Non-uniform projections of granule cells to the cerebellar molecular layer. An autoradiographic tracing study in a turtle.

Injections of radioactive amino acids and wheat germ agglutinin were made into the cerebellar cortex of the turtle, Pseudemys scripta elegans. Labeled parallel fibers were always found to project along the entire mediolateral extent of the corpus cerebelli, regardless of the location of the injection site. In fact, lateral cerebellar portions of both the ipsi- and the contralateral hemisphere often appeared more heavily labeled than intermediate regions. Particularly impressive was the differential pattern of labeling throughout the depth of the molecular layer. Projections coursing towards the midline and the contralateral side were located predominantly in the basal half of the molecular layer while those oriented laterally showed a tendency to occupy primarily superficial portions of the molecular layer. Regional variations along the rostrocaudal extent of the cerebellar plate were also noted. These findings are in an apparent conflict with the reported parallel fiber organization of mammals where granule cells are commonly thought to project to basal and superficial portions of the molecular layer respectively according to a basal-superficial gradient in their location within the granular layer.