Serotonin and serotonin-like substances as regulators of early embryogenesis and morphogenesis

The problem of pre-nervous neurotransmitter systems arose from studies carr ied out on different groups of invertebrates and vertebrates in the late 19 50s to early 1960s. These investigations were motivated by an hypothesis fo rmulated by K. S. Koshtoyants concerning the similarity between pre-nervous control processes and neuronal functions. Here, we review new data related to the embryogenetic and morphogenetic functions of serotonin (5-HT) and 5 -HT-like substances in early embryos of sea urchins, mouse, and other speci es. Accumulating evidence across animal phyla indicates that 5-HT, together with other classical neurotransmitters, regulates basic developmental proc esses, including cell proliferation, migration, differentiation, and morpho genesis. Future investigations of cellular and molecular mechanisms underly ing phylogenetically old functions of neurotransmitters could provide new i nsights into the evolutionary emergence of the vertebrate nervous system.