Morphology of retinal axons induced to arborize in a novel target, the medial geniculate nucleus. I. Comparison with arbors in normal targets

Abstract

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 responsible for this plasticity are W cells. The present study continues this work 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 the optic tract, and individual axons were serially reconstructed from sagittal sections. The control population consisted of fine-caliber axons arborizing in the superior colliculus (SC) and in the ventral C laminae of the lateral geniculate nucleus (LGN) of normal ferrets. We also compared the axons in the MGN of lesioned ferrets to intracellularly 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 axons in the lesioned ferrets do not resemble X or Y axons in normal ferrets in axon diameter, arbor volume, bouton number, or bouton density. However, they do resemble the fine-caliber, presumed W axons arborizing in the C laminae of the LGN and in the SC of normal ferrets. Thus, this study, in combination with previous studies, suggests strongly that W retinal ganglion cells are responsible for the retinal input to the MGN in lesioned animals. In addition, we find that the retino-MGN axons are of two types, branched and unbranched, which may correspond to different subtypes of retinal W cells.

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections

“…1). Under these conditions, retinal axons project to the contralateral medial geniculate nucleus (MGN) and to a lesser extent to the ipsilateral MGN (11,(23)(24)(25)(26).…”

Cortical GABAergic Interneurons in Cross-Modal Plasticity following Early Blindness

“…Nevertheless, the molecular and cellular mechanisms involved in the formation of these new ectopic connexions are still unanswered. In the ferret, the morphology of the retinal synapses in the MG are similar to the ones found in the visual CS and dLGN in control animals [85, 86]. Further, retinal afferents conserve their visual organization in the auditory relays [87, 88].…”

Dendritic orientation and laminar architecture in the rabbit auditory thalamus

“…Interestingly, there were discontinuities in the terminal bands creating patches 300 -500 m in diameter (Anderson et al, 1980), a feature shared by thalamocortical axons to AI (McMullen and deVenecia, 1993;Romanski and LeDoux, 1993;de Venecia et al, 1998;Cetas et al, 1999). More recent studies of tectothalamic axons using horseradish peroxidase bulk filling (Pallas and Sur, 1994;Wenstrup et al, 1994) or iontophoretic injections of biocytin ) have also reported oriented terminal arbors that paralleled the fibrodendritic laminae. Interestingly, axonal terminal arbors in these distinctly different species were consistent and quite narrow, with a width of only 75-100 m (Pallas and Sur, 1994;Wenstrup et al, 1994;Malmierca et al, 1997).…”

“…More recent studies of tectothalamic axons using horseradish peroxidase bulk filling (Pallas and Sur, 1994;Wenstrup et al, 1994) or iontophoretic injections of biocytin ) have also reported oriented terminal arbors that paralleled the fibrodendritic laminae. Interestingly, axonal terminal arbors in these distinctly different species were consistent and quite narrow, with a width of only 75-100 m (Pallas and Sur, 1994;Wenstrup et al, 1994;Malmierca et al, 1997). If similar dimensions are present in the tectothalamic terminal arbors in the rabbit, then the dendritic field width of MGV neurons (ϳ300 m) would allow the formation of synaptic or functional domains similar to those of the ICC (Oliver and Schneiderman, 1991;Oliver and Huerta, 1992).…”