Intraretinal projection of retinal ganglion cell axons as a model system for studying axon navigation

Abstract

The initial step of retinal ganglion cell (RGC) axon pathfinding involves directed growth of RGC axons toward the center of the retina, the optic disc, a process termed "intraretinal guidance". Due to the accessibility of the system, and with various embryological, molecular and genetic approaches, significant progress has been made in recent years toward understanding the mechanisms involved in the precise guidance of the RGC axons. As axons are extending from RGCs located throughout the retina, a multitude of factors expressed along with the differentiation wave are important for the guidance of the RGC axons. To ensure that the RGC axons are oriented correctly, restricted to the optic fiber layer (OFL) of the retina, and exit the eye properly, different sets of positive and negative factors cooperate in the process. Fasciculation mediated by a number of cell adhesion molecules (CAMs) and modulation of axonal response to guidance factors provide additional mechanisms to ensure proper guidance of the RGC axons. The intraretinal axon guidance thus serves as an excellent model system for studying how different signals are regulated, modulated and integrated for guiding a large number of axons in three-dimensional space.

Figure 1

Schematic diagrams of intraretinal axon pathfinding. Proper guidance of RGC axons inside the retina requires cooperation of a multitude of factors. A. Optic vesicles are derived from the evagination of the neural tube. NT: neural tube. B. Invagination of optic vesicles results in formation of the optic cup and also a ventral groove. The ventral groove is called the “optic fissure” that serves as the passage way for the exiting of the first axons and inward migration of the mesodermal cells for formation of retinal artery. The optic fissure fuses and the inner layer of the optic cup becomes the neural retina. Signals emitting from the optic fissure/disc may be required for guiding the early RGC axons to the optic fissure/disc. C. At later stages, when RGC axons are projected at a substantial distance away from the optic fissure/disc, a combination of positive guidance factors (shown in green) and negative factors (shown in pink) ensure the correct orientation of the RGC axons. Once the axons are oriented correctly, they can grow along with the other axons of the RGC cells at more central positions. OD: optic disc. D. Mechanisms are also present to restrict the RGC axonal growth within the OFL layer of the retina. The undifferentiated neuroepithelial cells are shown as white cells with bipolar processes, whereas the RGCs are shown as grey cells with their soma in the GCL. Glial/neuroepithelial endfeet and basal lamina (BM) provide positive/permissive substrates whereas Slit2 provides inhibitory signals to the RGC axons. Slit1 is a positive (or permissive) factor in chick (c), not in mouse (m).

Figure 2

The trajectory of newly extended RGC axons at the peripheral chick retina. Retinas from E7 chick embryos were flat-mounted and the RGC axons were immunostained by an antibody specific for neurofilament, 270.7. A. Assembled photos taken at the dorsal peripheral retina are shown. The large arrow indicates the direction to the optic disc. Enlarged images are also shown for selected areas (B, C, D). B. In the most peripheral areas, the newly emerged axons exhibit a certain degree of randomness in orientation (arrows indicate the directions of axon projection). C. At slightly more central positions, the axons are a little longer, still unfasciculated, but with a clear orientation toward the optic disc (arrows). D. Further toward the retinal center, the RGC axons are now well oriented toward the optic disc (arrows), and grow along together briefly then apart, giving rise to the “honeycomb” appearance. E. At approximately 1/3 of the radius distance to the optic disc, the RGC axons appear straighter, like stretched “honeycomb”. F. In close proximity to the optic disc, axons are more heavily fasciculated and project straight toward the optic disc. Scale bar in A (same scale in E and F): 50 μm; scale bar in B (same for C and D): 20 μm.