Purkinje cell maturation in the frog cerebellum during thyroxine-induced metamorphosis

Purkinje cell maturation during thyroxine-induced metamorphosis in premetamorphic bullfrog tadpoles was studied using electron microscopy and Golgi (silver-impregnated) preparations. Cerebella from tadpoles were examined following 1, 2, or 3 weeks of thyroxine treatment. Particular attention was paid to possible differences between the two populations of Purkinje cells previously described, i.e. (i) the smaller population located in the dorsal part of the cerebellum, where the Purkinje cells show dendritic arborization long before the appearance of the external granular layer, and (ii) the larger population located in the middle and ventral regions of the cerebellum, where the Purkinje cells begin to undergo maturation during metamorphosis when the external granular layer is established. Following thyroxine treatment, both populations of Purkinje cells showed rapid maturational change. In the mature (dorsal) group, dendritic growth resumed in the presence of an external granular layer increasing the complexity of their dendritic arbors. Moreover, climbing fiber synapses translocated from contacts on the soma to the thorns of growing dendrites, and somatic processes often disappeared. The immature (ventral) group showed dramatic differentiation of the perikaryon including polarization of cytoplasm with subsequent dendritic outgrowth and formation of somatic processes in the presence of climbing fibers. Stellate cell contacts appeared on the smooth portion of the soma of many Purkinje cells. Dendritic growth during thyroxine-induced metamorphosis was characterized by growth (elongation) with minimal branching, which is initially observed during spontaneous metamorphosis. Typically, these growing dendrites ended in growth cones, some with one or several filopodia. Developing Purkinje cell dendritic spines formed synapses with parallel fibers. The present study has provided an example of the dramatic nature of thyroxine's action in inducing the complex series of detailed maturational changes in the cerebellum 1-2 yr ahead of schedule. In addition, the results show that thyroxine-induced Purkinje cell maturation is more rapid and synchronous than that seen during spontaneous metamorphosis. It is concluded that Purkinje cell maturation during metamorphosis is largely dependent on thyroid hormone.