Climbing fiber projection to the turtle cerebellum: longitudinally oriented terminal zones within the basal third of the molecular layer.

Injections of various radiolabeled amino acids were made into the caudal rhombencephalic tegmentum in the turtle, Pseudemys scripta elegans. In animals in which injections encompassed the entire area previously identified as the possible source of cerebellar climbing fibers, the basal portion of the molecular layer was labeled almost throughout the contralateral cerebellum. In cases of restricted injections, labeled climbing fibers terminated in quite distinct longitudinally oriented zones. Control injections immediately caudal and rostral to the mentioned rhombencephalic region resulted in labeling of only the granular layer. The organization of the climbing fiber projection in the turtle is compared with the organization of the climbing fiber projection in representatives of birds and mammals as described in the literature. Reptiles, birds and mammals appear to be similar to each other in that they all have climbing fiber projections organized in longitudinal zones; whereas they differ with respect to the extent of climbing fiber penetration into the superficial portions of the molecular layer, mammals having climbing fibers that project significantly more superficially than those of birds or reptiles.