Biology Reference
In-Depth Information
A
B
C
Counteract force
Push force
Proliferating cell
Figure 6.8 Schematic drawing to explain eye migration in flatfish. (A) Before eye migra-
tion has initiated, cells located in the suborbital area exhibit asymmetric proliferation
between the left and right sides. (B) Once the migrating eye receives sufficient pushing
force from proliferating cells in its suborbital tissue, it begins moving in the dorsal di-
rection. (C) By the end of metamorphosis, both the migrating eye and the nonmigrating
eye have experienced a 90
rotation from their initial orientations, with the migrating
eye facing a dorsal trajectory, and the nonmigrating eye facing a ventral trajectory.
Reproduced in altered form with permission from
Bao et al. (2011)
.
the eye can no longer translocate. It is possible that inhibition of suborbital
proliferation also prevents the development of the postlateral ethmoid (der-
mal bone produced by the suborbital tissue),
indirectly inhibiting eye
migration.
Endocrine signals clearly mediate the progression of metamorphic skull
remodeling. Insulin-like growth factor I receptors have been shown to be
expressed in fibroblasts, frontal bone osteocytes, and dorsal chondrocytes
at the beginning of halibut metamorphosis (
Hildahl, Power, Bjornsson, &
Einarsdottir, 2008
), as well as in the dense cell population of dermal fibro-
blasts under the migrating eye (
Zhang, Shi, Cheng, & Chen, 2011
). Ulti-
mately, elevated levels of TH are necessary to induce eye migration and
skull remodeling, and if endogenous levels of TH are chemically ablated,
flatfish retain a bilaterally symmetrical skull (
Okada, Tanaka, et al., 2003;
Schreiber, 2006
). Interestingly,
Tagawa and Aritaki (2005)
have shown that
in the spotted halibut (
V. variegatus
) the timing of responsiveness to TH dif-
fers between the left and right sides of the larval body. Hypothyroid larvae
(held in developmental stasis by treatment with the goitrogen thiourea)
metamorphosed with two distinct bilaterally symmetrical phenotypes when
treated with exogenous TH at different times: (1) an ambicolorate pheno-
type in which both sides possessed characteristics of the ocular side when
treated with TH prior to 15 days after hatching (DAH) or after 60 DAH
and (2) a symmetrical pseudoalbino phenotype where both sides possessed
characteristics of the blind side when treated with TH beginning 25
DAH (
Tagawa & Aritaki, 2005
). These findings suggest that both sides of
