MYOFILBRILLOGENESIS AND Z-BAND DIFFERENTIATION

Developing myotubes in skeletal muscle tissue from the limbs of larval newts have been examined with respect to the ultrastructure and sequence of events accompanying myofibril formation. A population of filaments having a diameter of 100 Å is found to occur throughout and beyond the period of myofibrillogenesis. This population is in addition to developing actin and myosin filaments and probably does not contribute directly to myofibril formation. Rather it may represent a cytoskeletal network which ultimately becomes principally disposed around and at right angles to older myofibrils at the level of their Z‐bands. Assembly of thick and thin filaments into myofibrils seems to occur, in this muscle, predominantly near the periphery of the cell with registration of these components into A‐, I‐, and Z‐bands being accomplished as they assume progressively more internal locations. Z‐bands appear to develop by coalescence of Z‐bodies which in turn are earlier related to skeins of fine filamentous material which commonly occupy the most peripheral cytoplasm of these and other mesenchymally derived cells. Fine structural details of these skeins, Z‐bodies, and Z‐bands have been analyzed with regard to the several prevailing concepts of Z‐band architecture. An hypothetical sequence for myofibril formation and Z‐band differentiation is presented which takes into account several observations and relates them to the looping filament configuration previously proposed for mature Z‐band structure. Copyright © 1969 Wiley‐Liss, Inc.