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Fig. 1. Excess anterior FGF8 downregulates WNT gene expression in the hem. (A)
E11.5 cerebral hemisphere viewed from the medial face, anterior towards the
left, processed for two color in situ hybridization. The cortical hem is
marked by Wnt3a expression (blue); Fgf8 expression (brown)
is separated from the hem by a short gap (arrow). (B-L) Brains electroporated
anteriorly at E11.5 with Ap or Fgf8 and analyzed at E13.5.
B-D and K,L are coronal sections; E-J are hemispheres viewed from the medial
face. (B,C) In adjacent sections from the same brain, ectopic expression of
FGF8 (B) obliterates Wnt3a expression in the hem (C, black arrow
indicates normal Wnt3a expression site). (Note endogenous
Fgf8 expression ventrally in B.) Wnt2b expression fills the
cortical hem of an Ap electroporated brain (E) but is lost anteriorly
in a brain electroporated with Fgf8 (arrow, F). Wnt5a shows
a similar loss of expression (I, arrow in J). In contrast to WNT gene
expression, Bmp4 expression is not downregulated at this age by
excess FGF8 (D, white arrow indicates site of excess FGF8), nor is expression
of Msx1, a indicator of BMP activity (G,H). Overexpression of FGF8
(L) has no apparent effect on Foxg1 expression at this age,
indicating that the experimental protocol does not cause general damage. The
boundary between the hem and the rest of the cortical neuroepithelium not
obviously affected (K). Scale bar: 0.8 mm for A,E-J; 0.17 mm for B-D; 0.12 mm
for K,L.
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