Intrinsic determinants of retinal axon collateralization and arborization patterns

Permanent, novel retinal projections to the principal thalamic somatosensor y (ventrobasal) or auditory (medial geniculate) nuclei can be produced in a dult hamsters if the superior colliculus is ablated bilaterally and the som atosensory and auditory lemniscal axons are transected unilaterally on the day of birth. We studied the development of those novel projections by labe ling retinal axons with the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-t etramethylindocarbocyanine perchlorate to examine the relative roles of int rinsic factors and axon-target interactions in the specification of retinal axon connections. Our principal findings are as follows: (1) In hamsters o perated on the day of birth to produce the novel retinal projections, retin al ganglion cell axons projecting to the ventrobasal or medial geniculate n uclei develop in three morphologically distinct stages, i.e., elongation, c ollateralization, and arborization, as do retinal axons projecting to the d orsal lateral geniculate nucleus, the principal thalamic visual nucleus, in normal hamsters. (2) In both the ventrobasal and medial geniculate nuclei of operated hamsters, as in the dorsal lateral geniculate nucleus of normal hamsters, collateral branches were initially formed by retinal ganglion ce ll axons in both the superficial and internal components of the optic tract and only collaterals from the superficial component formed permanent proje ctions. (3) The retinofugal axon terminal arbors in the ventrobasal and med ial geniculate nuclei of mature, operated hamsters resemble the same three morphologic classes that are observed in the lateral geniculate nucleus of normal hamsters, although their absolute size appears to be altered. These data suggest that both superficial and internal optic tract axons can produ ce thalamic collaterals during development but that only superficial optic tract axons can permanently retain thalamic collaterals. Furthermore, the s ame morphologic types of retinofugal axone appear to contribute to normal a nd surgically induced retinal projections. (C) 1999 Wiley-Liss, Inc.