Myofibrillogenesis in the skeletal muscle cells of the zebrafish

We have proposed a three step model of myofibrillogenesis based on fixed and living cultured avian muscle cells. We have used Fluorescence Recovery After Photobleaching (FRAP) to study the dynamics of Z‐band proteins in these cells. To determine if similar processes exist inside living animals we have examined embryonic zebrafish where there is a temporal progression of myofibril formation that can be followed as myotomes increase in length from the youngest to the oldest myotomes. The earliest zebrafish myotomes exhibited small periodic z‐bodies of alpha‐actinin that were arrayed along overlapping actin fibers in patterns identical to premyofibrils in avian muscles. In young myotomes, myosin II was present in overlapping bundles that resembled myosin patterns in avian nascent myofibrils. A‐bands and Z‐bands were present in mature myofibrils in the elongated cells in older myotomes. However, at the ends of these cells the patterns of alpha‐actinin and myosin II resembled the arrangement seen in younger myotomes, suggesting that as the cells elongated, premyofibrils and nascent myofibrils were involved in the extension of myofibrils. FRAP experiments in zebrafish muscle showed recovery profiles of the proteins that were similar to those found for avian Z‐bands. Our experiments indicate that a stepwise model of myofibrillogenesis and the dynamics of Z‐band proteins occur in zebrafish. Grant support by MDA and NIH.