Microscopic and ultramicroscopic localizations and quantitative analysis of 5-HT receptors in human placentas

The localizations of 5-hydroxytryptamine receptor (5-HTR) at light and electron microscopic levels and its quantitative analysis in human placentas were studied by using immunohistochemistry and in situ hybridization. Both syncytiotrophoblast and cytotrophoblast in placental villi and fetal white blood cells in villose capillary cavity showed 5-HT receptor immunoreactivity, with 5-HT 1A receptor mRNA hybridized signal detected in cytoplasm. But the stromal cells and capillary endothelium in placental villi showed 5-HT receptor immunoreactivity in cytoplasm, without 5_HT-1A receptor mRNA hybridized signal detected. This suggested that two layers of trophoblast cells may produce 5-HT 1 and 5-HT 2 receptors, that the stromal cells and capillary endothelium in placental villi may only produce 5-HT 2 receptor. By immunohistochemistry at electron microscopic level, the small flattened vesicles and large dense cored vesicle within trophoblast cells showed 5-HT receptor immunoreactivity. This suggested that it may be the result of 5-HT receptors internalization and transportion. Using a quantitative immunohistochemical method, the contents of 5-HT receptor in placenta were higher during the 6th week of gestation, and decreased in 7th and 8th, reoccurred the second peak in the 9th, reduced gradually during the 10th, 20th and 40th of the gestation period. These changes paralleled the contents of 5-HT in the authors' studies, reflecting that 5-HT may be one of the most important bioactive substances in placental self-regulation.