Biology Reference
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growth (McMenamin and Parichy, unpublished). In flatfish, GH receptors
peak in expression immediately prior to the onset of metamorphosis, as do
concentrations of IGF (
Hildahl et al., 2007
). The hormone prolactin antago-
nizes GH, and appears to have a negative influence on salinity tolerance and
overall smoltification in salmonids (
Bj
ยจ
rnsson, Stefansson, & McCormick,
2011; Madsen & Bern, 1992
). The GH/IGF axis thus plays important roles
in promoting growth and regulating metabolism during postembryonic de-
velopment in teleosts (
Yousefian & Shirzad, 2011
), and its potential roles in
directly stimulating morphogenetic events merit further investigation.
Another factor that may contribute to metamorphic progression is cortisol,
known primarily for its roles in stress response. Cortisol titer increases during
salmon smoltification, and may directly contribute to physiological metamor-
phosis (
Richman, de Diaz, Nishioka, & Bern, 1985; Specker, 1982
). Flatfish
also exhibit peak cortisol levels at metamorphic climax, potentially indicating
a stimulatory role in this process (
de Jesus et al., 1991
). Indeed, stress hormones
are known to contribute tometamorphosis of some anuran amphibians (
Denver,
1993, 1997
). In eel leptocephali, however, cortisol levels decrease prior tometa-
morphosis and remain low throughout metamorphosis (
Yamano, Tagawa,
et al., 1991
). Thus, stress hormones may play synergistic roles with TH during
the metamorphosis of some teleosts (
Dufour & Rousseau, 2007
), but these
roles require further investigation and particularly experimental manipulation.
In summary, it is clear that several hormonal axes are activated immedi-
ately before, during, and following metamorphic climax in the teleosts that
have been studied to date. These hormonal axes, and specifically TH path-
ways, likely play roles in orchestrating metamorphosis, and may integrate
external environmental cues into coordinated sets of disparate morphoge-
netic events. Nevertheless, the evolutionary conservation of these roles,
the interactions between the hormonal axes, and the proximate mechanisms
of endocrine response remain unclear.
4. LOCAL MECHANISMS OF MORPHOGENETIC CHANGES
The morphogenetic changes that occur at metamorphosis include ma-
jor remodeling of existing features as well as the formation of entirely new
tissues and organs; thus, metamorphosis requires extensive differentiation as
well as the morphogenetic processes of cellular migration, proliferation,
growth, and death. For the overwhelming majority of metamorphic events,
however, the underlying cellular mechanics and genetic mechanisms remain
unexplored by modern methods. Here, we discuss the relatively few areas in
