Transient Pioneer Neurons Are Essential for Formation of an Embryonic Peripheral Nerve

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Abstract
In developing nervous systems, many peripheral and central pathways are established by early arising populations of pioneer neurons. The growth cones of these pioneer neurons can migrate while embryonic distances are short and while intervening tissue is relatively uncomplicated. Are these pioneers necessary? In grasshopper embryos, a pair of pioneer neurons arise at the tips of limb buds and extend axons through the limb to the central nervous system. Growth cones of later arising sensory neurons migrate along the pioneer axons. After ingrowth of sensory axons, the pioneer neurons die. If the pioneer neurons are prevented from differentiating by heat shock, then the sensory growth cones that would have migrated along them are blocked and fail to reach the central nervous system. Thus, the pioneer axons are necessary for successful migration of these sensory growth cones. By crossing a segment boundary early in embryogenesis, the pioneers circumvent an incompatibility between differentiated segment boundary cells and growth cone migration. Pioneer neurons may resolve similar problems in many systems.