Biomedical Engineering Reference
In-Depth Information
zebrafish larva is amenable to medium-to-high-throughput in vivo screening due the
following attributes:
.
Relative ease of maintaining large stocks of animals and their high fecundity.
.
Larvae can live in only 200
L of fluid; therefore, screening can be undertaken
in multiwell plates and only milligrams of compound are needed.
.
Zebrafish are DMSO tolerant and can readily absorb compounds from the
water in which they swim.
.
Rapid embryonic development ex utero, which facilitates experimental ma-
nipulation and allows the direct observation of organ function in vivo.
m
These properties have established the zebrafish as an in vivo model system that is
relevant to studies of human diseases (Zon and Peterson, 2005; Lieschke and
Currie, 2007) and for the assessment of safety liabilities (Barros et al., 2008). Thus,
in vivo analysis of the effects of compounds, for example, on visual function, can be
undertaken at much earlier stages in the drug development process (hit to lead or lead
optimization) with lower compound requirement and at a higher throughput in
zebrafish larvae than is usually possible with rodents.
15.2 DEVELOPMENT OF VISUAL SYSTEM IN
ZEBRAFISH
The zebrafish retina is structurally very similar to the human retina and therefore the
zebrafish visual system has been evaluated for both the modeling of eye diseases
(Goldsmith, 2001) and the assessment of the effects of drugs on visual function.
Zebrafish have a cone dense retina and thus, like humans, have rich color vision,
providing a potential advantage over testing compounds for effects on visual function
in nocturnal rodents, which have rod-dominant retinas. Figure 15.1 shows the cell
layout in an embryonic zebrafish retina (a) and a comparison of sections through an
adult human (left) and embryonic zebrafish (right) retina (b). The relative positions of
the various cell types are the same in both human and zebrafish retinas.
Visual system development is very rapid in zebrafish embryos and is imperative
for orientation and to enable predator avoidance and feeding behavior. The initial
event of eye development is the evagination of the optic lobes from the diencephalon
at around 10 h post fertilization (hpf). The optic primordium appears at about 12hpf
(Schmitt and Dowling, 1994) and the first ganglion cells appear in the ventronasal
retina at 30hpf (Schmitt and Dowling, 1994; Burrill and Easter, 1995). Cone outer
segments appear at 60hpf in a restricted area of ventral retina (Branchek and
Bremiller, 1984) and then gradually elongate. Signal transmission from photorecep-
tors to second-order neurons starts around 3.5 days post fertilization (dpf) and is fully
functional at 5dpf (Biehlmaier et al., 2003). The earliest quantifiable visual behavior is
the visual startle response, whereby larvae respond to a sudden decrease in illumi-
nation with a rapid body movement. This behavioral response starts at around 68hpf,
just at the time when outer segments of photoreceptors and synaptic ribbons have
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