ORGANIZATION AND POSTNATAL-DEVELOPMENT OF COMMISSURAL PROJECTION OF RAT SOMATIC SENSORY CORTEX

Anterograde and retrograde autoradiographic tracing experiments were used to demonstrate the origin and terminal distributions of commissural fibers in the 1st somatosensory cortex (SI) of the rat. The commissural fibers originate from pyramidal cells of all layers, but predominantly from layers III and V. The fibers terminate in a series of approximately vertical bands. In each of these there are concentrations of terminals extending from the inner portion of the molecular layer to the deep portion of layer III as well as in the superficial part of layer V, and in layer VI. Discrete vertical bands of cortex are reciprocally connected across the midline to give both the origin and terminal regions of the projection a patchy or columnar appearance. The commissural fibers arise from and terminate in areas of the cortex that lie between and alongside the aggregations of granule cells that distinguish SI of the rat. No commissural fibers terminate within the aggregations of layer IV cells themselves but the more superficial terminal ramifications may come to overlie these aggregations. A heterotopic projection to the contralateral second somatosensory cortex was observed and is similar in form to the homotopic projection to SI. Many commissural fibers have crossed the midline in the corpus callosum by the day of birth but lie in the underlying white matter and do not enter the cortical plate until at least the 3rd postnatal day. During the 1st postnatal wk these fibers grow somewhat diffusely into the maturing cortex and their topographic and laminar pattern of distribution attains its adult characteristics by the end of the 1st wk. commissural axons arise from immature cells but the maturation of cell form seems to precede in ingrowth of these axons and the acquisition of commissural synapses.