Reassemblage of primary cell aggregates and modulation of subcortical connections in the thalamic relay nucleus: Effects of vibrissal damage in the developing whisker-to-barrel pathway in the mouse

Abstract

To investigate the mechanisms underlying the reorganization of barrels in the whisker-tobarrel pathway, the facial vibrissae of mice were damaged by electrocauterization at alternate positions on either postnatal day 0 (P0) or P3, before or after the onset of cell aggregation in the thalamus and cortex. Animals were subsequently killed on P8, topographical changes were examined by cytochrome C oxidase histochemistry, and afferent connections were identified using DiI tracer. The cytoarchitecture was characterized with bisbenzimide counterstain. Regardless of when damage was done, the reorganized barreloids and barrels in the thalamus and cortex, respectively, were integrated in an array that represented the topography of undamaged vibrissae. In the brainstem, although the original framework of the array was preserved, defective cell aggregates remained, possibly still in contact with damaged vibrissae. During normal development, on P0 cell aggregates are formed only in the brainstem, and begin to be organized at the other levels of the pathway on P3. Therefore, when damage is induced on P3, the primary cell aggregates are replaced by new, possibly recombined, cell aggregates in the thalamus and cortex to represent the new peripheral topography. The presumably recombined aggregates indicate that cell reassemblage occurred between neighboring cell aggregates. Concomitant with these changes, afferent fibers originating in the brainstem and thalamus extended their terminal arborizations to delineate the new cell aggregates in the thalamus and cortex, respectively. These findings indicate that activity-dependent competitive interactions of afferents may play a crucial role in organizing the topography of cell aggregation and reassemblage in response to vibrissal damage at each level of the pathway.