Fig. 4. Middle-ear skeletal phenotype of Hoxa2^-/-; Six2^-/- and Hoxa2^-/-;Six2^+/- mutant mice. (A-E) Dissected middle-ear skeleton from wild-type (A), Hoxa2^-/- (B), Hoxa2^-/-; Six2^-/- (C,D) and Hoxa2^-/-; Six2^+/- (E) E18.5 embryos. (A) Malleus (m), incus (i), gonial bone (g) and tympanic ring (t) are derived from first arch; stapes (s) is second arch-derived. (B-E) In the absence of Hoxa2, the stapes disappears and duplicated first arch elements form in the second arch (asterisks). The gonial bone abnormally extends to connect the wild-type tympanic ring with its duplicated counterpart. (C) Removal of Six2 rescues the ectopic growth of the gonial bone. The malleal duplication is also reduced (compare double-ended arrows in B and C). (D) The extent of the rescue of the ectopic growth of the gonial bone in Hoxa2^-/-; Six2^-/- was variable, but the ectopic gonial bone, when present, never extended to reach the duplicated tympanic ring (arrowhead), as invariably observed in Hoxa2^-/- embryos (arrowhead in B). (E) Complete reversal to a wild-type gonial bone was observed also in Hoxa2^-/-; Six2^+/- embryos.