The Questions of Developmental Biology

The mechanism by which Hox genes could specify the proximal-distal axis is not yet
understood, but one clue comes from the analysis of chicken Hoxa-13. Ectopic expression of this
gene (which is usually expressed in the distal ends of developing chick limbs) appears to make
the cells expressing it stickier. This, in turn, would cause the cartilaginous nodules to condense in
specific ways (Yokouchi et al. 1995; Newman 1996).
As the limb grows outward, the pattern of Hox gene expression changes. When the
stylopod is forming, Hoxd-9 and Hoxd-10 are expressed in the progress zone mesenchyme
(Figure 16.15; Nelson et al. 1996). When the zeugopod bones are being formed, the pattern shifts
remarkably, displaying a nested sequence of Hoxd gene expression.