Cortical organization of the postero-medial barrel-subfield in mice and its reorganization after destruction of vibrissal follicles after birth.

The postero-medial barrel-subfield (PMBSF) of the SI cortex of normal adult mice contains clusters of cells called "barrels". Each barrel histochemically shows increased activity of succinate, lactate, and glucose-6-phosphate dehydrogenase and also GABA-T activity. Some neuronal perikarya in the barrel walls show GABA-T activity. Mitochondrial alpha-GPDH and AChE show equal activities in the hollows and in the walls of the barrels. On this cortical vibrissa field, the contralateral and ipsilateral vibrissae project somatotopically in a way which coincides with the barrels. The mystacial vibrissae and the common fur of the muzzle project to different loci. The cortical surface area for the normal fur is 0.025 mm2, whereas the cortical vibrissal area is 1.0 mm2. In mice with lesioned whisker pads the succinate-dehydrogenase activity of the IVth layer in the vibrissal area becomes a continuous sheet similar to the adjacent IVth layer, the thickness of the cortex is relatively preserved, and the total enzyme activities, biochemically assayed, are unchanged. These features can be explained by a functional substitution. In mice with whisker pads lesioned since birth, the vibrissal area can still be identified by the projections from ipsilateral vibrissae (undamaged side). This vibrissal area, and this alone, is found to be invaded by projections from the contralateral common fur of the muzzle. Experimental data suggest that the compensatory process may result from an invasion of the vibrissal area by a new set of ascending fibers, and not merely from axonal sprouting either at the periphery or at the cortical level.