Visualizing the lateral somitic frontier in the Prx1Cre transgenic mouse.

Changes in the organization of the musculoskeletal system have accounted for many evolutionary adaptations in the vertebrate body plan. The musculoskeletal system develops from two mesodermal populations: somitic mesoderm gives rise to the axial skeleton and all of the skeletal muscle of the body, and lateral plate mesoderm gives rise to the appendicular skeleton. The recognition of embryonic domains resulting from the dynamics of morphogenesis has inspired new terminology based on developmental criteria. Two mesodermal domains are defined, primaxial and abaxial. The primaxial domain includes musculoskeletal structures comprising just somitic cells. The abaxial domain contains somitic myoblasts in connective tissue derived from lateral plate mesoderm, as well as lateral plate-derived skeletal structures. The boundary between these two domains is the lateral somitic frontier. Recent studies have described the developmental relationship between these two domains in the chick. In the present study, we describe the labelling pattern in the body of the Prx1/Cre/Z/AP compound transgenic mouse. The enhancer employed in this transgenic leads to reporter expression in the postcranial, somatic lateral plate mesoderm. The boundary between labelled and unlabelled cell populations is described at embryonic day (E)13.5 and E15.5. We argue that the distribution of labelled cells is consistent with the somatic lateral plate lineage, and therefore provides an estimate of the position of the lateral somitic frontier. The role of the frontier in both development and evolution is discussed.