Growth of the rat somatic sensory cortex and its constituent parts during postnatal development.

We have compared the size and arrangement of the primary somatic sensory cortex (SI) and its constituent parts in juvenile (1 week old) and mature (10-12 weeks old) rats using succinic dehydrogenase histochemistry and digital image analysis. Our goal was to determine whether some regions of the maturing cortex grow more than others. To this end, we examined (1) the growth of barrels and the surrounding (interbarrel) cortex, (2) the growth of the major somatic representations within SI, and (3) the overall growth of SI compared to the neocortex as a whole. With respect to the first of these issues, SI barrels and barrel-like structures grow more than the intervening cortex. The growth of these elements varies according to region: barrels in the head representation more than double in size, whereas the barrel-like structures in the paw representations increase by only about half this amount. The growth of the major somatic representations within SI is also heterogeneous, the representation of the head enlarging to a greater extent than the representations of the paws. Thus, the ratio of the total area of head representation to the combined paw representation is 15% greater in adults than in juveniles. Finally, the primary somatic sensory cortex grows to a somewhat greater extent than the neocortex as a whole. These observations demonstrate that postnatal cortical growth is not uniform; it varies among cortical barrels and the immediately surrounding (interbarrel) cortex, among the representations of different body parts, and between SI and the rest of the neocortex. As an explanation of this differential growth, we suggest that the neuropil of metabolically (and/or electrically) more active cortical regions grows to a greater extent during maturation than that of less active regions.