ABSTRACT
In an adult mammal, the uterus undergoes waves of cell proliferation, differentiation and re-modeling in preparation for receiving a blastocyst, during the development of the embryo and following parturition. These processes are under the overall regulation of ovarian sex steroid hormones that act through their nuclear transcription factor receptors. In the most commonly studied species, rats and mice, adult animals have an estrous cycle that is without a true luteal phase and is, therefore, dominated by estradiol-17β (E2). Only very small amounts of progesterone (P4) are synthesized following ovulation and what is synthesized is mostly metabolized in situ to 20α-hydroxy progesterone, an inactive metabolite (Bazer et al., 1998). However, if the animal undergoes copulation, the corpus luteum is maintained and P4 synthesis continues. This P4 acts to cause differentiation of the uterus in preparation for blastocyst implantation. In contrast to these species that have estrous cycles, in old-world primates a luteal phase ensues after ovulation, P4 continues to be synthesized, and the stromal cells begin to decidualize (Brenner and Slayden, 1994). If pregnancy does not occur and, consequently, chorionic gonadotropin is not synthesized to maintain P4 synthesis, then the uterine lining is sloughed off during menstruation followed by cell proliferation to replenish the lost functionalist layer. These two extremes will be discussed in this article.
