Pioneer Axon - Mechanisms of Growth

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.