chapter 21
Muscle and Nonmuscle Contractile Systems
T A B L E 2 1 - 3
Factors Known to Influence Muscle Development
Regulatory Factor
Role in Development
Can initiate the differentiation of stem
cells to myoblasts, may stimulate
proliferation of myoblasts
Same as
m yf
5 must be present:
deletion of both prevents myoblast
Required for terminal differentiation
of myoblasts, myoblast fusion, and
development of myotubes
Required for development of normal
amount of myofibrils; may be needed
for maintenance of mature muscle cells
Deletion of myogenin gene results in
accumulation of myoblasts instead of
formation of muscle cells
Inhibits proliferation of myoblasts and
formation of myotubes; plays major role
in terminating the myogenic program in
small animals, lesser role in large animals
Defect in myostatin gene in cattle explains
the unusual degree of muscularity in
Belgian Blue and Piedmontese breeds
F I G U R E 2 1 -1
Structure of skeletal muscle, (a) A myofibril is a long cylinder consisting of tandemly arranged sarcomeres. A sarcomere
consists of a darkly staining central region, the A-band, and two adjacent lightly staining regions, the I-bands. Very
darkly staining Z-lines mark the borders of successive sarcomeres, (b) Many myofibrils are packed inside one muscle
cell (myofiber). The myofibrils are in parallel register and impart the sarcomere banding pattern to the muscle cell.
(c) A bundle of muscle cells. Muscle cells are multinucleated. (d) Bundles of muscle cells are in turn arranged into
larger bundles within a muscle.
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