Fig. 6. (A-F) Misexpression of dominant-negative Tbx5 leads to downregulation of anterior mesenchymal markers. (A) In the contralateral control, uninjected limb Msx is expressed in the distal mesenchyme of the limb just under the AER at stage 21. (B) In the dominant-negative Tbx5-(Tbx5^en) injected limb, Msx is downregulated in the distal mesenchyme, most obviously in the anterior of the limb. (C) Lhx9 is expressed in the anterior limb mesenchyme in the control limb at stage 19 and (D) in the Tbx5^en-injected limb, expression is downregulated (arrowed). (E) Hoxc4 is expressed in the anterior limb mesenchyme of a control limb (stage 20) and (F) is downregulated in the Tbx5^en-injected limb. (G-J) Misexpression of dominant-active forms of Tbx5 (Tbx5^vp16) leads to an anterior expansion of the limb. (G) Fgf8 expression in the uninjected control limb (stage 20). (H) After misexpression of Tbx5^vp16, the domain of cells expressing Fgf8 in the AER is expanded anteriorly (arrow). (I) Expression of Lhx9 in the anterior of the control limb at stage 20. (J) After misexpression of Tbx5^vp16, Lhx9 expression is expanded anteriorly (arrow). In A,C,E,G,I, the image of the contralateral uninjected control limb has been reversed to provide a clearer comparison with the respective experimental, injected limbs in B,D,F,H,J. A, anterior; P, posterior. (K) A model for Tbx5 function in the developing limb bud. Tbx5 is first expressed as limb bud initiation commences in response to axial cues. Tbx5 is acting genetically upstream of Fgf10, and is required for Fgf10 expression in the limb mesenchyme. Later, Tbx5 is also required for the positive inductive loop between Fgf8, which is expressed by cells in the AER, and Fgf10, which is expressed by cells of the distal mesenchyme.