Development of myotomal cells in Xenopus laevis larvae.

Electron microscopic appearances of Xenopus laevis axial myotome cells were examined through Stages 22-41 (Nieuwkoop & Faber, 1956). Differentiated contractile structures were not observed in myotome cells at or before Stage 22. At Stages 24-25, myofibrils appeared both as disordered strands in association with tubular membranes, and in different degrees of assembly into ordered sarcomeres. By Stage 28, all contractile organelles observed were organised into sarcomeres. The latter extended the length of each myocyte by Stage 35. Myofibrils were initially laid down adjacent to the cell membrane in each myocyte close to its lateral surface. They filled most of each cell by Stage 41. Membrane structures known to be associated with contractile activation in adult muscle appeared early in development. Transverse tubular, and sarcoplasmic reticular membranes, and 'triad' complexes could be demonstrated from the outset of sarcomere formation at Stage 24. In places, establishment of a regular repeating tubular system appeared to precede myofilament organisation. Examination of sections stained with ruthenium red during preparation suggested that tubular and surface membranes were continuous even at Stage 24 and all subsequent developmental stages studied here. Earlier work (Huang, 1986) has suggested that excitation-contraction coupling in embryonic muscle involves release of stored calcium in response to tubular voltage changes as in adult muscle. These findings corroborate such physiological observations.