Placenta an organ exceptional to mammals helps in connecting the developing fetus to the uterine wall.
Placenta has a thin layer of composite extra-fetal tissue that separates the maternal and fetal circulation systems called as ‘Placental membrane’ or sometimes as ‘placental barrier’.
Role of Placental barrier:
The physiological role of placental membrane is to protect the fetus from various harmful substances and help in its development by allowing transport of useful substances from mother.
Cloaking of fetus from maternal immune system:
The placenta and fetus is recognized as ‘foreign allograft’ by the maternal immune system because of which immune responses are produced to kill and remove them from the body. So immune tolerance in pregnancy also termed as ‘Gestational immune tolerance’, is required for fetus to continue its development inside mother and to prevent spontaneous termination of pregnancy i.e., abortion or miscarriage which is most common in first few weeks of conception.
The placental barrier thus functions as immunological barrier mainly by two different mechanisms namely:
1) Secretion of neurokinin-B which contains phosphocholine molecules which acts as suppressant for immune responses from mother.
2) Fetus contains small lymphocytic suppressor cells which by inhibiting interleukin-2 stop maternal cytotoxic cells.
Transport of substances from maternal circulation to fetus:
The placental membrane is a semi permeable membrane which allows the selective transport of molecules from maternal blood to fetus blood.
Water is easily transported to fetus. The amount transferred increases as pregnancy advances.
Gases: oxygen, carbon dioxide transport occurs by simple diffusion. Interruption of this transport is very dangerous to the embryonic or fetal survival.
Nutrients: These are essential for fetal development. Water soluble vitamins are transported more quickly than fat soluble vitamins. Free fatty acids enter in small amounts.
Glucose can easily pass the placental membrane.
Hormones: Unconjugated steroid hormones can freely pass through the membrane compared to protein hormones. Thyroxin and Triiodotyroxine are transported in small amounts.
Antibodies: Trans-placental transfer of some antibodies mostly gamma globulins like IgG class occurs frequently. This confers passive immunity to certain diseases like measles, smallpox etc.
Electrolytes: These are widely transported through placental membrane. Intravenous administration of electrolytes to mother may interfere with water and electrolyte status to fetus.
Usage of drugs should be monitored during pregnancy:
Most of the lipid soluble drugs freely pass the placental membrane. This can sometimes lead to harmful effects.
Anti-epileptic drugs such as carbamazepine, valproate and Phenobarbitals in combination with caffeine significantly increases congenital malformations or endanger the fetal survival itself.
Anti-cancer drugs containing ‘thalidomide’ are potentially harmful to fetus
Certain antibodies known as ‘teratogens’ like tetracycline effect fetal bone growth.
Anti-convulsants produce detrimental effects to fetus like mental retardation.
Sulpha drugs increase the bilirubin concentration and may cause death.
Excess alcohol can also interfere with the fetal growth.
Placental membrane as ‘Placental barrier’:
It acts as barrier based on size, configuration and charge.
Macromolecules such as cholesterol, triglycerides and phospholipids cannot pass through placental membrane.
Some metabolites, toxins though present in the maternal circulation don’t get through the membrane in amounts to affect fetus.
Heparin and bacteria cannot pass the placental barrier.
Alpha and Beta globulins are not easily transported across placental membrane.
So placental membrane plays a pivotal role in healthy fetal development.