Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

GENESIS AND REGENERATION OF SKELETAL MUSCLE
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(A)
(B)
(D)
skeletal muscle fiber
skeletal muscle fibers
nerve fibers
heart muscle cell smooth muscle cell myoepithelial cell
bundle of smooth muscle cells
10 µm
50 µm
differently arranged in the cell, and are associated with different sets of proteins
that control contraction.
We focus in this section on MBoC6 skeletal m23.47/22.18 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.
(C)
(E)
heart muscle cells
50 µm
myoepithelial cell
milk-secreting cell
10 µm
10 µm
Figure 22–18 The four classes of
muscle cells of a mammal. (A) Schematic
drawings (to scale). (B–E) Scanning
electron micrographs. Skeletal muscle
fibers (B, from a hamster) are giant cells
with many nuclei and are formed by cell
fusion. The other types of muscle cells
are more conventional, generally having
only a single nucleus. Heart muscle cells
(C, from a rat) resemble skeletal muscle
fibers in that their actin and myosin
filaments are aligned in very orderly arrays
to form a series of contractile units called
sarcomeres, so that the cells have a
striated (striped) appearance. The arrows
in (C) point to intercalated discs—end-toend
junctions between the heart muscle
cells; skeletal muscle cells in long muscles
are joined end-to-end in a similar way.
Smooth muscle cells (D, from the urinary
bladder of a guinea-pig) are so named
because they do not appear striated; they
belong to the connective-tissue family
and are closely related to fibroblasts. Note
that the smooth muscle is shown here
at a lower magnification than the other
muscle types. The functions of smooth
muscle vary greatly, from propelling food
along the digestive tract to erecting hairs
in response to cold or fear. Myoepithelial
cells (E, from a secretory alveolus of a
lactating rat mammary gland) also have no
striations, but unlike all other muscle cells
they lie in epithelia and are derived from
the ectoderm. They form the dilator muscle
of the eye’s iris and serve to expel saliva,
sweat, and milk from the corresponding
glands. (B, courtesy of Junzo Desaki;
C, from T. Fujiwara, in Cardiac Muscle in
Handbook of Microscopic Anatomy
[E.D. Canal, ed.]. Berlin: Springer-Verlag,
1986; D, courtesy of Satoshi Nakasiro;
E, from T. Nagato et al., Cell Tissue Res.
209:1–10, 1980. With permission from
Springer-Verlag.)
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.