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by expression of sonic hedgehog among other factors (
Sire & Akimenko,
2004
). The processes of skin stratification have been studied in greatest
detail in zebrafish, but appear at least structurally similar in cichlids
(
Sire & G´raudie, 1983
). Keratins mediate some of the skin restructuring
events, and in flatfish, keratin expression is regulated directly by TH
(
Infante et al., 2007
), decreasing after metamorphic climax (
Campinho,
Silva, Sweeney, & Power, 2007
). The metamorphic transformation of
flatfish skin resembles the metamorphic restructuring seen in amphibians
(
Power et al., 2008
), in which keratin loci are also under TH control (
Page
et al., 2007
). Nevertheless, further research is needed to determine the
precise roles of TH in promoting integumentary metamorphosis of tele-
osts. In this regard, genetically tractable species such as zebrafish offer out-
standing potential for studying skin metamorphosis; indeed, a variety of
mutants with defects in the formation of scales and other postembryonic
integumentary cell lineages have been identified (
Harrisetal.,2008;Lang,
Patterson, Gordon, Johnson, & Parichy, 2009
). As many features of skin
development are conserved even with mammals, and mammalian skin un-
dergoes a similar period of increased stratification and barrier function
acquisition during fetal stages, studies of teleost skin metamorphosis may
have translational relevance as well (
Hoath & Maibach, 2003; Rakers
et al., 2010
).
4.3. Pigmentation
The formation of adult pigmentation is another common feature of teleost
metamorphosis. Some of the genetic mechanisms underlying metamorphic
pigmentation have been dissected in zebrafish, in which adult pigment pat-
terns result from the spatial arrangements of neural crest-derived black me-
lanophores, yellow xanthophores, and iridescent iridophores. During
embryonic stages of neural crest migration, precursors to adult pigment cells
are established in part owing to ErbB signaling (
Budi et al., 2008; Hultman
et al., 2009
). Subsequently, these precursors are associated with peripheral
nerves and, at metamorphosis, migrate to the skin along stereotypical path-
ways to form the adult pigment pattern, a process that can be directly visu-
alized owing to the existence of fluorescent lineage reporters and the relative
transparency of even metamorphic stage zebrafish (
Budi et al., 2011
).
Embryonic and adult pigment cells have partially nonoverlapping genetic
requirements, demonstrated by the phenotypes of several zebrafish mutants
in which normal embryonic pigment cells develop yet adult precursors
