Studies on the localization of 5-hydroxytryptamine and its receptors in human placenta

The localization of 5-hydroxytryptamine (5-HT) and its receptors in human placenta was studied under light and electron microscopy using immunohistochemistry and in situ hybridization. The syncytiotrophoblast, cytotrophoblast, stromal cells and decidual cells in human placentae all appeared to be 5-HT immunoreactive. The 5-HT immunoreactive material was distributed in cytoplasm with negative nuclei. The 5-HT immunoreactive material was also found in capillary endothelium. Trophoblast cells cultured in serum-free medium also showed 5-HT immunoreactivity in the cytoplasm. Fetal white blood cells and both syncytiotrophoblast and cytotrophoblast in human placenta showed 5-HT receptor immunoreactivity and 5-HT1A receptor mRNA hybridized signal was also detected in cytoplasm. The stromal cells and capillary endothelium in placental villi and maternal decidual cells all showed 5-HT receptor immunoreactivity in cytoplasm. The small flattened vesicles and large dense cored vesicles within trophoblast cells showed electron-dense 5-HT receptor immunoreactivity using immunoelectron microscopy. These results suggest that human placenta may not only produce 5-HT but also be a 5-HT target organ, and that 5-HT may not only play roles in placental development and pregnancy maintenance by paracrine and autocrine interactions but may also take part in regulating fetal development.