Expression of steroidogenic genes in maternal and extraembryonic cells during early pregnancy in mice

The ontogeny and functional role of steroidogenesis during early gestation in rodents is poorly understood. In previous studies, we have shown that ex pression of messenger RNAs (mRNAs) encoding two key enzymes indispensable f or de novo synthesis of steroid hormones, i.e. cholesterol side chain cleav age cytochrome P450 (P450scc) and a newly identified isoform of murine 3 be ta-hydroxysteroid dehydrogenase/isomerase type VI (3 beta HSD VI), is initi ated upon decidualization of the uterine wall induced by implantation. In s itu hybridization and immunohistochemical visualization of 3 beta HSD VI mR NA and protein shows high expression of this enzyme in the antimesometrial cells of the decidua of days 6.5-7.5 post coitum (p.c.). Thereafter, expres sion of 3 beta HSD VI in the decidual zones disappears and is replaced by a high expression of mRNA and protein in the embryonal giant trophoblast cel ls. At the peak of their development on day 9.5 p.c., the mouse giant troph oblast cells also express Steroidogenic Acute Regulatory (StAR) protein, wh ich is required for steroidogenesis in the gonads and adrenal cortex. Our f indings also suggest that the declining levels of P450scc, 3 beta HSD VI, a nd StAR proteins between days 10.5-14.5 p.c, in the developing placenta is consistent with previous reports that the mouse placenta is not involved in de novo synthesis of steroids during the second half of pregnancy. Collect ively, the results of the present study suggest that, during early phases o f pregnancy, local progesterone synthesis in the maternal decidua and the t rophoblast layers surrounding the embryonal cavity is important for success ful implantation and/or maintenance of pregnancy. We propose that the local production of progesterone acts as an immunosuppressant at the fetal mater nal interface preventing the rejection of the fetal allograft.