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.