The stages of axonal differentiation described below (Figure 1.8) were originally determined from the analysis of callosal axons originating from, and terminating in, the primary visual areas of the cat, areas 17 and 18 (Aggoun-Zouaoui and Innocenti, 1994; Aggoun-Zouaoui et al., 1996). However they also apply to the projection from the primary visual areas to the peristriate areas (Bressoud and Innocenti, 1999) and perhaps to all cal-losal connections. The stages are schematized and purely descriptive. For individual axons there can be some temporal overlap between the events ascribed to two successive stages. Furthermore, at any age point, different axons can be at different stages of their development.
The First Stage: Axonal Elongation This is essentially a prenatal event in the visual areas of the cat, although during the first postnatal week, axons from the primary visual areas continue to be added to the corpus callosum (see Aggoun-Zouaoui and Innocenti, 1994). The details of this important process are unknown. Apparently, a glial sling forms at the midline before the first callosal axons begin to cross and may be a prerequisite for the formation of the commissure (Silver et al., 1982; Silver, Edwards, and Levitt, 1993). The congenital or experimentally induced destruction of this bridge leads to callosal agenesis. The first callosal axons in the rat grow out of neurons in the cingulate cortex and might pioneer later growing axons (Koester and O'Leary, 1994). Radial glia seems to be involved in guiding callosal axons toward the cerebral cortex (Norris and Kalil,
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