Ultrastructural studies on Purkinje cell maturation in the cerebellum of the frog tadpole during spontaneous and thyroxine-induced metamorphosis.

The maturation of Purkinje cells in the cerebella of both thyroxine (T4)-induced and normally metamorphosing tadpoles was studied by transmission electron microscopy, with particular reference to the perikaryal changes. During the latter part of the prometamorphic phase, many Purkinje cells showed hypertrophied apical cones filled with mitochondria, Golgi elements and rosettes of ribosomes. In early metamorphic climax, the perikaryal cytoplasm displayed stratification, with an inner zone of perinuclear Nissl bodies and an outer region of neurotubules. At the onset of metamorphic climax, there was an abrupt appearance of numerous somatic processes, as well as climbing fiber boutons which synapsed with them on some cells. However, many Purkinje cells did not display somatic processes. Stellate cell synapses also were seen in considerable numbers. As metamorphic climax progressed to completion, the somatic processes steadily grew scarce with a concomitant increase in climbing fiber synapses on the major dendrites. Glial ensheathment of the Purkinje cell soma was also rapidly accomplished during metamorphic climax. In addition, premetamorphic bullfrog tadpoles were induced to metamorphose prematurely following treatment with T4 and then compared to normally metamorphosing tadpoles. Following 3 weeks of T4 treatment, large numbers of Purkinje cells displayed somatic processes. These processes were observed to be postsynaptic to climbing fibers and similar to those seen normally. Additionally, Purkinje cell hypertrophied apical cones were observed in treated tadpoles. These observations indicate that some aspects of Purkinje cell maturation during metamorphosis, especially the interaction of climbing fibers and somatic contacts, are T4-dependent. In both normal and induced metamorphosis, changes in frog Purkinje cells thus proceed at a tempo comparable to that of such gross morphological transformation as hindlimb growth.