Regulation of Actin Filaments During Neurite Extension and Guidance

Behavior and other neural functions are based on neural circuits that develop over a prolonged period, which lasts in humans from the second fetal month through postnatal years. These circuits develop as neurons extend complex cytoplasmic processes, long axons and highly branched dendrites, expressing intrinsic morphogenetic behaviors while interacting with other cells and molecules of the developing organism. Actin-based motility dominates this morphogenetic process of developing neural circuits. Actin, always one of the most abundant intracellular proteins in neurons, is expressed at its highest levels during neuronal morphogenesis (Santerre and Rich 1976). An analysis of the actin content of embryonic sympathetic neurons indicated that actin comprises up to 20% of total cell protein (Fine and Bray 1971). Much of this actin is used in the motility that drives the formation of axons and dendrites. This chapter describes the roles of actin in the intrinsic mechanisms of morphogenesis of axons and dendrites and the extrinsic environmental features that regulate where and when axons and dendrites grow.