Hox-A2 protein expression in avian jaws cartilages and muscle primordia development

•By studying the expression of the Hox-A2 protein in chicken middle ear ossicle homologues, we confirm the potential involvement of the second arch in the construction of these elements, and further support the Reichert-Gaupp theory.•In the course of this work, we fortuitously observed expression of this protein in muscle primordia, suggesting a role for Hox-A2 in muscle development. To elucidate the branchial origin of the articular and the square (homology of the malleus and the incus of mammals), we used immunohistochemistry to analyse the expression of the Hox-A2 protein during cephalogenesis in chickens. Immunohistochemistry on paraffin sections of embryos from stage HH16 to HH40. In addition to the columella (equivalent to the mammalian stapes), the joint between the articular and the quadrate bones, and the retro-articular process of the articular (homologous to the short process of the malleus) express Hox-A2, suggesting an intervention of the 2nd arch in their formation. However, we fortuitously observed very intense expression within the early muscle plate of the second arch, which then generalized to all cephalic muscles, and extended to the trunk's myotomes. In the cartilage, the presence of the protein disappeared at stage 35. The present results, while confirming the contribution of the second arch to the development of avian equivalents of the mammalian ear ossicles, strongly suggest that the Hox-A2 gene plays a role in muscle development, which remains to be elucidated by more sophisticated techniques.