EXPRESSION AND CELLULAR-LOCALIZATION OF UTERINE SIDE-CHAIN CLEAVAGE CYTOCHROME-P450 MESSENGER-RIBONUCLEIC-ACID DURING EARLY-PREGNANCY IN MICE

Very little is known about steroidogenic capacities in the uterus duri ng the early stages of pregnancy in rodents. Cholesterol side-chain cl eavage cytochrome P450 (P450scc) is the enzyme catalizing the first an d key regulatory reaction controlling the production of steroid hormon es. Using a cRNA probe, we made use of in situ hybridization analysis to evaluate the spatial and temporal patterns of P450scc mRNA expressi on in the mouse uterus until midgestation. Unexpectedly, we found that upon implantation on day 4.5, maternal cells of both decidua capsular is and decidua basalis expressed P450scc mRNA. Only later, and no earl ier than day 6.5 of gestation, were high levels of P450scc mRNA also d etected in the trophoblast giant cells surrounding the embryonal cavit y. Analysis of pseudopregnant mice revealed that the induction of P450 scc mRNA can be coupled to the decidual reaction evoked by intrauterin e injection of mineral oil. These results, therefore, unambiguously co nfirmed the capacity of the decidualized cells of maternal origin to e xpress P450scc mRNA and, thus, ruled out any direct role of the blasto cyst involvement in P450scc induction. The dual localization of P450sc c mRNA in maternal and trophoblast cells, expressing this cytochrome e arlier than the previously suspected onset of uterine steroidogenesis, suggests an unexpected role for steroid hormones locally produced at the site of implantation and the surrounding milieu of the embryo duri ng the first half of pregnancy.