Effects of thyroidectomy and season on the external granular layer of the cerebellum in metamorphosing bullfrog tadpoles (Rana catesbeiana).

Transient formation of the cerebellar external granular layer (EGL) occurs during metamorphosis in frog tadpoles and is dependent on thyroid hormone. Late prometamorphic bullfrog tadpoles at similar stages of development were thyroidectomized and then killed after 1 month and 2 months during the fall and winter. The cerebellum was studied by qualitative and quantitative light microscopy. Thyroidectomy resulted in an inhibition of external metamorphic changes such as hind limb growth. Examination of the cerebellum after thyroidectomy showed the EGL greatly decreased in thickness after 1 month and in some cases was altogether absent after 2 months. Because the disappearance of EGL cells was a result of their inward migration into the IGL, we conclude that thyroid hormone was not necessary for granule cell migration. In addition, metamorphic change normally occurs in the late spring and summer in bullfrog tadpoles, and is inhibited during the fall and winter (metamorphic stasis). During the winter, the volume of the EGL was significantly less compared with tadpoles undergoing active metamorphosis. Although lowered temperature appears to contribute to the large decline in EGL thickness, significant seasonal differences in the EGL volume ratio were still observed in tadpoles maintained under conditions of constant temperature and light. Thyroxine administered to thyroidectomized tadpoles during the winter increased the thickness of the EGL. Therefore, the seasonal decline in the EGL thickness reflects a decrease in thyroid hormone activity, which in turn appears controlled by both internal (hypothalamic) and environmental (e.g., temperature) factors.