ABSTRACT
Estrogen and progesterone are ovarian sex steroid hormones that are required in maturational and functional changes in the primate endometrium. They are essential for the establishment and maintenance of pregnancy. After menstruation, a rapid proliferation of both epithelial and stromal cells occurs under the influence of estrogen and is thus termed as the proliferative phase. Following ovulation, the secretory or luteal phase begins, when progesterone levels start to rise and proliferation in glands and stroma cease while differentiation begins. Secretory activity in glands is maximal on the 5th to 7th day post-ovulation when large amounts of mucin, glycogen and glycoprotein are observed in the gland lumina. Stromal cell differentiation occurs around 9 to 10 days post-ovulation when cells around the spiral arteries and beneath uterine epithelium become plump and rich in glycogen. In the human, spontaneous decidualization of endometrial stroma is initiated in the normal menstrual cycle and will occur even in the absence of pregnancy, a situation that differs from that in other species such as baboon, rats, mice, guinea pigs and rabbits in which the decidual reaction only begins at implantation. This decidualized tissue eventually breaks down with bleeding at the end of the cycle at menstruation. The biological effects of sex steroid hormones are mediated in part by specific receptor proteins that have specificity and high affinity for their respective ligands. The estrogen and progesterone receptor gene knockout (ERKO, PRKO) studies have provided invaluable information on the role of these hormones in reproductive processes. For example, studies in the estrogen receptor-α (ERα) knockout mice demonstrated a lack of epithelial proliferative response to estrogen, no induction of mRNAs of estrogen-responsive genes such as glucose-6-phosphate dehydrogenase, lactoferrin and progesterone receptor (Lubahn et al., 1993; Couse et al., 1995). Other knockout systems, such as progesterone receptor (PR)(−/−), leukemia inhibitory factor (LIF)(−/−), cycloxygenase (COX-2)(−/−), and HOXa-10(−/−) have demonstrated a deficiency in the decidualization process. In this review, we will focus primarily on the human and non-human primate models. Current information regarding steroid hormone action through its receptors and the different receptor isoforms that are differentially expressed in various cell types of reproductive tissues will be discussed. In addition, regulation of Steroidogenic enzymes involved in steroid hormone biosynthesis, and the role of non-steroidal hormones, i.e. prolactin in the female reproductive tract will be summarized.
