Position of axons in the cat's optic tract in relation to their retinal origin and chiasmatic pathway

The positions of the crossed and uncrossed optic axons of distinct diameter classes has been examined in the optic tract of the adult cat. In addition, the retinal origin of axons occupying different positions within the tract has been studied. Since the position of a fibre within the optic tract reflects its time of arrival during development, we have used axonal position as an indicator of age and have related this to the chiasmatic pathway choice of the axons. Cats were either monocularly enucleated, to reveal the position and diameter of surviving crossed and uncrossed optic axons in semithin and thin sections, or implants of horseradish peroxidase (HRP) were placed so as to retrogradely label the ganglion cells giving rise to axons within the deep (early arriving), or superficial (later arriving) parts of the tract selectively. This was accomplished by either 1) surgically implanting HRP into the superficial portion of the optic tract, via a transbuccal approach, or 2) making such a transbuccal transection of the superficial fibres, followed by intracerebral injections of HRP to retrogradely label the surviving, deeper, optic axons from their target nuclei. The deep parts of the optic tract contain fine and medium, crossed and uncrossed axons arising from mainly medium sized cells in the contralateral nasal and the ipsilateral temporal retina; there is a clear line of decussation. In contrast, the superficial parts of the tract contain mainly fine diameter axons arising from small cells in the whole contralateral retina, and a small proportion of large diameter axons arising from large, alpha cells in the whole contralateral retina and in the ipsilateral temporal retina. The likelihood that axons from the temporal retina will project contralaterally therefore increases as development proceeds, since these axons are found in the superficial parts of the tract only. This suggests that a time-dependent signal that weakens with age is responsible for directing early arriving optic axons from the temporal retina to take an exclusively uncrossed course