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Serotonin [5-hydroxytryptamine (5-HT)] is essential to intrauterine development, but its source is debated. We used immunocytochemistry to gauge 5-HT, its biosynthetic enzyme tryptophan hydroxylase 1 (TPH1); an importer (serotonin transporter, 5-HTT/SERT/SLC6A); other transporters [P-glycoprotein 1 (P-gp/ABCB1), OCT3/SLC22A3, and gap junction connexin-43]; and the 5-HT degradative enzyme monoamine oxidase A (MAOA) in sections of placentas. In humans, 5-HT was faintly stained only in first-trimester trophoblasts, whereas TPH1 was not seen at any stage. SERT was expressed in syncytiotrophoblasts and, more strongly, in cytotrophoblasts. MAOA was prominent in syncytiotrophoblasts, OCT3 and gap junctions were stained in cytotrophoblasts, and P-gp was present at the apical surfaces of both epithelia. 5-HT added to cultured placental explants accumulated in the trophoblast epithelium and reached the villus core vessels. Trophoblast uptake was blocked by the SERT inhibitor escitalopram. Inhibition of gap junctions with heptanol prevented the accumulation of 5-HT in cytotrophoblasts, whereas blocking OCT3 with decynium-22 and P-gp with mitotane led to its accumulation in cytotrophoblasts. Reducing 5-HT destruction by inhibiting MAOA with clorgyline increased the accumulation of 5-HT throughout the villus. In the mouse fetus, intravascular platelets stained prominently for 5-HT at day 13.5, whereas the placenta and yolk sac endoderm were both negative. TPH1 was not detected, but SERT was prominent in these mouse tissues. We conclude that serotonin is conveyed from the maternal blood stream through syncytiotrophoblasts, cytotrophoblasts and the villus core to the fetus through a physiological pathway that involves at least SERT, gap junctions, P-gp, OCT3, and MAOA.

Pathway of Maternal Serotonin to the Human Embryo and Fetus