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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

Comments

myoD

myf5

Can initiate the differentiation of stem

cells to myoblasts, may stimulate

proliferation of myoblasts

Same as

myoD

Either

myoD

or

m yf

5 must be present:

deletion of both prevents myoblast

formation

myogenin

mrf4

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

myostatin

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.