The morphology of optic tract axons arborizing in the superior colliculus of the hamster.

Single axons innervating the superficial layers of the hamster's superior colliculus (SC) were visualized using an HRP-filling technique. Five types of axons were distinguished. Experiments involving the removal of retinal and/or cortical input showed that three of these axon types originated in the contralateral retina with the fourth type most likely originating in the visual cortex. The origin of the fifth type, a widely branched varicose axon, is apparently subcortical. The two major types of presumed retinotectal axons (types U and L1) project to the stratum griseum superficiale (SGS) in a bilaminar pattern. Type U axons take relatively direct paths from the layer of optic fibers to form dense terminal arbors in the upper half of the SGS. Terminal fields for type U fibers showed fairly consistent dimensions. Their rostrocaudal extent ranged from 90 micron to 190 micron, averaging about 120 micron. Type L1 axons were thicker than type U axons and terminated in deeper regions of the SGS and in the stratum opticum (SO). Single axons of this type often gave rise to multiple branches which took separate, circuitous paths to a common terminal field. Terminal fields for type L1 axons varied more in extent than did type U fields, but 58% of them had fields 90-150 micron in extent. Each of the axon types found can be related to previous studies of populations of tectal afferents. The two major types of retinofugal axons fit a scheme of parallel ascending pathways. The findings also have interesting implications for the study of axonal development.