Development of interhemispheric connections through the anterior commissure in hamsters

The development of interhemispheric connections through the anterior commissure was studied in hamsters by use of the postmortem fluorescent tracer diI. Labelled commissural axons were seen to approach midline and cross it on E14 (E1, day of conception), extend into the opposite hemisphere and reach the terminal targets on E15, and start to arborize on E16. On P1 (day of birth), a few labelled neurons could already be seen in the anterior olfactory nucleus, olfactory tubercle, piriform and insular cortices, nucleus of the lateral olfactory tract, bed nucleus of the stria terminalis, amygdaloid complex, temporal, perirhinal and entorhinal cortices. In all these regions, labelled neurons became progressively more numerous on P3, P5 and P10, but their distribution did not change. Since no evidence of topographical exuberance of connections could be found, it is hypothesized that the development of anterior commissure connections is entirely progressive, lacking the regressive events that characterize callosal ontogenesis.