Induction of the ipsilateral retinothalamic projection in Xenopus laevis by thyroxine

Hormones are important in the development of behaviour and there is now abundant evidence that they also affect the morphological development of the nervous system. In principle, hormones could act by inducing new patterns of connectivity between widely separated structures in the nervous system or by influencing local connectivity. Most available work documents effects of the latter sort. We present here evidence for the former. Our results indicate that thyroxine, the hormone which causes metamorphosis in the frog Xenopus laevis, can induce precociously in a pre-metamorphic tadpole the ipsilateral retinothalamic projection, a retinofugal pathway which normally develops during metamorphosis. Our evidence suggests that the hormone's presence in one eye alone is sufficient to cause axons of some ganglion cells in that eye to grow to targets in the ipsilateral thalamus and to form terminal fields of appropriate morphology. Since the axons of the induced pathway project ipsilaterally, unlike axons in the normal premetamorphic tadpole, virtually all of which project contralaterally, our results are also relevant to questions concerning the control of axonal trajectory in the optic chiasm.