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

1234 Chapter 22: Stem Cells and Tissue Renewal
single
myoblasts
two fused
myoblasts
multinucleate muscle fibers
Figure 22–19 Myoblast fusion in culture.
The culture is stained with a fluorescent
antibody (green) against skeletal muscle
myosin, which marks differentiated
muscle cells, and with a DNA-specific
dye (blue) to show cell nuclei. (A) A short
time after a change to a culture medium
that favors differentiation, just two of the
many myoblasts in the field of view have
switched on myosin production and have
fused to form a muscle cell with two nuclei
(upper right). (B) Somewhat later, almost
all the cells have differentiated and fused.
(C) High-magnification view, showing
characteristic striations (fine transverse
stripes) in two of the multinucleate muscle
cells. (Courtesy of Jacqueline Gross and
Terence Partridge.)
(A)
100 µm
(B)
100 µm
(C)
25 µm
Some Myoblasts Persist as Quiescent Stem Cells in the Adult
Even though humans do not normally generate new skeletal muscle fibers in adult
life, they still have the capacity to do so, and existing muscle fibers can resume
growth when the need arises. MBoC6 Cells m23.48/22.19 capable of serving as myoblasts are retained as
small, flattened, and inactive cells lying in close contact with the mature muscle
cell and contained within its sheath of basal lamina (Figure 22–20). If the muscle
is damaged or stimulated to grow, these satellite cells are activated to proliferate,
and their progeny can fuse to repair the damaged muscle or to allow muscle
growth. Satellite cells, or some subset of the satellite cells, are thus the stem cells
of adult skeletal muscle, normally held in reserve in a quiescent state but available
when needed as a self-renewing source of terminally differentiated cells.
The process of muscle repair by means of satellite cells is, however, limited
in what it can achieve. In one form of muscular dystrophy, for example, a genetic
defect in the cytoskeletal protein dystrophin damages differentiated skeletal muscle
cells. As a result, satellite cells proliferate to repair the damaged muscle fibers.
This regenerative response is, however, unable to keep pace with the damage, and
connective tissue eventually replaces the muscle cells, blocking any further possibility
of regeneration. A decline of capacity for repair likewise contributes to the
weakening of muscle in the elderly.
satellite cell
muscle progenitor cells
satellite cell
activated to
divide
cell fusion and
muscle fiber
regeneration
multinucleate
muscle
fiber
(A)
50 µm
(B)
damage to muscle fiber
Figure 22–20 Satellite cells repair skeletal muscle fibers. (A) The specimen is stained with an antibody (red) against a muscle
cadherin, M-cadherin, which is present on both the satellite cell and the muscle fiber and is concentrated at the site where their
membranes are in contact. The nuclei of the muscle fiber are stained green, and the nucleus of the satellite cell is stained blue.
(B) Schematic of the repair of a damaged muscle fiber by proliferation and fusion of satellite cells. (A, courtesy of Terence Partridge.)
MBoC6 m23.51/22.20