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. 2014 Jan 28;5:14. doi: 10.3389/fphys.2014.00014

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Copyright © 2014 Raeker, Shavit, Dowling, Michele and Russell.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Lateral integration of new myofibrils involves coordinate remodeling of the sarcomeres, the cellular cytoskeleton and the surrounding membranes. (A) New skeletal myofibrils in a 20 hpf zebrafish embryo undergoing lateral fusion show progressive alignment at the level of the Z disks (arrowheads). Note the paucity of myofilaments in the intervening region between the Z disk involved in the active lateral incorporation (arrow) with intact sarcomeres on either end of the remodeling sarcomere. (B) A higher magnification of the image in (A) demonstrating the area of remodeling with interruption of the myosin filaments (arrowhead). (C) Newly assembled sarcomeres demonstrating recent alignment of the Z disks in register one end of the sarcomere (arrow) and misalignment at the other end (arrowhead). (D) Lateral integration of myofibrils with loss of defined sarcomere structure of the fusing myofibril (arrow) with progressive organization of the Z disk (arrowhead). (E) Higher magnification image of the image in (D) demonstrating remodeling of the T tubule/sarcoplasmic reticulum with loss of the membranes (white arrow) as the myofibrils fuse (black arrow). Note the disturbance of sarcomere architecture as the myofibrils fuse (arrowhead). (F) Transverse sections reveal that, as myofibrils (M1 and M2) fuse, the sarcoplasmic reticulum (arrowhead) withdraws from the line of apposition, the hexagonal array of myofilaments is disordered (^*) on either side of the interface suggesting remodeling of the supporting cytoskeleton. Note that the new myofibrils are “separated” from the rest of the sarcoplasm by the membranes of the SR. Scale bars are 500 nm (A–E) and 100 nm (F).