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Fig. 8. Model for FGF3 function in otic morphogenesis initiation. Oblique
view of the developing hindbrain and otic vesicle during initiation of otic
morphogenesis. The model integrates present data with those published
previously. Otic vesicle molecular patterning and morphogenesis are initiated
by WNT signals provided redundantly by roofplate-expressed Wnt3a and
Wnt1 (dark blue). This signal induces otic Gbx2, which in
turn induces Wnt2 and Dlx5 at a minimum, and is required for
dorsal outgrowth of the EDS anlage. Fgf3, induced in r5-6 by
Hoxa1 and Mafb [Mafb (Kr)], serves to
maintain otic Gbx2 and its downstream genes, but has no direct effect
on ventral otic genes (Pax2 and Otx2). In addition,
Fgf3 prevents medial expansion of otic Hmx3 and ventral
expansion of hindbrain Wnt3a. The role of sonic hedgehog (SHH) in
ventral otic patterning via induction of Pax2 and Otx2 is
illustrated, but was not addressed here.