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ET & Rx1
Pax6, Six3
ET & Rx1
Pax6, Six3
Lhx2, tll
Neural induction
stage 10.5
Fore-/Midbrain specification
stage 11
Eye field specification
stage 12.5
FIGURE 2.21 Summary model of eye field induction in the anterior neural plate. The top of the figure
shows dorsal views of the neural plate of Xenopus embryos at successively later stages of development from
left to right. Light blue indicates the neural plate, blue shows the area of Otx2 expression, and dark blue rep-
resents the eye field. The diagram shows the complex relationships among the eye-determining transcription
factors, including pax6, Rx1, Lhx2, Six3, Otx2, and tll . These genes act together to coordinate eye development
in this specific region of the neural plate. The bottom panels show examples of in situ hybridizations for
several eye transcription factors to show their specific patterns of expression in the presumptive eye-forming
region of the embryo. (Modified from Zuber et al., 2003)
eye field, the region of neural plate fated to become the
eye. Each of these factors is necessary for specification
and growth of the eye. Mutations in any one of these
transcription factors, including pax6, Rx1, Lhx2, Six3,
ET , all have devastating effects on the development of
the eye. Whereas overexpression of some of these
factors on their own can cause the formation of ectopic
eyes in Xenopus frogs, overexpression “cocktails” of
several of the factors together have much more
potency in inducing ectopic eyes (Zuber et al., 2003)
(Figure 2.21). Thus, several different transcription
factors may be necessary to control the expression of
genes necessary for development of such a complex
sensory organ as the eye.
and developing spinal cord are fluid filled chambers.
The surface of the tube, adjacent to the lumen, is known
as the ventricular surface, since eventually the lumen of
the neural tube goes on to form the ventricular system
of the mature brain. The progenitor cells for neurons
and glia of the CNS have a simple bipolar morphology
and initially span the width of the neural tube. As these
cells undergo mitotic divisions, they typically go
through the M-phase of the cell cycle at the ventricular
surface. The postmitotic immature neurons generated
from the progenitor cells migrate away from the ven-
tricular zone toward the margin of the spinal cord to
form the mantle layer (see Chapter 3).
At the neural plate stage, several mechanisms are
set in motion that will define the overall organization
of the neural tube. First, the most ventral part of the
neural tube becomes flattened into a distinct “floor-
plate.” Second, the most dorsal aspect of the neural
tube develops into a tissue known as the roofplate.
Third, a distinct fissure, the sulcus limitans , forms
between the dorsal and ventral parts of the neural tube
along most of its length (Figure 2.22). These structures
are an early sign that the neural tube is differentiating
The early neural tube consists only of undifferenti-
ated neural and glial progenitor cells. The neural tube
is essentially a closed system, and the brain vesicles
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