Biology Reference
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a thyroid-stimulating hormone in lampreys and the close relationship of
the hormone to gonadotropin (Sower et al
.
, 2009). This area of study is of
signifi cance for any interpretation of the interplay between the reproductive
and thyroid axes that may have infl uenced the evolution of metamorphosis
as a developmental strategy in lampreys (Youson and Sower, 2001; Youson,
2004). There is some evidence, from both laboratory and fi eld observations,
that there is certain degree of plasticity in the initiation of the onset of
metamorphosis in non-parasitic species that may refl ect their pathway of
evolution form parasitic species (Youson, 2004).
There are likely many factors other than THs that are critical to lamprey
metamorphosis (Fig. 4). One of these is undoubtedly water temperature
and, in particular, the stimulus from a rise in temperature, rather than
the magnitude of the rise. Neither photoperiod nor animal density
seem to be directly responsible for triggering metamorphosis, although
experiments with the photosensitive pineal organ seem to suggest some
involvement of this organ. Physiological preparation for the non-trophic
phase of metamorphosis has been clearly demonstrated for some species
where lipogenesis and fat storage are characteristic of immediately pre-
metamorphic larva and early stages of metamorphosis. Whether the
sites of fat storage release a factor, or hormone, as a signal of preparation
that stimulates a cascade of events affecting both the brain, thyroid and
reproductive systems remains to be resolved.
That lamprey metamorphosis is a true vertebrate metamorphosis
(Youson, 1988) is demonstrated by the magnitude of the internal and
external changes in all species. This organism has proven to be a useful
model system for studying developmental change in many systems and
organs. These changes include development of a new oesophagus that is
independent of the pharynx to permit tidal ventilation through the pharynx
when the adults are attached by the suctorial disc. The pharyngeal endostyle
of larva transforms into a thyroid gland with follicles. The remainder of the
alimentary canal is modifi ed for increased rates of absorption and ingestion
of a high protein diet of host fl esh or blood even if the post-metamorphic
animals never feed, as in non-parasitic species.
The liver loses its bile ducts and intrahepatic gall bladder in a biliary
atresia that mirrors some of the events of a human disorder of the same
name. Adult principal islet tissue develops during metamorphosis with
new endocrine cell types. A caudal principal islet in northern hemisphere
species is a consequence of transformation of bile duct cells. The larval
kidneys (opisthonephroi) undergo a complete regression and are replaced
by a defi nitive pair of opisthonephroi that develop from anlagen that
have resided within a nephrogenic cord behind the larval kidneys since
embryogenesis. The cartilaginous head and branchial skeleton of larva is
modifi ed and expanded to accommodate the suctorial disc, the appearance
