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 project into the medial geniculate nucleus (MGN). In the accompanying paper (Pallas et al., this issue), we described the morphology of these retinal ganglion cell axons. Those results and others (Roe et al. [1993] J. Comp. Neurol. 334:263) suggest that these axons belong to the W class 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 colliculus (BIG). We first sought to determine further the extent to which a novel target might influence retinal axon arbor morphology. The second issue concerns retinal topography. Ferrets with retinal projections to the MGN have a two-dimensional retinotopic map in the MGN and the primary auditory cortex rather than the one-dimensional tonotopic map normally present (Roe et al. [1990] Science 250:818). To investigate whether there might be an anatomical substrate for a two-dimensional retinotopic map in the MGN, we compared the space-filling characteristics of the retino-MGN axons with the IC-MGN axons. Our results show that 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 rostrocaudal (isofrequency) axis, whereas the branched retino-MGN axons are more spatially restricted, suggesting an anatomical substrate for a retinotopic map in the MGN of the rewired ferrets. (C) 1994 Wiley-Liss, Inc.