Hox Gene Expression within the Embryonic Skin Is Dynamic

Hox genes are usually expressed temporally and spatially in a colinear manner with respect to their positions in the Hox complex. Previous studies have shown that within the skin Hox genes are expressed either in a regionally restricted or in a regionally unrestricted manner. To test whether other genes of the Hox family display expression patterns which are spatially restricted (Hoxc-8) or spatially unrestricted (Hoxc-13) we studied the expression of Hoxa-7, a-11, b-4, c-6, c-8, and d-4 to d-13 within the dorsal skin of chick embryos. We confirmed the findings for Hoxc-8 and found that Hoxb-4 and Hoxa-7 are also expressed in a spatially restricted manner in the skin. These genes exhibit colinearity. In contrast, the Hoxd genes as well as Hoxa-11 and Hoxc-6 are expressed ubiquitously within the skin. Expression of the restricted genes is initiated in the epidermis by E5. Epidermal expression of the unrestricted genes is not temporally colinear as they are all expressed concomitantly at E6.25. Dermal expression for both groups occurs later at E6.5, but maintains the same anterior-posterior patterning as that seen in the epidermis. During E7-E8, expression for all genes is up-regulated within the dense dermis while being reduced within the interbud regions. Later, expression within the bud mesenchyme is down-regulated while high levels of transcriptional activity are detectable within the epidermal sheath of each feather bud. These results suggest that Hox genes may play an important role in patterning the embryonic skin by providing regionally restricted positional cues as well as by imparting generic signals necessary for feather morphology.