Local action of thyroxine on amphibian metamorphosis. V. Cell division in the eye of anuran larvae effected by thyroxine‐cholesterol implants

To determine whether or not thyroxine stimulates cell division in the tadpole eye by acting directly upon ocular tissues, thyroxine‐cholesterol and cholesterol control pellets were implanted into the vitreous chambers on opposite sides of young Rana larvae. From 1 to 20 days later some of the larvae were exposed to tritiated thymidine. Scintillation counting indicated more radioactivity in the thyroxine‐treated eyes than in the contralateral control eyes, and radioautography showed that it was located in nuclei in the ora serrata of the retina and in the epithelia of the lens and cornea. Mitotic figures were found in similar locations in the eyes of tadpoles fixed at five day intervals, up to 45 days after pellet implantation; striking increases in mitotic activity were induced in the eyes containing thyroxine implants. As long as 35 days post implantation there was little, if any, advancement of metamorphic changes in other regions of the tadpoles including such structures as the hind limbs and the control eyes; even development of accessory ocular structures near the thyroxine‐treated eyes (such as eyelids, nictitating membrane and extrinsic ocular muscles) was not enhanced. The precocious unilateral stimulation of cell division in the retina, lens, and cornea of eyes containing thyroxine implants indicates a direct, local action of thyroxine upon ocular tissues. This suggests that during normal metamorphosis cell division in the eye is probably brought about by the direct, peripheral action of thyroid hormone.