Oestrogen and human implantation

There is no clear consensus on whether the presence and/or concentration of estrogen is necessary for successful human implantation in the uterus. Observation of the effect of an estrogen-free environment upon the implantation process would be necessary to ascertain the hormone's absolute necessity. This is not possible. Researchers can instead consider clues as to how the interacting systems involved in the implantation process behave with respect to the putative role of estrogen. Evidence indicates that the maternal and embryonic estrogen-secreting mechanisms work both individually and in concert to produce an implantation environment with a raised estrogen level. Endometrial cells, however, have a metabolic profile which seems to work toward nullifying the effects of any estrogenic influence. These cells are also likely to have a reduced sensitivity to any residual influence due to a reduction in endometrial estrogen receptors at the peri-implantation period. In trying to understand why the embryo increases the local estrogen concentration during the peri-implantation period, the author suggests that the secretion of specific products or the activity of specific metabolic enzymes are simply reflections of gene expression patterns associated with relatively undifferentiated cells. It is also possible that each biochemical event has some as yet undetermined function.