Growth Responses of Opposite Sign Among Different Neuron Types Exposed to Thyroid Hormone

Abstract

Tadpoles of Rana pipiens, Xenopus laevis and Pseudacris versicolor were chosen to study the reasons for size reduction after metamorphosis in the single pair of Mauthner''s cells (M-cells), found in the hind brain of the tadpole and commonly considered to be involved in the swimming movements of the tail and trunk. The possibility that the regression is due to tail resorption was ruled out by (1) removal of the tail bud in the embryo, (2) later amputation and reamputation of the tail and (3) spinal cord hemisection in the 2d trunk region with prevention of reconnection. In all expts. the M-cells did not regress. To test the action of the thyroid, fragments of rat thyroid gland or flakes of agar about 0.5 mm. across, soaked for 20 min. in 1:1000 soln. of thyroxine (synthetic "Roche Organon") was placed in the vicinity of the hindbrain. There resulted an extensive mitotic activity in the ependymal layer lining the brain ventricles and a marked growth of nerve cells with the exception of the neighboring M-cells which atrophied. The phenomenon was the same as the usual occurrence in normal metamorphosis.