MORPHOLOGY OF RETINAL AXON ARBORS INDUCED TO ARBORIZE IN A NOVEL TARGET, THE MEDIAL GENICULATE-NUCLEUS .2. COMPARISON WITH AXONS FROM THE INFERIOR COLLICULUS

Specific neonatal lesions in ferrets can induce retinal axons to proje ct into the medial geniculate nucleus (MGN). In the accompanying paper (Pallas et al., this issue), we described the morphology of these ret inal ganglion cell axons. Those results and others (Roe et al. [1993] J. Comp. Neurol. 334:263) suggest that these axons belong to the W cla ss of retinal axons. In this paper, the retino-MGN axons are compared with the normal inputs to the MGN from the brachium of the inferior co lliculus (BIG). We first sought to determine further the extent to whi ch a novel target might influence retinal axon arbor morphology. The s econd issue concerns retinal topography. Ferrets with retinal projecti ons to the MGN have a two-dimensional retinotopic map in the MGN and t he primary auditory cortex rather than the one-dimensional tonotopic m ap normally present (Roe et al. [1990] Science 250:818). To investigat e whether there might be an anatomical substrate for a two-dimensional retinotopic map in the MGN, we compared the space-filling characteris tics of the retino-MGN axons with the IC-MGN axons. Our results show t hat the branched retino-MGN axons resemble normal retinal W axons much more closely than they resemble the normal inputs to MGN. In addition , most of the axon arbors from the BIC are elongated along the rostroc audal (isofrequency) axis, whereas the branched retino-MGN axons are m ore spatially restricted, suggesting an anatomical substrate for a ret inotopic map in the MGN of the rewired ferrets. (C) 1994 Wiley-Liss, I nc.