Patterning of fast and slow fibers within embryonic muscles is establishedindependently of signals from the surrounding mesenchyme

During embryonic development, and before functional innervation, a highly s tereotypic pattern of slow- and fast-contracting primary muscle fibers is e stablished within individual muscles of the limbs, from distinct population s of myoblasts, A difference between the fiber-type pattern found within ch icken and quail pectoral muscles was exploited to investigate the contribut ions of somite-derived myogenic precursors and lateral plate-derived mesenc hymal stroma to the establishment of muscle fiber-type patterns. Chimeric c hicken/quail embryos were constructed by reciprocal transplantation of somi tes or lateral plate mesoderm at stages prior to muscle formation. Muscle f ibers derived from quail myogenic precursors that had migrated into chicken stroma showed a quail pattern of mixed fast- and slow-contracting muscle f ibers. Conversely, chicken myogenic precursors that had migrated into quail stroma showed a chicken pattern of nearly exclusive fast muscle fiber form ation. These results demonstrate in vivo an intrinsic commitment to fiber-t ype on the part of the myoblast, independent of extrinsic signals it receiv es from the mesenchymal stroma in which it differentiates.