First trimester utero-placental circulation: maternal-fetal interaction.

Progressive development of embryonal and maternal circulations is essential for normal pregnancy outcome. The early embryo and trophoblast are very sensitive to the conditions under which they grow and differentiate. The most crucial event in human ontogeny is trophoblast invasion, a process that will enable chorionic and maternal circulations to develop in a synchronized fashion, balancing the metabolic needs with the capability to defend against oxidative stress. Trophoblast invasion transforms the spiral arteries into low resistance vessels, a change necessary for establishment of continuous forward blood flow once the villi are anchored and have obtained defense mechanisms against oxidative insult. Based on a review of current knowledge of human trophoblast and embryonic growth and development of embryonic and maternal placental circulations it seems that human ontogeny occurs in a low oxygen environment during the initial stages of pregnancy. This is similar to the environment in which the first single cell organisms appeared and it remains to be seen if the development process of the primitive single cell organisms, and its energy generating processes, is an appropriate model for studying early human ontogeny.