Developmental Mechanisms of Neurogenesis

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Most of what we know about the embryonic development of nervous systems comes from studies of flies or mice. As those studies revealed similar signaling systems and molecular pathways, it was proposed that nervous systems arose once, early in the history of multicellular animals. Subsequent studies in several types of invertebrates support this idea, but the striking diversity in how nervous systems develop makes it difficult to rationalize a single origin. A more refined explanation is that there has been independent evolution of the cellular organization in many groups and neurogenesis has drawn upon conserved signaling systems and molecular pathways in various ways as animals diversified. We lack the models necessary to test this hypothesis, as the molecular and cellular mechanisms underlying neurogenesis need to be carefully analyzed to make useful comparisons in groups of related animals.