Brain Development
105
Host
Embed a bead
containing FGF8
or
Graft the isthmus in
the forebrain
fb
mb
hb
Isthmus
Donor
FGF8 expression
(a)
Induction of the ectopic tectum
Grafted tissue
or a bead
Aa
p
P
Pax2
FGF8
Wnt1
En1
A-P patterning molecule in the midbrain
(e.g., Axon guidance molecules such as Eph/ephrins)
Emx2
Pax6
OB
cad8
cad6
cad8
Mouse neocortical map
M2
M1
S1
V2
V1
A1
S2
Wild type
Emx2 -/-
Pax6 -/-
FGF8 is normally expressed at
the anterior neocortical primordium
Experimental side
Reversed S1 is induced
Ectopic FGF8 expression
at the posterior primordium
by electroporation
Control side
(b)
(d)
(c)
Fig. 8
A–P patterning of the midbrain and mammalian neocortex
(a) The role of the isthmic organizer in midbrain patterning. Fgf8 is a
component of the organizer. See details in the text. fb, forebrain; hb,
hindbrain; mb, midbrain, (b) Dorsal view of the mouse postnatal
neocortex. The mouse neocortex can be subdivided into several
functional areas, which are delineated by differential expression of type II
cadherins. A, auditory cortex; cad, cadherin; M, motor cortex; OB,
olfactory bulb; S, somatosensory cortex; V, visual cortex,
(c) Countergradient of Pax6 and Emx2 in the neocortical primordium is
important to bring out differences among neocortical areas later in the
development, (d) Fgf8 expressed at the anterior neocortical primordium
is a crucial organizer to pattern the mouse neocortical areas.
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