Juvenile visual callosal axons in kittens display origin- and fate-related morphology and distribution of arbors

In kittens, callosal axons originating either from medial area 17 (transient axons) or near the 17/18 border (mostly permanent axons) were labelled with anterogradely transported biocytin; they were reconstructed by computer from serial sections, and their morphologies compared at different ages. During the first and second postnatal weeks both sets of axons branched profusely in the white matter of the lateral gyrus and the number of branches increased with age. The most common type of axon ending was the growth cone; others may have been collapsing growth cones, branches in the process of elimination or early synaptic boutons. Axons from medial area 17 distributed over a broad territory, including the 17/18 border where callosal axons terminate in the adult cat, but without aiming specifically at any one area. The majority of axons and their branches terminated in the white matter or at the bottom of layer VI; exceptionally they extended further into the cortex. Most of the axons originating near the 17/18 border were different from those described above, and the difference increased with age. Although they also terminated profusely in the white matter of the lateral gyrus, most of the branches terminated near the contralateral 17/18 border; they frequently entered the grey matter up to the superficial layers and branched into it. During the third week, axons from medial area 17 were rarely found to extend beyond the corpus callosum, probably because they were in the process of being eliminated. In contrast, axons originating near the 17/18 border had increased their number of branches in the grey matter. In conclusion, during the first and second postnatal weeks axons grew and differentiated according to their origin, and this anticipated whether they would be maintained or eliminated. Neurotrophic signals, possibly from the white matter or the subplate, and growth-inhibiting signals from area 17 may be involved in this process.