MORPHOLOGY OF RETINAL AXONS INDUCED TO ARBORIZE IN A NOVEL TARGET, THE MEDIAL GENICULATE-NUCLEUS .1. COMPARISON WITH ARBORS IN NORMAL TARGETS

Ferret retinal axons can be induced to innervate the medial geniculate nucleus (MGN) by a combination of brain lesions early in development. Our previous work suggests that the retinal ganglion cells responsibl e for this plasticity are W cells. The present study continues this wo rk with a morphological investigation of normal retinal ganglion-cell axons and retinal ganglion-cell axons induced to arborize in the MGN. Retinal axons were bulk filled with horseradish peroxidase placed in t he optic tract, and individual axons were serially reconstructed from sagittal sections. The control population consisted of fine-caliber ax ons arborizing in the superior colliculus (SC) and in the ventral C la minae of the lateral geniculate nucleus (LGN) of normal ferrets. We al so compared the axons in the MGN of lesioned ferrets to intracellularl y filled X and Y axons from normal ferrets as reported by Roe et al. ( [1989] J. Comp. Neurol. 288:208). We have found that the retino-MGN ax ons in the lesioned ferrets do not resemble X or Y axons in normal fer rets in axon diameter, arbor volume, bouton number, or bouton density. However, they do resemble the fine-caliber, presumed W axons arborizi ng in the C laminae of the LGN and in the SC of normal ferrets. Thus, this study, in combination with previous studies, suggests strongly th at W retinal ganglion cells are responsible for the retinal input to t he MGN in lesioned animals. In addition, we find that the retino-MGN a xons are of two types, branched and unbranched, which may correspond t o different subtypes of retinal W cells. (C) 1994 Wiley-Liss, Inc.