Neural Crest Cells and Axonal Specificity
The adhesion and growth of retinal ganglion cell (RGC) axons along the inner surface of the retina may be governed by the retina’s laminin-containing basal lamina. The embryonic lens and the periphery of the retina secrete inhibitory factors (probably chondroitin sulfate proteoglycans) that repel the RGC axons, preventing them from traveling in the wrong direction (Figure 1; Hynes and Lander 1992; Ohta et al. 1999). Neural cell adhesion molecule (NCAM) may also be especially important here because the directional migration of the retinal ganglion growth cones depends on the NCAM-expressing glial endfeet at the inner retinal surface (Stier and Schlosshauer 1995). In the mouse retina, RGCs express Robo1 and Robo2 receptors, and Slits are expressed in both the ganglion layer and lens epithelium. Functional analysis of Slit and Robo during intraretinal pathfinding of RGC axons suggests that Slits and Robo2 play a role in repelling RGC axons out of the retina (Niclou et al. 2000; Thompson et al. 2006, 2009). The secretion of netrin-1 by the cells of the optic disc (where the axons are assembled to form the optic nerve) plays a role in this migration, as well. Mice lacking the genes for either netrin-1 or for the netrin receptor (found in the retinal ganglion axons) have poorly formed optic nerves because many of the axons fail to leave the eye and instead grow randomly around the disc (Deiner et al. 1997). The role of netrin may change in different parts of the eye. At the entrance to the optic nerve, netrin-1 is co-expressed with laminin on the surface of the retina. Laminin converts netrin from having an attractive signal to having a repulsive signal. This repulsion might “push” the growth cone away from the retinal surface and into the head of the optic nerve, where netrin is expressed without laminin (Mann et al. 2004; see Figure 1).
Upon their arrival at the optic nerve, the migrating axons fasciculate (form a bundle) with axons already present there. NCAM and L1 cell adhesion molecules are critical to this fasciculation, and antibodies against L1 or NCAM cause the axons to enter the optic nerve in a disorderly fashion, which in turn causes them to emerge into the tectum at the wrong positions (Thanos et al. 1984; Brittis et al. 1995; Yin et al. 1995).
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