FIGURE 4.

Fgf8 and immediate responsive gene expression is lacking during SCE ontogeny. (a) Fgf8 expression in E9.5 wild-type and Fgf8^null/Neo littermates. Expression in the commissural plate (cp) is diminished (green arrowhead) while ventro-lateral ectodermal (vle) expression is completely abrogated (red arrowheads). (b, c) FNP expression of Spry1 (b) and Pea3 (c), two Fgf8-responsive genes, is reduced or lost at E10.5 (compare yellow arrowheads) and weaker in the cp (green arrowhead). (d) Lateral and frontal views of E9.5 wild-type and Fgf8^null/Neo littermates showing expression of Alx3, a marker of the ectomesenchyme yielding the elements of the frontonasal and rostral trabecular skeleton, which is clearly expressed in a ring around the eye (orange arrowheads) and subjacent to the Fgf8-less vle, although Alx3-positive cells were found ectopically placed across the midline (red arrowhead) in mutants.