Of minds and embryos: left-right asymmetry and the serotonergic controls of pre-neural morphogenesis

Serotonin is a clinically important neurotransmitter regulating diverse aspects of cognitive function, sleep, mood, and appetite. Increasingly, it is becoming appreciated that serotonin signaling among non-neuronal cells is a novel patterning mechanism existing throughout diverse phyla. Here, we review the evidence implicating serotonergic signaling in embryonic morphogenesis, including gastrulation, craniofacial and bone patterning, and the generation of left-right asymmetry. We propose two models suggesting movement of neurotransmitter molecules as a novel mechanism for how bioelectrical events may couple to downstream signaling cascades and gene activation networks. The discovery of serotonin-dependent patterning events occurring long before the development of the nervous system opens exciting new avenues for future research in evolutionary, developmental, and clinical biology.