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

Abstract

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 i n the contralateral control eyes, and radioautography showed that it was located i n nuclei i n the ora serrata of the retina and i n the epithelia of the lens and cornea. Mitotic figures were found i n similar locations i n the eyes of tadpoles fixed at five day intervals, up to 45 days after pellet implantation; striking increases in mitotic activity were induced i n the eyes containing thyroxine implants. As long as 35 days post implantation there was little, if any, advancement of metamorphic changes i n 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. 1 100 (cpm right eye-cpm left eye)/cpm left eye. fi Range of total number of counts about 14,000 to 160,000. 7 Counts from right eyes significantly higher than those from left eyes ( P < 0.001. one-sided sign test), 8 Range of total number of counts about 25,000 to 160, 000, 9 No sienificance according to either the one-sided sign test or the t-test ( P > 0.1).after H3-thymidine injection).

Thyroid Hormone Acts Locally to Increase Neurogenesis, Neuronal Differentiation, and Dendritic Arbor Elaboration in the Tadpole Visual System

“…These tadpoles typically do not survive longer than 7-10 d, likely because precocious TH-induced changes to the mouth, gut, and lungs are not temporally coordinated with other metamorphic changes, such as limb development. To determine whether T 3 mediates changes in tectal development by acting directly on tectal cells and to circumvent ambiguities arising from systemic T 3 treatment, we adapted methods from previously published studies (Kaltenbach, 1970;Kaltenbach and Hobbs, 1972;Leatherland, 1987). We injected T 3 emulsified in coconut oil directly into the midbrain ventricle.…”

Control of retinal growth and axon divergence at the chiasm: lessons from Xenopus

“…Kaltenbach and Hobbs first showed in Rana pipiens that implanting an intraocular TH-releasing pellet in tadpoles causes an increase in retinal cell proliferation. (12) Similarly, in Xenopus laevis, slow release of TH into the eye of premetamorphic tadpoles stimulates cell proliferation restricted to the treated eye. (9) In the latter case, although cells in both dorsal and ventral retina are capable of responding to exogenous TH, a greater proliferation response occurs in the ventral retina, giving rise prematurely to an asymmetric pattern of retinal proliferation.…”

“…This could be accounted for by the mitogenic activity that TH has on the retinal margins. (9,12,32) In the injected eye, TH induced the proliferation and the production of Eph-expressing retinal ganglion cells that are guided along the ipsilateral retinal pathway, whereas in the non-injected eye of PTU-reared animal, the reduced proliferation rate was not sufficient to give rise to a significant number of ipsilaterally projecting ganglion cells.…”

Mitotic Variations in the Lens Epithelium of the Frog

“…Serum TSH has yet to be assayed. Kaltenbach and Hobbs [1972] have shown that eye implants of choles terol-thyroxine pellets stimulate mitotic activity in larval lens, retina and cornea; the part played by the thyroid has been quite rigorously demonstrated for amphibian metamorphosis [Etkin, 1968;F rieden, 1967] in general. Fur ther radioimmunologic assays are planned since our past work has been done at only one interval.…”