Growth of Thalamic Afferents into Mouse Barrel Cortex

We studied thalamocortical afferent (TCA) growth into somatosensory cortex as the whisker barrels emerge in postnatal mice. Ingrowing fibers from the ventrobasal (VB) thalamus were selectively labeled by two means. Under direct vision, individual axons and populations of axons were labeled in vitro with HRP, or in fixed tissue with Dil (1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), in pieces of brain containing both the source nucleus in the thalamus and its cortical target. Many simple thalamocortical afferents are already within the upper cortical plate at birth postnatal day one (PND1). Initially, TCAs from each point in the thalamus distribute in the cortex as two-dimensional "Gaussians," which overlap laterally to constitute a uniform projection pattern. The projection is topographic, because adjacent focal injections within VB label adjacent cortical loci. Subsequent development of barreloids (thalamic representations of the whiskers) partitions the TCA projection into a set of whisker-related Gaussians, centered on cortical targets whose collective topography reflects that of the source pattern. After barreloids form on about PND3, but before barrels appear in cytoarchitecture on about PND5, the overlapping TCAs segregate into dense terminal clusters in layer IV, around which barrels later mature. Time series of single fibers traced with camera lucida explain this transformation that is so noticeable at the population level. As early as PND1, individual TCAs emit multiple ascending collaterals on their horizontal run through white matter and oblique ascent into upper cortex. Subsequently, by PND4, and proceeding at least through PND7, there is accelerated terminal arborization of selected appropriate collateral branches and pruning back of other inappropriate ones. The selection mechanism appears to result from within-group reinforcement events that are stronger for branches toward the center of each whisker-related Gaussian distribution.