Ultrastructural studies on cerebellar histogenesis in the frog: the external granular layer and the molecular layer.

Maturational changes of the cerebellum of frog tadpoles were studied with the electron miscroscope. In the premetamorphic tadpole, parallel fiber-like processes (PFP) were present in the incipient molecular layer, long before the appearance of the external granular layer (EGL). These PFP showed synaptic contacts with the precociously developed Purkinje cell dendrites. It appears that these PFP may be responsible for inducing the precocious elaboration of the Purkinje cell dendritic arborization. In the metamorphosing tadpoles, the EGL cells migrating into the internal granular layer were frequently seen in close association with the ependymoglial cell processes, which extend from the pia down toward the ependymal surface. This observation lends support to the hypothesis that glial processes guide the migrating EGL cells.