Published online by Cambridge University Press: 10 October 2008
Not only does the placenta function as an organ of exchange between mother and fetus, it also has an endocrine role, producing a number of hormones necessary for the maintenance of pregnancy and the initiation of parturition. There is a large amount of information available which describes the synthesis and secretion of steroids, prostaglandins, human chorionic gonadotrophin (hCG)and placental lactogen by the placenta and fetal membranes. However, the placenta also contains a number of counterparts of hypothalamic-releasing factors and pituitary peptides. Why the human placenta should be the site for the synthesis and secretion of these hypothalamic and pituitary factors is something of a mystery. In man and other vertebrates the nervous system is derived from embryonic ectoderm. In particular, the hypothalamus and pituitary are in part derived from the neural plate and in part from the neural crest. Embryonic ectoderm in the early stages of embryonic development is intimately related spatially to trophoblast, and it is therefore possible that the cells of the ectoderm and trophoblast may be similarly programmed. It is unclear, however, why the trophoblast cells assume similar endocrine functions to those of the pituitary and hypothalamus rather than of other regions of the brain. Furthermore, whether the pituitary and hypothalamic-like peptides are of physiological importance or serve only as an evolutionary and embryological residue remains to be determined.
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