MULTIPLE FACTORS GOVERN INTRARETINAL AXON GUIDANCE - A TIME-LAPSE STUDY

In this study, the multiple factors that govern the unidirectional path of intraretinal axons, as well as the cellular movements prior to and during early axonogenesis, were investigated using time-lapse videomicroscopy. For several hours prior to overt axon elongation, young retinal ganglion cells send out transient minor processes in all directions at the pial surface. Time-lapse analysis of the chondroitin sulfate (CS)-containing matrix that has been suggested to play an important role in regulating this early differentiative event revealed the dynamic, wavelike properties of this extracellular matrix component. As the CS matrix dissipates across the immature ganglion cells, only one minor process, away from the highest concentration of CS peripherally and in the direction of the optic fissure centrally, is retained and becomes the mature axon. Focal concentrations of L1 appear at points of neurite contact with previously established axons, suggesting that this growth-promoting molecule is also involved with establishing the precise, unidirectional outgrowth pattern of retinal ganglion cell axons. NCAM was diffusely distributed on neural elements and on the neuroepithelial endfeet in the central and peripheral retina and, thus, may not be an essential unidirectional axon growth cue. Growth cones mechanically deflected 180 degrees from the optic fissure after the CS wave had receded from the central retina had morphologies and rates of elongation similar to those oriented in the proper direction. Growth cones deflected obliquely toward the ventral retinal periphery entered a territory of increasing CS-containing proteoglycan matrix and neurons with minor processes. As these deflected axons entered more deeply into this region they slowed down and sent out long transient branchlike processes. These observations illustrate the complex organization of the changing cell surface and matrix components within the retina during axonogenesis and axon outgrowth. The results also elucidate the potential importance of a cell state where immature neurons probe their environment via minor processes. These specialized neurites may provide the neuron with a way to sample a full 360 degrees of terrain around them. This method of exploring the environment could afford the cell a mechanism with which to sample, summate, and respond to physical structures as well as simultaneously occurring negative and positive molecular influences that are distributed unequally on either side of the cell body.