The human placenta is a transitional organ, a mediator between the mother and the fetus for physiologic exchange processes. It is genetically programmed to last for 9 months. Since it consists of maternal and fetal parts, its cells are of two different genotypes. This biologic situation has important immunologic consequences, since the feto-placental complex can be seen as a natural, allogenic transplant that is resistant to rejection.
Even though the maternal and fetal blood circulation systems are very close to each other in the placenta, they remain separated by tissue layers. This delimitation is termed the placental barrier. Oxygen and nutrients pass from maternal into fetal blood, and carbon dioxide and a multiplicity of metabolic waste products of the fetus are delivered into maternal blood. During pregnancy, this important exchange interface serves as the fetal lungs, kidneys and intestines. In addition, the placenta takes on an important role as an endocrine gland, which steers the hormone secretions of the hypothalamus, anterior hypophysis and ovaries of mother and child. The placenta is thus functionally autonomous and during pregnancy it takes over important regulatory functions.
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Autogenic transplant: donor and receiver are identical.
Allogenic transplant:
donor and receiver are genetically different individuals that, however, belong to the same species.
Xenogenic transplant: Donor and receiver come from different species.
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