5-hydroxytryptamine: a cytospecific growth stimulator of cultured fibroblasts.

5-Hydroxytryptamine (serotonin) in micromolar amounts increased the growth of fibroblasts in culture while not affecting five other cell lines. Serotonin appeared to shorten the lag phase of cell growth. The effect was less when serotonin was added to the fibroblast culture after the initial 24-hour period. The two functional groups of the serotonin molecule were required for growth enhancement. Serotonin in millimolar concentrations was toxic to fibroblasts.