Ontogeny of 17beta-hydroxysteroid dehydrogenase type 2 mRNA expression in the developing mouse placenta and fetus

17beta-Hydroxysteroid dehydrogenase type 2 (17HSD type 2) catalyzes the inactivation of estradiol, testosterone and dihydrotestosterone into biologically less active 17-keto forms. Our recent Northern analysis indicated that the enzyme is expressed both in mouse placenta and fetus. The present data indicate that in the placenta the distribution of enzyme expression changes during pregnancy. In the choriovitelline placenta (day 8) 17HSD type 2 was expressed both in mural and polar giant cells. Later, on days 9-12.5, the mRNA was also detected in the junctional zone, and in late gestation (days 14.5-17.5), 17HSD type 2 mRNA was predominantly expressed only at the labyrinth region. In the fetus, 17HSD type 2 expression appears in the liver on day 11. At day 12 the expression was strongly increased in the liver, and at the same time moderate mRNA expression was also detected in the esophagus and intestine. In these tissues, high constitutive expression of 17HSD type 2 was then maintained throughout pregnancy. At later stages of development (days 15-16) the mRNA was, furthermore, detected in epithelial cells of the stomach, tongue, oropharynx and nasopharynx as well as in the kidney. We conclude that the expression pattern of 17HSD type 2 in the developing placenta and fetus suggests a role for the enzyme in maintaining a barrier to the transfer of active 17-hydroxy forms of sex steroids between the fetus and maternal circulation.