Thalamocortical connections in newborn mice.

Thalamocortical axons reach the developing neocortex and become distributed within the cortical subplate during the third week of gestation. The present study is an analysis of the organization of connections that link thalamus and cortical subplate (corresponding to future layers V and VI) at birth. This age antedates the ascent of thalamic axons to contact cells of the supragranular layers, their principal targets in the adult cortex. At birth thalamic nuclear subdivisions are explicit; field-characteristic cytoarchitectonic features, relating principally to the infragranular layers, delineate the majority of neocortical fields. The projection of principal relay nuclei upon the majority of fields of the cerebral convexity has been mapped by means of retrograde transport of HRP. Nucleus-to-field interrelationships as well as topologic order of the overall thalamic projection prove to be identical to that in the adult animal. The neonatal projection appears to be somewhat more divergent than that of the adult.