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transverse optical cross-section at the level of the dorsal margin at 60% epiboly
Fig. 2 At the onset of gastrulation the expression of Fgf8 disappears progressively from the ventral margin and increases in dorsal marginal cells establishing a dorsal to ventral gradient of expression. At 60% epiboly a weak expression is still observed in EVL cells that are now located vegetaly to the marginal cells. Dorsally a strong expression is observed in presumptive paraxial mesodermal cell and in the deepest cells of the epiblast in direct contact with the involuted mesoderm. At mid-gastrulation, in addition to the marginal EVL and the embryo margin a novel site of expression appears in the presumptive hindbrain and the underlying cephalic paraxial mesoderm. A strong expression is also observed in ventral neurectoderm (presumptive floor plate). Expression of Fgf8 increases in presumptive hindbrain and disappears from paraxial cephalic mesoderm at late gastrula. Near the margin expression is strong in presumptive neurectodermis as well as in dorsal marginal cells (that give rise to axial structures).
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