ABSTRACT
In vertebrate animals, muscle precursor cells, or myoblasts, come from the somites, a series of tissue masses segmentally arranged on either side of the body. The migration of muscle precursors can be traced by marking them using either a nontoxic dye or a heritable genetic label. Studies in which cells were grafted from quail embryos into chick embryos, for example, provided an important source of information. The entire series of events leading to muscle formation, including cell fusion, is under the control of master genes. The role of microtubules is especially clear in development of the indirect flight muscle of Drosophila, described later in the chapter. As this muscle forms, uniformly striated myofibrils appear first within 'sleeves' of microtubules. The variations of muscle type so far encountered depend on the selective expression of different genes, but an even richer source of variation is provided by tissue-specific RNA splicing. The flight muscles of many insects contract at an amazingly rapid rate.
