Development of the thalamocortical system: transient-crossed projections to the frontal cortex in neonatal rats

The developmental remodeling of thalamic projections to frontal and prefrontal cortical fields was investigated in the rat by using a double retrograde tracing technique. Bilateral cortical injections of fluorescent tracers were made either in neonatal (first or second postnatal day) or in adult animals. In neonates, the cell populations retrogradely labeled from each cortical injection overlapped in a medial thalamic region that included the midline nuclei and the medial part of the mediodorsal nucleus, ventral medial nucleus, and nucleus gelatinosus. In adults, the overlap region was confined within the boundaries of the midline nuclei. Quantitative analysis showed that this overlap area was three times as wide in neonates as in adults. The neurons located in this region projected unilaterally both in neonatal and adult animals; bilaterally projecting cells were virtually absent. In neonates, a second set of contralaterally projecting neurons was found in more lateral thalamic regions. This population consisted of cell clusters in the dorsal part of the central lateral nucleus and in the lateral part of the ventral medial nucleus; scattered cells were also observed throughout other nuclei. This second cell population was represented in part by neurons bifurcating bilaterally. In adult animals, neurons projecting contralaterally were observed only occasionally in the lateral thalamus. The present results demonstrate that the bilaterality of thalamocortical projections undergoes a reduction during postnatal development. The mechanisms underlying this remodeling and the possible functional role of the transient-crossed thalamocortical system are discussed.