Tissue- and site-specific gene expression of type 2 17 ss-hydroxysteroid dehydrogenase: In situ hybridization and specific enzymatic activity studiesin human placental endothelial cells of the arterial system

Progesterone and estradiol are the most potent human sex steroid hormones o f placental origin and are essential to the maintenance of pregnancy, the t iming of parturition, the maturation of many fetal organs, and the preparat ion of the maternal reproductive system. Naturally, regulatory mechanisms m ust be in place to coordinate the synthesis and inactivation of these two h ormones. We have previously shown that the highest levels of type 1 and typ e 2 17 beta -hydroxysteroid dehydrogenase (17 beta HSD) messenger ribonucle ic acids (mRNAs) occur in the placenta, particularly in the villi. However, in contrast to type 1 17 beta HSD mRNA, type 2 17 beta HSD mRNA was not de tectable in cell cultures of human cytotrophoblasts or syncytiotrophoblasts . Using in situ hybridization, we unequivocally identified endothelial cell s as the only cell type expressing the type 2 17 beta HSD gene in fetal vil li. Moreover, type 2 17 beta HSD mRNA was specifically detected in the endo thelial cells of the arterial system, and at higher levels in the villi com pared with endothelial cells of the cord arteries when the two tissue secti ons were cohybridized. In fact, both mRNA levels and enzymatic activity are at their highest levels in arterial endothelial cells. In conclusion, the endothelial cells of the villous arterioles are the primary site of type 2 17 beta HSD gene expression. This suggests a regulatory role for these cell s in the control of progestin, androgen, and estrogen levels during pregnan cy, thus opening a whole new way of viewing regionalization and localizatio n of steroidogenesis in the human villi.