Is there a genuine exuberancy of callosal projections in development? A quantitative electron microscopic study in...

The main tract of interhemispheric connections, the corpus callosum, is now suspected to contain more axons at birth than in adulthood. This notion is based on results obtained with retrograde pathway tracing techniques, but this indirect approach has several shortcomings. Since the elimination of projections during development now seems to be a general phenomenon, probably a crucial one in the establishment of connections, we have examined the development of the corpus callosum using quantitative electron microscopy. An average of 70% of the callosal axons present at birth are eliminated by adulthood in the cat. We have also calculated a new figure of 23 million axons in the adult cat corpus callosum, which is over 4 times greater than the currently accepted figure.