The development of myofibrils in cultured muscle cells: A whole-mount and thin-section electron microscopic study

The development of myofibrils in cultured myotome cells from Xenopus embryos was studied with whole-mount and thin-section electron microscopy. For whole mount, the cells were grown on Formvar-coated grids, fixed, dehydrated, critical-point dried, and examined with a conventional (100 kV) or a high-voltage (1000 kV) electron microscope. Nonstriated bundles of 6- to 8-nm microfilaments, similar to stress fibers in nonmuscle cells, appear prior to nascent myofibrils. These bundles run the whole length of the cell and are inserted into the cell cortex. The transition from striated region to nonstriated region on a single nascent myofibril can be seen in both whole-mount and thin-section images. New sarcomeres appear to be added at the distal end of existing ones. Our data also indicate that these new sarcomeres are formed on a preexisting bundle of thin filaments. This suggests that the bundles of microfilaments are precursors to myofibrils. Evidence for this hypothesis came from the following observations. (1) Nascent myofibrils are anchored to the cell cortex via thin filaments similar to microfilament bundles. (2) Thin filaments in newly formed sarcomeres are often continuous through the middle of the A band. Later they break to form the H zone. (3) Thin filaments appear to be continuous through the developing Z band. Later they interact with the filaments in the Z band to form the staggered appearance.