THE ROLE OF THYROID-HORMONE IN ZEBRAFISH AND AXOLOTL DEVELOPMENT

Exogenous thyroid hormone (TH) induces premature differentiation of th e zebrafish pectoral fins, which are analogous to the forelimbs of tet rapods. It accelerates the growth of the pelvic fins but not precociou sly, Goitrogens, which are chemical inhibitors of TH synthesis by the thyroid gland, inhibit the transition from larva to juvenile fish incl uding the formation of scales, and pigment pattern; they stunt the gro wth of both pectoral and pelvic paired fins. Inhibition by goitrogens is rescued by the simultaneous addition of thyroxine. The effect of ad ding TH to the rearing water of the postembryonic Mexican axolotl was reinvestigated under conditions that permit continued growth and devel opment, In addition to morphological changes that have been described, TH greatly stimulates axolotl limb growth causing the resulting larva to be proportioned as an adult in about two months, This study extend s the known evolutionary relatedness of tetrapod limbs and fish fins t o include the TH stimulation of salamander limb and zebrafish fin grow th, and suggests that TH is required to complete the life cycle of a t ypical bony fish and a salamander at the same developmental stage that it controls anuran and flounder metamorphosis.