Abstract
The corticotectal projection in adult cats has a precise topographic and laminar organization. Yet this projection initially grows beyond these adult targets. To begin to understand how the growing cortical axons achieve this precision, the morphological development of axons growing from the posteromedial lateral suprasylvian area (PMLS) to the superior colliculus was studied by injecting the anterograde tracer biocytin into the PMLS of cats between postnatal day (PO) and adulthood. The labeling patterns showed that (1) axons grow independently towards the colliculus and (2) the first axons from the PMLS arrive in the colliculus by P1 and continue to arrive over several days. Labeled growth cones were seen within the colliculus up to P15. (3) After reaching the colliculus, the axons undergo several morphological changes. Initially, they are unbranched and beaded, then short side branches are formed and finally extensive arborizations appear. Comparing the timing of these events with results from electron microscopic27 and electrophysiological46 studies that the appearance and increase in labeled axons with short side brances roughly coincides with the appearance and increase in number of synapses in the colliculus, whereas the elaboration of extensive arbors (and hence a corresponding increase in synapses) is well underway before visual influences on the colliculus can be measured. Thick sinuous axons are also labeled during maturation, usually in areas of the colliculus where they would be considered exuberant and may represent degenerating axons. (4) A coarse topography develops as the axons grow into the colliculus and becomes more precise in the following weeks. Initially, some axons extend well beyond their correct terminal zone, growing into the contralateral colliculus, caudally into the inferior colliculus and reaching all laminae of the ipsilateral superior colliculus. Similar targeting 'errors' have been reported during the growth of retinotectal axons, suggesting that cortical, retinal and perhaps other sources as well, may use the same extracellular cues to establish an initial coarse topography within the colliculus.