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Fig. 8. Summary and model for Fgf-dependent retinal NT patterning. (A) In normal
embryos, fgf8 is expressed in the telencephalon (tel, in blue) at the
5-somite stage, medial and asymmetric to the evaginating optic vesicle (ov).
The future nasal (N) retina (green) lies proximal to and the future temporal
(T) retina (pink) lies distal to the telencephalic signaling source. Fgf8 from
the telencephalon activates nasal cell fates in the proximal optic vesicle and
thereby positions the NT boundary. (B) In ace, nasal fates are
reduced and the retinal NT boundary shifts proximally/nasally owing to the
lack of the Fgf8 signal, but an unknown Fgf signal (Fgfx) maintains some NT
patterning. (C) Inactivation of all Fgf-receptor (Fgfr) signaling leads to a
complete expansion of the temporal/distal cell fates throughout the retina at
the expense of all nasal/proximal fates, as the telencephalic Fgf signal
cannot activate the Fgf pathway in the optic vesicle. (D) Ectopic expression
of Fgf8 by bead implantation locally induces nasal/proximal fates at the
expense of temporal/distal fates. (E) Fgf8 activates spry4 in the
diencephalon and proximal optic vesicle, which indicates active Fgf signaling
and a possible propagation route for Fgf8 along the dorsal-ventral axis of the
forebrain and proximal-distal axis of the optic vesicle. npp, nasal placode
precursors.
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