90
Brain Development
tissue can be generated by the serial induc-
tive actions of surrounding tissues prior to
gastrulation without the organizer and/or
axial mesoderm signals. For example, even
surgical ablations of the notochord as well
as genetic manipulations to mutate genes
expressed in the organizer, that harbors a
role in the formation of the notochord, re-
sult in neural tissue formation. Recently,
using early chicken embryos as a good
Primitative
streak
Primitive streak
Node(EGO)
Extra-
embryonic
mesoderm
Axial mesoderm
Lateral mesoderm
Lateral mesoderm
Axial mesoderm
Paraxial
mesoderm
Paraxial mesoderm
EGO
ne
Anterior
epiblast
AVE
Primitive
streak
Extraembryonic ectoderm
Epiblast
VE
Cripto
Proximal
Distal
Posterior
Anterior
ap
Node
Bottle cel s
may help
involution
Blastpore
Dorsal lip
Blastula
Gastrula
Donor
Transplanted tissue mainly contributes
to the secondary notochord
Formation of secondary neural axis
by host cel s
Host
Transplanted
dorsal lip
Neural
Epidermal
Epidermal
BMP
BMP
BMP
BMP
BMP
BMP
Chordin
Noggin, Cerberus,Fol istatin etc.
' Organizer"
(a) Amphibian
Neural
Neural
Epidermal
Epidermal
Area opaca
Hensen's node
Primitive
streak
Late-gastrula
Blastula
Mid-gastrula
a
n
n
N
p
Medial epiblast (Wnt
)
Lateral epiblast (Wnt
++
)
Fgf
Fgf
FGF
FGF
WNT
WNT
Bmp
Bmp
BMP
BMP
(b) Chicken
(c) Mouse
E5.5
E6.5
E7.5
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