A new view of patterning domains in the vertebrate mesoderm.

The musculoskeletal system of vertebrates is derived from the embryonic mesoderm. Its structures are categorized as epaxial or hypaxial based on their adult position and innervation. The epaxial/hypaxial terminology is also used to describe regions of the embryonic somites based on fate mapping of somitic derivatives. However, the adult, functional distinctions are not fully consistent with the changing embryonic environments of mesodermal populations during morphogenesis, and the traditional terminology loses accuracy when used to describe certain mutant phenotypes. Here we describe a new terminology naming two mesodermal environments defined by the lineage of the included cells. We discuss how mutant phenotypes may be better explained by consideration of the embryonic context in which genes take their effect and argue that the recognition of these embryonic territories clarifies description and discussion of the morphogenesis and patterning of the musculoskeletal system.