Ganglion cell differentiation and axon guidance in the ocular retardation mouse retina

Purpose. Ocular retardation results from a null allele in the Chx10 homeobox gene, and is characterized by abnormal retinal development with reduced proliferation of retinal progenitors, an absence of differentiated bipolar cells, and optic nerve aplasia associated with a failure of retinal ganglion cell (RGC) axons to exit the globe. To begin to address the mechanism of optic nerve aplasia, we examined RGC differentiation and the guidance of RGC axons. Methods. Neuron-specific β-tubulin (TuJ1) antibodies were used to examine RGCs in retinal whole-mounts and sections from mutants containing the or^J allele (or^J/or^J). This data will be correlated with ³H-thymidine birthdating studies. Results. Immunoreactivity for TuJ1 in retinal whole-mounts was seen by E11 in wild-type (+/+) retina but not until E13 in the or^J/or^J retina. The +/+ RGC axons showed a predominant orientation toward the optic fissure, whereas those in the or^J/or^J retina showed variable orientation - cells either had long neurites directed toward the periphery or had short, curved neurites without clear orientation. In sections, TuJ1⁺ cells in the +/+ retina were detected at the ventricular margin, were present as radial cells, and along the vitreal margin (usually with a short ventricular process and a vitreal (axonal) process). In mutants, however, TuJ1⁺ cells appeared only rarely along the ventricular margin, with most TuJ1⁺ cells located along the vitreal margin in the position of the developing ganglion cell layer. In addition, TuJ1⁺ cells in or^J/or^J retina were confined to the central retina, although their neurites frequently projected into the peripheral retina. Conclusions. Although mutant retinae contain RGCs which expressed characteristic cell markers, their appearance was delayed compared to normal. The ability to form a [rudimentary] optic fiber layer suggests that the cues which direct RGC axons to form and maintain an OFL are present. The aberrant orientation of the RGC axons, however, indicates that the guidance cues which direct them to the optic nerve are lacking.