Movement of retinal terminals in goldfish optic tectum predicted by analysis of neuronal proliferation

Quantitative, computer-assisted autoradiography was used to assess the relative rate and pattern of growth of retina and tectum in larval and early juvenile goldfish. 3H-thymidine was used to mark the boundary of retina and tectum, and the location of this boundary was charted as the eye and brain grew and added more cells. The pattern of growth is at all times discordant. The original (larval) retina becomes surrounded by annuli of new tissue, whereas the larval tectum remains adjacent to the rostral edge as crescents of new tissue are added to the caudal end. After 2 years of growth, more than 95% of the total surface area of retina and tectum in goldfish derives from cells born after larval stages. Computer-aided reconstructions of 3H-thymidine labeled retina and tecta were used to predict the direction and magnitude of displacement of the retinotopic map. It was estimated that retinal terminals can shift 1.5-1.8 mm caudally at a rate of 5 micron/d during the first 2 years of growth. The terminals that move the farthest are those from temporal retina that project to rostral tectum. The magnitude and direction of the predicted movements matches certain features of HRP-filled retinal axons that others have assumed represented the history of displacements of the terminal arbors.