The neurotransmitter serotonin (5-HT) was localized in the ectoplacental cone (EPC) and placenta of the day 9–12 (E9–12) mouse embryo in vivo and in whole embryo cultures, using immunocytochemistry with a specific 5-HT antiserum. In uncultured conceptuses, 5-HT immunoreactivity (5-HT IR) was most intense in the EPC at E9 (2–7 somites), particularly in giant cells around the periphery. Nuclear staining was observed in lightly staining giant cells and in small cells in the core of the cone. By E10 (18–24 somites) 5-HT IR in the placenta was less intense and almost exclusively limited to giant cells, where it was localized to chromatin-like material in nuclei. The same pattern and level of 5-HT IR persisted through E12. In the placenta, 5-HT IR appeared to be most intense in giant cells located near aggregations of platelets in decidual blood vessels. 5-HT IR was enhanced in cultured conceptuses, and further increased when exogenous 5-HT was added to the culture medium. Immunoreactivity was greatly reduced by adding the 5-HT uptake inhibitor fluoxetine to the culture medium, or culturing conceptuses in medium containing 5-HT depleted rat serum. Thus, 5-HT was apparently taken up from the culture medium. In conceptuses exposed to exogenous 5-HT, immunoreactivity in the placenta appeared as a gradient from the giant cells to the inner layers, suggesting that these cells may transport 5-HT toward the embryo. No evidence of 5-HT synthesis by the EPC/placenta was found. These results suggest that 5-HT present in the EPC/placenta is due to uptake, not synthesis. Possible sources and functions of 5-HT in the developing placenta are discussed. © 1993, Baillière Tindall. All rights reserved.
