Fig. 6.

T-box gene expression during limb development. T-box gene expression is present at the earliest stages of limb outgrowth when Tbx4 and Tbx5 are expressed in the hindlimb and forelimb buds, respectively. Tbx2 and Tbx3 are first expressed in the posterior margin of the forelimb bud and then in the anterior and posterior margins of both limbs, where they are largely overlapping. In addition, Tbx3 is expressed in the apical ectodermal ridge and T is expressed in the mesoderm immediately subadjacent to the apical ectodermal ridge. Tbx15 and Tbx18 are largely overlapping in the core of the limbs, whereas Tbx1 is expressed in the developing muscle masses. Finally, Eomes is expressed in a small domain of mesoderm at the base of the developing fourth digit, although no function has yet been assigned to this area of expression (Hancock et al., 1999). E9.5 and E12.5 are left lateral views and E10.5 is a transverse section through the hindlimbs. For clarity, expression in areas other than the limbs is not shown. AER, apical ectodermal ridge; DA, dorsal aorta; FL, forelimb; G, gut; H, heart; HL, hindlimb; NC, notochord; NT, neural tube; OV, otic vesicle.