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Figure 3. Thyrotropic axis in amphibians. Note the change in the brain stimulatory neurohormone
between juvenile (CRH=corticotropin-releasing hormone) and adult (TRH=thyrotropin-
releasing hormone). TSH=thyrotropin; T4=thyroxine; T3=triiodothyronine.
1.3.2 Thyroid-hormone induced metamorphosis in other
vertebrates and chordates
This triggering effect of THs on metamorphosis was afterwards found also
in teleosts, (elopomorphs, Chapter 3 and pleuronectiforms, Chapter 4).
Concerning secondary metamorphosis, smoltifi cation in salmonids is also
partially controlled by changes in THs (Chapter 6). This suggests that TH-
induced metamorphosis may be a common regulatory mechanism among
vertebrates (Fig. 4). However, as highlighted in Chapter 2, in the lamprey,
a petromyzontidae, larval metamorphosis is driven in contrast by a drop
of TH, and not by an increase of TH levels (Chapter 2).
Recent data in amphioxus, a cephalochordate, showed that
iodothyronines induced metamorphosis by binding to a receptor
homologous to vertebrate thyroid hormone receptors (Paris et al. 2008).
These fi ndings suggested an ancestral origin of thyroactive compound-
induced metamorphosis in chordates (Denver, 2008) and supported a
defi nition of metamorphosis based on its hormonal control by TH-like
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