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ZLI
P1
M
P2
P3
I
r1
r2
r3
r4
IsO
TEL
HY
Cephalic
flexure
ANR
r5
r6
Cervical
flexure
r7
r8/sc
Figure 3.19
Representation of a lateral view of an embryonic day-10.5 mouse neural tube
showing the main neuronal regions and the transverse segments of the neural tube in relation
to the secondary organizers.
Abbreviations
: ANR, anterior neural ridge; AVE, anterior ventral
endoderm; AM, axial mesoderm; DI, diencephalon; M, midbrain; HY, hypothalamus; I,
isthmus; IsO, isthmic organizer; RH, rhombencephalon; TEL, telencephalon; ZLI, zona
limitans intrathalamica; p1-p3, prosomeres; r1-r8, rhombomeres; sc, spinal cord.
Source
: From
Vieira et al. (2010)
.
In some species, like chick embryos, primary neurulation occurs in the anterior parts of
the trunk and secondary neurulation in the caudal parts (
Osorio et al., 2009
).
Just prior to the closure of the neural folds, the neural crest cells (NCCs) delami-
nate from the neural tube and begin their epigenetically programmed migration. The
anterior part of the neural tube then forms three bulges, the prosencephalon (fore-
brain), mesencephalon (midbrain), and rhombencephalon (hindbrain). The prosen-
cephalon differentiates into the anterior secondary prosencephalon (telencephalon
and hypothalamus) and the more caudal diencephalons (
Pombero and Martinez,
2009
), from which two secondary bulges extend laterally to form the eye Anlagen or
optic vesicles. At the boundary between the hindbrain and midbrain, in the isthmus
region, a secondary organizing center referred to as an
isthmic organizer
develops at
this stage.
The prosencephalon and the rhombencephalon consist of segmental units respec-
tively called
prosomeres
and
rhombomeres
(
Figure 3.19
).
At the beginning of the
Xenopus
late tailbud (phylotypic) stage (stages 29-44),
the maternal serotonin is almost exhausted. But immediately after stage 32, the level
of serotonin increases dramatically. This coincides with the formation of axons by
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