Tissue and Cell 41 (2009) 286–298
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Tissue and Cell
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A light and transmission electron microscope study of the distribution and
ultrastructural features of peripheral nerve processes in the
extra-retinal layers of the zebrafish eye
G.B. Chapman
a
, R. Tarboush
b,∗
, D.A. Eagles
a
, V.P. Connaughton
b
a
Department of Biology, Georgetown University, Washington, DC, United States
b
Department of Biology, American University, Washington, DC, United States
article info
Article history:
Received 29 August 2008
Received in revised form 6 December 2008
Accepted 24 December 2008
Available online 28 February 2009
Keywords:
TEM
Zebrafish
Peripheral nerve ultrastructure
abstract
The distribution and ultrastructural features of peripheral nerve processes in the extra-retinal layers of
the eyes of the zebrafish, Danio rerio (Hamilton), were investigated using light and transmission electron
microscopy. A comparative study of the quality of preservation provided by three different fixation proce-
dures revealed no consistently striking general differences. However, somewhat subjectively, the fixative
containing Millonig’s buffer did consistently provide better fixation of myelin. Overall, nerve processes,
depending on the site studied, were distributed as either (1) bundles (in the choroid near the optic nerve
head and in the choroid adjacent to the limbus), (2) linear arrays (in the junction between the sclera and
cartilage and in the choroid adjacent to the retina) or (3) individual units (in the choroid under the car-
tilage or in the sclera). Both myelinated and unmyelinated processes were identified in these locations.
Myelinated processes usually contained both neurofilaments and neurotubules, but a few apparently
contained only neurofilaments. Unmyelinated processes usually contained mainly neurotubules, but a
few apparently contained only neurofilaments. Taken together, these findings indicate innervation of
extra-retinal structures, as seen in zebrafish, is highly conserved among vertebrates, further supporting
the use of zebrafish as a model for the vertebrate visual system.
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
The vertebrate eye receives a rich supply of both somatic (sen-
sory) and autonomic innervations. In mammals, there are at least
six cranial nerves (CN) associated with the eye. Axons of retinal gan-
glion cell neurons gather slightly off the centre of the retina at the
optic disc to form the optic nerve (CN II), which carries visual infor-
mation to the brain. Cranial nerves III (oculomotor), IV (trochlear),
and VI (abducens) provide motor innervation to the extra-ocular
muscles and subserve eye movement. The oculomotor nerve also
provides parasympathetic input to the pupillary constrictor mus-
cle and the ciliary muscle of the lens, while the facial nerve (CN
VII) provides parasympathetic input to the choroid (reviewed by
Lutjen-Drecoll, 2006). Furthermore, it has been shown that the
cornea receives sensory innervation from the ophthalmic branch
of the trigeminal nerve (CN V) and that the choroid, iris and cornea
receive sympathetic innervation from the superior cervical gan-
glion (Marfurt et al., 1989; Cavalotti et al., 2005).
∗
Corresponding author. Tel.: +1 202 885 2176; fax: +1 202 885 2182.
E-mail address: ra1458a@american.edu (R. Tarboush).
Innervation of extra-retinal layers of the eye (which include
structures such as the iris, ciliary body, cornea, choroid, and blood
vessels) has been described in a number of higher vertebrate
species. Sensory inputs to the eye, which originate from the trigem-
inal nerve, have been studied in rats (Cavalotti et al., 2005), humans
(May, 2004), and cats (Heppelmann et al., 2001). Sympathetic and
parasympathetic innervations to the choroid, ciliary body, and iris
are known in humans (Trivino et al., 2002; May, 2004; Lutjen-
Drecoll, 2006), rats (Klooster et al., 1996), rabbits (Butler et al.,
1984), and birds (Schrodl et al., 2001).
In contrast, numerous studies in fish have focused exclusively
on describing the structure of the optic nerve as new axons are
added throughout life. In particular, the growth and organization
of optic nerve fibres have been described in adult striped mojarra,
Eugerres plumieri of the Perciformes order, family Gerreidae (Tapp,
1974), adult and juvenile goldfish, Carassius auratus (order Cyprini-
formes, family Cyprinidae; Easter et al., 1981, 1984), and the tench,
Tinca tinca (order Cypriniformes, family Cyprinidae; Lillo et al.,
1998). Further, the arrangement of retinotectal projections has been
described in a few teleost species from the order Cypriniformes,
family Cyprinidae, namely, the goldfish (Bunt, 1982), zebrafish
(Schmitt and Dowling, 1999) and carp (Wolburg and Bouzehouane,
1986), as has eye growth which, in zebrafish, follows stages of pat-
0040-8166/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tice.2008.12.003