Muscle and Cell Motility
Answers
123.The answer is a.(Kumar, pp 91, 94, 1327. Alberts, pp 1299–1300.)
Satellite cells in skeletal muscle proliferate and reconstitute the damaged part
of the myofibers. They are supportive cells for maintenance of muscle and a
source of new myofibers after injury or after increased load. There is no ded-
ifferentiation of myocytes into myoblasts (answer b),or fusion of damaged
myofibers to form new myotubes (answer c).Hypertrophy, not hyperplasia
(answer d),occurs in existing myofibers in response to increased load. Pro-
liferation of fibroblasts may occur in the damaged area but leads to fibrosis,
not repair of skeletal muscle. Fibroblasts do notdifferentiate into myocytes
(answer e).The multinucleate organization of skeletal muscle is derived
developmentally by fusion and not by amitosis (failure of cytokinesis after
DNA synthesis). Mitotic activity is terminated after fusion occurs. In the
development of skeletal muscle, myoblasts of mesodermal origin undergo
cell proliferation. Myocyte cell division ceases soon after birth. Myoblasts,
which are mononucleate cells, fuse with each other end to end to form
myotubes. This process requires cell recognition between myoblasts, align-
ment, and subsequent fusion.
124.The answer is a.(Alberts, pp 961–965. Junqueira, pp 186–191. High-
Yield Facts, p 22.)During contraction, the sarcomere, the distance between
adjacent Z lines, decreases in length, and the length of the A band is almost
constant. However, as the degree of overlap of thick and thin filaments is
altered, the thin filaments, which form the I band and are anchored to the
Z line, are pulled toward the center of the sarcomere. As this occurs, the I
band decreases in length and the H band is no longer visible. The filaments
themselves do not decrease in length; they slide past one another in the
sliding-filament model of muscle contraction. The average length of a sar-
comere is 2.5 μm. This distance is measured from one Z line to the next Z
line. If the resting length of the A band is 1.5 μm and the length of the I
band is 1.0 μm, then the resting length of the sarcomere is determined by
adding the length of the I band to the length of the A band. If there is a 20%
contraction of the muscle (contraction to 80% of its length), then the sar-
comere is reduced in length from 2.5 to 2.0 μm. The size of the A band
221