:
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Intake of oxygen and output of carbon dioxide occurs by simple diffusion, across the foetal membrane, i.e., oxygen from maternal blood diffuses into the foetal blood, whereas carbon dioxide from the foetal blood diffuses into the maternal blood.
3. Excretory Function of Placenta:
Excretory wastes such as urea, uric acid and creatinine from the foetus are excreted to the maternal blood through placenta by diffusion. The kidneys of the mother eliminate these wastes of foetus along with her own waste products.
4. The placenta during first half of pregnancy is impermeable to bacteria and macromolecules. Therefore, it reduces the chances of appearance of maternal ill health on the foetus. But in the later half of pregnancy, the placenta becomes more permeable and chances of passing the germs from mother to foetus become more. For example, syphilitic spirochetes enter the foetus during this period and causes congenital syphilis in the foetus.
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5. The antibodies, which have developed in the blood of a mother who has acquired immunity to certain diseases, such as diphtheria, scarlet fever, small pox and measles, are passed to the foetus, which, thus, becomes passively immunised and unsusceptible to these illnesses in the first period after birth. Certain pathogenic organisms and viruses can penetrate through the placental barrier and can infect the foetus, if the mother is infected by those pathogens. This is known to happen with syphilis, small pox, chicken pox, measles, and rubella or German measles.
Many drugs used medically may penetrate the placental barrier and sometimes cause most adverse effects on the embryo. Thus, it is believed that, the drug thalidomide, which was used as a sedative when taken by women in early pregnancy (25 to 44 days), caused very extensive deficiencies in the development of limbs, the alimentary canal (non-perforation of the anus) and the heart.
The children born to such mothers had flipper-like limbs (phocomelia) and were called thalidomide babies. Drugs like quinine and aspirin, exposure to X-rays and radium, alcoholic drinks and cigarette smoking, etc., are found to be potent teratogens. In horses, it also acts like an endocrine gland. It secretes many hormones such as estradiol, progesterone, chorionic gonadotropin and placental lactogen.
In some animals, such as rabbit, the placenta is a significant source of relaxin, which is a protein hormone for the relaxation of the pelvic ligaments to facilitate the birth of the young. Human placenta produces a variety of protein and steroid hormones such as oestrogens, and progesterones. Protein hormones are chorionic gonadotropin, placental lactogen, chorionic thyrotropin and corticotropin and relaxin, etc.