Transient intra-areal axons in developing cat visual cortex

Transient axons reaching the medialmost part of area 17 were demonstrated with anterogradely transported biocytin injected in the dorsal part of the lateral gyrus in kittens during the first and second postnatal weeks. The axons decreased in number during the third postnatal week and were only exceptionally found thereafter. Computer-aided reconstructions from serial sections demonstrated axons with different degrees of complexity. The most complex ones were found at postnatal days 7-9 and were characterized by multiple branches terminating with growth cones in the white matter. Characteristically, endings of axons that entered the cortex remained confined to the infragranular layers V and VI. A few axons entered the supragranular layers. Transient axons terminated with different endings, which may indicate different stages of maturation. A few, possibly permanent, axons were still found in the medial part of area 17 at the end of the first, and during the second postnatal months; they arborized widely in the infragranular layers, and modestly or not at all supragranularly.