Evidence for centripetally shifting terminals on the tectum of postmetamorphic Rana pipiens

In larval frogs the retina and tectum grow in topologically dissimilar patterns: new cells are added as peripheral annuli in the retina and as caudal crescents in the tectum. Retinotopy is maintained by the continual caudalward shifting of the terminals of the optic axons. After metamorphosis the pattern of growth changes. The retina continues to add new ganglion cells peripherally, but there is no neurogenesis in the tectum. To maintain retinotopy in postmetamorphic frogs, the terminals of the optic axons must continually shift toward the central tectum. We tested the proposal of centripetally shifting axons by making punctate injections of horseradish peroxidase (HRP) in the tectum of adult Rana pipiens and observing the patterns of filled cells in the contralateral retina, as was done in the goldfish (Easter and Stuermer, '84). Punctate applications of HRP in the tectum should be taken up: 1) by fascicles, and label a partial anulus of cells, 2) by terminals, and label a cluster of cells in the corresponding retinotopic site, and 3) by the extrafascicular axonal segments, and label a band of cells connecting the partial annulus to the cluster. If the terminals have shifted centripetally, the band of cells labeled through their extrafascicular segments should have a spoke-like orientation, with the center of the retina as the hub. As the tectal site moves from rostral to caudal, this band of cells should move, pendulum-like, from temporal to nasal retina. In general, the patterns of HRP-filled retinal cells we observed were consistent with our predictions. In addition, HRP taken up by the oldest (rostral) tectal axons produced more complex patterns of filled cells that indicated that these axons had shifted both caudally before metamorphosis and centripetally after.