Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats.

The thalamocortical projection to the rodent barrel cortex consists of inputs from the ventral posterior medial (VPM) and posterior medial (POm) nuclei that terminate in largely nonoverlapping territories in and outside of layer IV. This projection in both rats and mice has been used extensively to study development and plasticity of highly organized synaptic circuits. Whereas the VPM pathway has been well characterized in both rats and mice, organization of the POm pathway has only been described in rats, and no studies have focused exclusively on the development of the POm projection. Here, using transport of Phaseolus vulgaris leucoagglutinin(PHA-L) or carbocyanine dyes, we characterize the POm thalamocortical innervation of adult mouse barrel cortex and describe its early postnatal development in both mice and rats. In adult mice, POm inputs form a dense plexus in layer Va that extends uniformly underneath layer IV barrels and septa. Innervation of layer IV is very sparse; a clear septal innervation pattern is evident only at the layer IV/Va border. This pattern differs subtly from that described previously in rats. Developmentally, in both species, POm axons are present in barrel cortex at birth. In mice, they occupy layer IV as it differentiates, whereas in rats, POm axons do not enter layer IV until 1-2 days after its emergence from the cortical plate. In both species, arbors undergo progressive and directed growth. However, no layer IV septal innervation pattern emerges until several days after the cytoarchitectonic appearance of barrels and well after the emergence of whisker-related clusters of VPM thalamocortical axons. The mature pattern resolves earlier in rats than in mice. Taken together, these data reveal anatomical differences between mice and rats in the development and organization of POm inputs to barrel cortex, with implications for species differences in the nature and plasticity of lemniscal and paralemniscal information processing.