Major Basic Protein in Placenta and Eosinophils
The human, chimpanzee and rhesus monkey placentas produce major basic protein from their extravillous trophoblast. There are three named trophoblastic cells: a) the cytotrophoblast is a single layer of reproductively active cells; it produces the cytoplasm and nuclei for b) the syncytiotrophoblast, the layer covering all villi and whose nuclei can no longer divide, and c) the extravillous trophoblast. The latter forms columns whose centers are frequently cystic and it underlies the chorionic plate and membranes. This trophoblast produces normally substantial amounts of ‘major basic protein’ into the maternal circulation whose function is not understood. This protein is extremely similar to that of the granules in eosinophilic granulocytes. Associated with the extravillous trophoblast (often referred to as “X-cells” because their fetal/maternal origin had been disputed) are accumulations of fibrinoid that differs from fibrin. These cells are also responsible for the invasion of the decidua basalis and the maternal decidual arterioles. An excess of fibrinoid and MBP in humans is a cause of severe fetal growth restriction and occasional death; the condition may be repetitive. This has not been reported in "great apes"