Biology Reference
In-Depth Information
4. ASYMMETRIC CRANIOFACIAL REMODELING AND THE
BASIS OF EYE MIGRATION
The metamorphic remodeling of the flatfish skull is perhaps the most
striking example of asymmetric postembryonic development in the verte-
brate world, a process that ultimately accommodates eye migration and
the transition from a pelagic to a benthic lifestyle. In adult winter flounder
(
P. americanus
), the orbit of the migrated eye is formed entirely of bones
whereas that of the nonmigrated eye consists partially of jaw muscles
(
Ballard et al., 1987
). The orbital bones of the migrated eye induce a deflec-
tion in the path of the extraocular muscles prior to insertion, causing the
lateral and medial rectus muscles from the migrated eye to be shorter than
those of the opposite eye (
Ballard et al., 1987
). These extraocular muscle
asymmetries are clearly necessary to accommodate the final eye position,
though it is not known if changes in extraocular muscle length play an active
role in actually promoting eye migration. The rearrangement of cranial
elements to accommodate eye migration and transition to a benthic habitat
is known to be accompanied by the development of asymmetric movements
of the jaws during prey capture for some flatfish species (
Francis & Turingan,
2008; Gibb, 1995, 1996, 1997, 2003
).
Most of the metamorphic changes in cranial cartilage and bone appear to
be conserved across flatfish species (
Brewster, 1987; Chapleau, 1993; Saele,
Silva, & Pittman, 2006
), and the major events of asymmetric neurocranial
remodeling that have been described include (1) asymmetric growth of
the ethmoid plate and frontal bones that twist toward the future ocular di-
rection (
Brewster, 1987; Chapleau, 1993; Norman, 1934; Saele, Silva, et al.,
2006; Saele, Smaradottir, & Pittman, 2006; Schreiber, 2006; Wagemans,
Focant, & Vandewalle, 1998; Wagemans & Vandewalle, 2001
) (also see
Fig. 6.2
D-H); (2) differential resorption of the left and right cartilaginous
supraorbital bars (
Okada, Takagi, Seikai, Tanaka, & Tagawa, 2001;
Schreiber, 2006; Wagemans et al., 1998
); (3) the proliferation of a dense cell
population of dermal fibroblasts specifically under the migrating eye (
Bao
et al., 2011; Okada, Takagi, et al., 2003; Okada et al., 2001; Saele, Silva,
et al., 2006
; also see
Fig. 6.7
); (4) the deposition of osteoblasts within the
dense cell layer to form the anterior flange of the blind side frontal (dermal
bone), termed the “postlateral ethmoid” by
Hoshino (2006)
(and referred to
previously as the “pseudomesial bar”—see
Okada et al., 2001; Traquair,
1865
), a structure unique to flatfishes that eventually covers the area vacated
