17β-Hydroxysteroid Dehydrogenase Types 1 and 2 in Human Placenta: An Immunohistochemical Study with Correlation to Placental Development1

In estrogen metabolism, the enzymatic properties of the 17beta-hydroxysteroid dehydrogenase (17betaHSD) isozymes play very important roles in steroid hormone metabolism in various tissues, including the placenta. 17betaHSD type 1 catalyzes primarily the reduction of estrone (E1) to estradiol (E2), whereas 17betaHSD type 2 catalyzes primarily the oxidation of E2 to E1. In this study, we examined immunohistochemical localization of 17betaHSD types 1 and 2 in human placenta (31 cases) ranging from 4-40 weeks gestation. The immunoreactivity of 17betaHSD type 1 was exclusively detected in syncytiotrophoblast from 4 weeks gestation to term placenta. Immunoreactivity of 17betaHSD type 2 first appeared in endothelial cells of intravillous vessels at 12 weeks gestation, and the number of 17betaHSD type 2-positive endothelial cells markedly increased up to 19 weeks, then reached a plateau. We quantitatively evaluated the 17betaHSD type 2-positive endothelial cells in chorionic villi and determined the ratio of 17betaHSD type 2-positive endothelial cells using immunohistochemistry of CD34, an endothelial antigen, in serial mirror tissue sections and subsequent image analysis using CAS 200. CD34 was detected from 4 weeks gestation, and its positive areas continued to increase toward term. The 17betaHSD type 2-positive area per CD34-positive area markedly increased from 13 weeks gestation and reached a plateau at 19 weeks gestation, in which almost all endothelial cells were positive for 17betaHSD type 2. 17BetaHSD type 2, therefore, is considered to prevent the passage of excessive estrogens into the fetal circulation at endothelial cells of the intravillous fetal capillaries by catalyzing the inactivation ofE2 to E1.