Pioneer axon is the classification given to axons that are the first to grow in a particular region. They originate from pioneer neurons, and have the main function of laying down the initial growing path that subsequent growing axons, dubbed follower axons, from other neurons will eventually follow. Several theories relating to the structure and function of pioneer axons are currently being explored. The first theory is that pioneer axons are specialized structures, and that they play a crucial role in guiding follower axons. The second is that pioneer axons are no different from follower axons, and that they play no role in guiding follower axons. Anatomically, there are no differences between pioneer and follower axons, although there are morphological differences. The mechanisms of pioneer axons and their role in axon guidance is currently being explored. In addition, many studies are being conducted in model organisms, such grasshoppers, zebrafish, and fruit flies to study the effects of manipulations of pioneer axons on neuronal development.
History Santiago Ramon y Cajal, considered the father of modern neuroscience, was one of the first to physically observe growing axons. Moreover, he observed that axons grew in a structured, guided manner. He advocated that axons were guided by chemotactic cues. Indeed, later experiments showed that in both invertebrate and vertebrate models, axons grew along pre-determined routes to create a reproducible scaffold of nerves. Ramon y Cajal's views faced some competition from those of Paul Alfred Weiss, his contemporary neuroscientist during the 1920s and 1930s. Weiss argued that functional specificity did not depend on specific axon connections, and that nonspecific mechanical cues participated in guiding axons. Subsequent investigations into chemotactics cues that started in the 1970s eventually proved that Ramon y Cajal's initial ideas were intuitive and ahead of his time.
Mechanisms of growth The mechanism of growth of pioneer neurons has been investigated in the central and peripheral nervous systems of invertebrate animals. Observations of axon growth during the early embryonic period have led to conclusions that axons are actively guided to specific locations. Within these animal models, several factors have been identified as playing a role in determining the direction of growth. Guidepost cells are specialized early differentiating sensory cells. These cells are essential in providing navigational information to pioneer axons. Arrays of pioneer neurons create short segments of pioneer axons extending distal to proximal within an appendage. The resulting trajectories are due to pioneer axons growing from guidepost to guidepost cells. In addition, pioneer axons can act as guidepost cells to more distant pioneer neurons. Studies that involved selective destruction of guidepost cells resulted in pioneer axons becoming unable to navigate normally to the CNS from the PNS. Instead, the pioneer axons assumed alternate configurations and followed different trajectories. In addition, without the guidepost cells, the pioneer axons did not find the stereotyped route that pioneer axons would normally navigate. It has been shown that glial cells also play a role in axon guidance in various ways. In particular, glial cells demonstrate an interaction with the growth cones of pioneer axons. The route of extending growth cones has been shown to be abundant in glial cells, which are in turn part of a cellular mesh including other intermediate neurons and filopodia. Glial cells also participate in the fasciculation and defasciculation of axons, which are essential in shaping the pathways that are eventually followed. A proposed mechanism involves the creation of a scaffold made out of interface glia, which growth cones contact during the establishment of axon tracts. Ablation of the interface glia leads to a complete loss of longitudinal pioneer axon tracts. In addition, ablation of glia in later embryonic development also interfered with guidance of follower axons, showing that glial cells are necessary in maintaining scaffold needed for contacting growth cones.
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