title: Excessive numbers of axons after early enucleation and blockade of metamorphosis in the oculomotor nerve of Xenopus laevis.
abstract: The number of axons in the oculomotor (OM) nerve of Xenopus laevis tadpoles was counted in unoperated and in unilaterally enucleated animals, raised in 0.4 g/l thiourea (TU), a thyroid-blocking agent, which arrested their development at premetamorphosis. In unoperated animals the number of axons starts to decrease with metamorphosis. When raised in TU, the tadpoles do not metamorphose and show no axon loss; rather, there is a moderate increase in axon number (13%) after 6 months of thiourea-treatment. Thus metamorphosis is necessary for the initiation of axon loss. In unilaterally enucleated tadpoles, increased axon loss occurs at metamorphosis in the OM nerve of the operated side, but the contralateral OM nerve shows no loss at all. When these animals are treated with TU, there is, as compared with the effects of the TU-treatment in unoperated animals, an increase of 7% in the ipsilateral side and of 28% in the contralateral one. Thus thyroxine blockade prevents the effects of unilateral enucleation and induces an excessive number of axons during the period observed. We postulate that through blockade of metamorphosis, axon elimination is arrested, while their production continues, and that these effects are accentuated by manipulation of the axonal target.
mesh_terms: D000367:Age Factors; D000818:Animals; D001369:Axons / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D002452:Cell Count; D008675:Metamorphosis, Biological / Q000187:drug effects*; D008854:Microscopy, Electron; D024510:Muscle Development; D009186:Myelin Sheath / Q000187:drug effects / Q000502:physiology; D009473:Neuronal Plasticity / Q000187:drug effects*; D009799:Ocular Physiological Phenomena; D009801:Oculomotor Muscles / Q000187:drug effects / Q000254:growth & development; D009802:Oculomotor Nerve / Q000187:drug effects / Q000254:growth & development / Q000502:physiology*; D013890:Thiourea / Q000494:pharmacology*; D014982:Xenopus laevis / Q000502:physiology*
chemical_list: D013890:Thiourea