The term “muscle” includes many cell types, all specialized for contraction but in other respects dissimilar. As noted in Chapter 16, all eukaryotic cells possess a contractile system involving actin and myosin, but muscle cells have developed this apparatus to a high degree. Mammals possess four main categories of cells specialized for contraction: skeletal muscle cells, heart (cardiac) muscle cells, smooth muscle cells, and myoepithelial cells (Figure 22–18). These differ in function, structure, and development. Although all of them generate contractile forces by using organized filament systems based on actin and myosin II, the actin and myosin molecules employed have somewhat different amino acid sequences, are bundle of smooth muscle cells differently arranged in the cell, and are associated with different sets of proteins that control contraction.
We focus in this section on skeletal muscle cells, which are responsible for practically all movements that are under voluntary control. These cells can be very large (2–3 cm long and 100 μm in diameter in an adult human) and are often called muscle fibers because of their highly elongated shape. Each one is a syncytium, containing many nuclei within a common cytoplasm. In an intact muscle, they are bundled tightly together, with fibroblasts (and some fat cells) in the interstices between them and blood vessels and nerve fibers running through the tissue. The mechanisms of muscle contraction were discussed in Chapter 16. Here we consider the unusual strategy by which the multinucleate skeletal muscle cells are generated and maintained.
myoblasts Fuse to Form new Skeletal muscle Fibers
During development, certain cells, originating from the somites of a vertebrate embryo at a very early stage, become determined as myoblasts, the precursors of skeletal muscle fibers. After a period of proliferation, the myoblasts undergo a dramatic change of state: they stop dividing, switch on the expression of a whole battery of muscle-specific genes required for terminal differentiation, and fuse with one another to form multinucleate skeletal muscle fibers (Figure 22–19). Once differentiation and cell fusion have occurred, the cells do not divide and the nuclei never again replicate their DNA.