Topographic organization in the anterior commissure of developing hamsters

The present study has examined the existence of a topographic organization in the anterior commissure (AC) of developing hamsters. Fluorescent carbocyanine crystals (DiI and/or DiA) were implanted into different rostrocaudal and dorsoventral sectors of the paleocortex of hamsters ranging in age from E15 to P10 (E16 = P1 = date of birth). The cerebral hemispheres of each brain were cut horizontally and sagittally, respectively, and the sections were observed under a fluorescence microscope coupled to a computerized reconstruction system. A distinct topographic organization of AC fibers was observed along the rostrocaudal axis starting at E15, and continued unchanged thereafter. These results support the hypothesis that the orderly pattern of AC fibers is achieved by active positioning during the first days after crossing the midplane rather than by a regressive sculpting from an initially disorganized pattern.