What mechanisms control neoteny and regulate induced metamorphosis in urodeles?

The Mexican axolotl, like a number of other urodele species, is an obligatory neotene, completing its full life cycle without metamorphosis. Metamorphosis can be induced with thyroid hormone, thyroid stimulating hormone, or stimulation of hypothalamic neurons. Thus, neoteny represents a deviation from the standard course of amphibian ontogeny, affecting the thyroid axis at one or more levels. Analysis of the thyroid axis at strategic ontogenic stages and after completed neotenic development suggests that there are a number of deviations, and that the deviations may be temporal and/or quantitative in nature. A surge of thyroxine secretion occurs early in larval life but does not lead to metamorphosis, apparently because the enzyme which deiodinates thyroxine to the active form, triiodothyronine, is not yet present. Later in ontogeny, activity in the thyroid axis is low. Hormone treatment can reactivate the thyroid axis at all levels, but some singularities remain. Inhibition at central nervous or peripheral levels may be involved in axolotl neoteny.