Immunohistochemical Localization of Aromatase Cytochrome P-450 and Estradiol Dehydrogenase in the Syncytiotrophoblast of the Human Placenta*

Abstract
Immunohistochemistry employing immunoglobulin G fractions raised against aromatase cytochrome P-450 and antiserum against 17.beta.-estradiol dehydrogenase was used to localize these two steroid-coverting enzymes in the human placenta. Immunostaining for both enzymes was found exclusively in the syncytiotrophoblast, while the underlying cytotrophoblast and the villus core did not stain. Ultrastructural examination of aromatose cytochrome P-450- and 17.beta.-estradiol dehydrogenase-labeled sections disclosed immunoreactive product in the membranes of the endoplasmic reticulum; the nucleus, mitochondria, Golgi apparatus, and secretory granules were free of staining. These findings suggest that the syncytiotrophoblast is actively involved in the synthesis and metabolism of estrogens and in their role in placental endocrine function.