Migratory appendicular muscles precursor cells in the common ancestor to all vertebrates

In amniote embryos, skeletal muscles in the trunk are derived from epithelial dermomyotomes, the ventral margin of which extends ventrally to form body wall muscles. At limb levels, ventral dermomyotomes also generate limb-muscle precursors, an Lbx1-positive cell population that originates from the dermomyotome and migrates distally into the limb bud. In elasmobranchs, however, muscles in the paired fins were believed to be formed by direct somitic extension, a developmental pattern used by the amniote body wall muscles. Here we re-examined the development of pectoral fin muscles in catsharks, Scyliorhinus, and found that chondrichthyan fin muscles are indeed formed from Lbx-positive muscle precursors. Furthermore, these precursors originate from the ventral edge of the dermomyotome, the rest of which extends towards the ventral midline to form body wall muscles. Therefore, the Lbx1-positive, de-epithelialized appendicular muscle precursors appear to have been established in the body plan before the divergence of Chondrichthyes and Osteichthyes.