Second branchial arch lineages of the middle ear of wild-type and Hoxa2 mutant mice.

Our current understanding of the evolution of the mammalian middle ear was first suggested by embryological studies from the 19th century. Here, site-specific recombinase-mediated lineage tracing was used to define the second branchial arch contribution to the middle ear of wild-type and Hoxa-2 mutant embryos. The processus brevis of the malleus was found to arise from second arch tissues, making it the likely homologue of the retroarticular process of nonmammalian tetrapods. The second arch also formed a portion of the otic capsule. In light of avian lineage studies, second arch cells were probably incorporated into the otic capsule before avian and mammalian lineages diverged. In Hoxa2 mutant embryos, middle ear skeletal duplications occurred at sites where first and second arch elements are normally apposed. The dorsoventral positions at which second arch skeletal elements formed and the early migration of second arch neural crest cells were not altered by the absence of Hoxa2 function.