Organized growth of thalamocortical axons from the deep tier of terminations into layer IV of developing mouse barrel cortex.

The thalamocortical projection to rodent somatosensory ("barrell") cortex is highly ordered in both the radial and the tangential dimensions. During a brief period of postnatal development, thalamocortical axons establish two tiers of terminations, in the deep layers and in layer IV, and form whisker-specific clusters within layer IV; however, little is known about the cues that guide them to their appropriate radial and tangential positions. To gain insight into potential mechanisms underlying this process, we studied the development of thalamocortical termination patterns in mouse barrel cortex at high spatial resolution. Developing thalamocortical axons were labeled in fixed slices with the lipophilic carbocyanine dye Dil and imaged with a laser scanning confocal microscope. On the day of birth (postnatal day 0, P0) axons coursed through layers VI and V, with little or no branching. By P2 the lower tier of terminations, at the border of layers VI and V, was clearly identifiable. Below this tier axons coursed obliquely or tangentially, forming a dense meshwork of intersecting fibers, but with no apparent branching. By P4 the upper tier of terminations, in layer IV, was clearly recognizable, and consisted of periodic, dense clusters of terminal arborizations. In marked contrast to the oblique and apparently disorderly course followed by axons in layer VI and lower layer V, axons in upper layer V heading toward the upper tier were organized in loose bundles running radially, suggesting that axons destined to terminate in a particular layer IV barrel had already reached their appropriate tangential coordinates within the lower tier. Thus, the pattern of thalamocortical terminations in layer IV seems to be projected from the deep tier of terminations, and does not develop from an initially profuse arborization pattern through pruning of inappropriate branches.