[The biography of a neuron]

A summary review of current concepts regarding the major events in the long life cycle (the "biography") of a typical human brain neuron, and the main cellular and molecular mechanisms involved, is provided. Once born in the embryo, neurons never undergo cell division: Their differentiated phenotype, which includes the multiple neuronal networks that each cell establishes, is the cumulative result, over years of development, if many cell-autonomous (intrinsic) and intercellular (extrinsic) signaling events that regulate gene expression. The effect of such signals is not just qualitative, but dependent on their precise timing and dosage. Moreover, most intercelullar signals are powerfully regulated by tissue spatial constraints, or by the patterned bioelectrical activity (spontaneous and/or experience-related) of the developing neuronal networks. Thus, a major part of the biological information required to build each adult neuron is coded by a myriad of temporally-, spatially- or activity-dependent signaling events that occur within the developing neuronal networks themselves over a very protracted period. These facts cast serious doubt on the biological soundness of cell-replacement strategies as substitutes for damaged adult brain neurons.