Studies using retrogradely transported axonal tracers have demonstrated that callosal connections develop through a phase of exuberance in which more axons project into the corpus callosum than will be present in the adult (reviewed in Innocenti, 1986, 1991). The first unequivocally transient projections to be discovered were those originating from parts of the cortex that are no longer callosally connected in the adult such as most of area 17 and parts of the primary somatosensory areas. Transient callosal projections were also found in the cat between the primary auditory area and the visual area. Comparable results were reported in several other areas and species, including the rhesus monkey (Chalupa and Killackey, 1989).
The elimination of the transitory projections is due to selective elimination of axonal branches rather than neuronal death. The elimination is massive. For the whole corpus callosum it was estimated, on the basis of electron microscopic counts of callosal axons, to amount to at least 70% of the axons produced in both cats (Ber-bel and Innocenti, 1988) and monkeys (LaMantia and Rakic, 1990b). These findings were tentatively extrapolated to the developing human corpus callosum on the basis of measurements of corpus callosum sectional area (reviewed in Innocenti, 1991).
The production of transient projections, often topographically different from the adult ones, was found to be a very general phenomenon, not restricted to the callosal connections but applying also to intrahemispheric and corticosubcortical projections (reviewed in Innocenti, 1991). The overproduction of projections clearly provides the potential substrate for plastic changes in the developing brain. Indeed, neither the fate of normally maintained axons nor that of normally eliminated axons seemed rigidly predetermined. Instead, it was found to be modifiable by epigenetic events, including activity, integrity of the sensory periphery and of other brain regions, and hormonal and dietary influences (reviewed in Innocenti, 1991; Zufferey et al., 1999).
Studies with retrograde transport techniques, however, provided only limited and indirect information on the axons concerned. They failed to clarify the morphology of the developing axons and which relations they establish with the target regions. Both questions are important for understanding the functional role played by the transient connections in development and for clarifying the nature of the control that determines the fate of the juvenile axons.
Therefore, the methods for visualization, three-dimensional reconstruction and analysis of single axons, already used for the adult, were applied to developing callosal axons from the primary visual areas of the cat (Aggoun-Zouaoui and Innocenti, 1994; Aggoun-Zouaoui, Kiper, and Innocenti, 1996). The results clarified a number of factual and theoretical issues. The overall conclusion is that the formation of connectional maps includes a process of axonal differentiation involving both productive and regressive events. In this process, several growth stages can be identified. At each stage the axon exhibits exuberant growth, but the growth is progressively constrained within territories that gradually approach the adult distribution of the axon.
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