Regeneration and Formation of Skeletal Muscle

Question 1

What is a somite?

Answer 1

blocks of cells formed from mesodermal cells adjacent to the neural tube.

Question 2

How do skeletal muscle fibres form?

Answer 2

By fusion of myoblasts

Question 3

Outline the steps of skeletal muscle fibre formation.

Answer 3

1. Cells originating from the somites become determined as myoblasts (precursors of skeletal muscle fibers).’
2. Myoblast differentiation
3. As they differentiate, they fuse with one another to form multinucleated skeletal muscle fibers. Fusion involves specific cell-cell adhesion molecules that mediate recognition between newly differentiating myoblasts and fibers.

Question 4

What does the commitment of the myoblasts depend on?

Answer 4

gene regulatory proteins of at least two families—the MyoD family of basic helix-loop-helix proteins, and the MEF2 family of MADS box proteins.

These act in combination to give the myoblast a memory of its committed state, and, eventually, to regulate the expression of other genes that give the mature muscle cell its specialized character.

Question 5

What is myoblast differentiation?

Answer 5

After a period of proliferation, the myoblasts undergo a dramatic switch of phenotype that depends on the coordinated activation of a whole battery of muscle-specific genes

Question 6

Once differentiation has occurred, do the cells divide?

Answer 6

No, they do not and the nuclei never replicate their DNA again

Question 7

What can keep myoblasts proliferating, while they retain their ability to fuse to form muscle cells?

Answer 7

signal proteins such as fibroblast or hepatocyte growth factor

Question 8

What happens when you culture myoblasts in culture without HGF and FGF?

Answer 8

The cells rapidly stop dividing, differentiate, and fuse.

Question 9

What is another factor important to myoblast differentiation?

Answer 9

attachment to the extracellular matrix

Question 10

How do skeletal muscle fibres modulate their properties according to their functional requirements?

Answer 10

By changing the protein isoforms they contain

Question 11

When are the adult number of muscle fibres attained

Answer 11

Before birth

Question 12

How long do skeletal muscles survive?

Answer 12

A lifetime

Question 13

How is the postnatal increase in muscle bulk attained?

Answer 13

Cell enlargement as individual muscle nuclei are added.

Question 14

How does growth in muscle length occur?

Answer 14

recruitment of more myoblasts into the existing multinucleated fibers, which increases the number of nuclei in each cell.

Question 15

How does growth in muscle girth occur?

Answer 15

involves both myoblast recruitment and an increase in the size and numbers of the contractile myofibrils that each muscle fiber nucleus supports.

Question 16

What is a mechanism that controls muscle cell number and size?

Answer 16

Extracellular signal protein - myostatin

Question 17

What happens to myostatin loss-of-function mice?

Answer 17

have enormous muscles—two to three times larger than normal. Both the numbers and the size of the muscle cells seem to be increased.

Question 18

What other animal has an myostatin related mutation?

Answer 18

Belgian Blue cattle lack myostatin and thus have giant muscles “double-muscled”

Question 19

What is myostatin and where is it made?

Answer 19

Myostatin belongs to the TGFβ superfamily of signal proteins

It is normally made and secreted by skeletal muscle cells.

An autocrine/paracrine inhibitor of skeletal muscle growth

Question 20

What is the function of myostatin?

Answer 20

to provide negative feedback to limit muscle growth.
myostatin may act as a negative regulator of muscle growth in adult life as well as during development.
It inhibits proliferation, differentiation and enhances degradation

Question 21

What is the muscle structure hierarchy?

Answer 21

Myofilaments –> myofibril –> myofiber/myotube (aka. muscle fiber) –> fascicle –> muscle

Question 22

Define a satellite cell

Answer 22

a stem cell that lies adjacent to a skeletal muscle fiber and plays a role in muscle growth, repair, and regeneration.

Question 23

Where are the satellite cells?

Answer 23

They 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.

Question 24

What happens when muscle is damaged/ must repair after exercise?

Answer 24

satellite cells are activated to proliferate, and their progeny can fuse to repair the damaged muscle.

Question 25

Where does most muscle tissue in the body come from?

Answer 25

embryonic mesoderm

Question 26

How are muscle cells in the head and limbs different from other skeletal muscle cells?

Answer 26

They originate from the general mesoderm not the mesodermal somites

Question 27

What is a myoblast?

Answer 27

muscle-forming stem cell that migrates to different regions in the body and then fuse(s) to form a syncytium, or myotube.

Question 28

How do neuromuscular junctions form along muscle cells?

Answer 28

In skeletal muscles, ACh receptors are initially present along most of the surface of the myoblasts, but spinal nerve innervation causes the release of growth factors that stimulate the formation of motor end-plates and NMJs.

Question 29

How limited are the regenerative capabilities of satellite cells?

Answer 29

Satellite cells can regenerate muscle fibers to a very limited extent, but they primarily help to repair damage in living cells.

Question 30

What happens if a cell is damaged to a greater extent than satellite cells can regenerate?

Answer 30

the muscle fibers are replaced by scar tissue in a process called fibrosis.
Scar tissue cannot contracts and muscle that has sustained significant damage cannot produce the same amount of power or endurance.

Question 31

What might increase the mass and bulk of skeletal muscle?

Answer 31

hormones and stress (and artificial anabolic steroids)

Question 32

What might decrease skeletal muscle bulk and mass?

Answer 32

Disuse (due to being in a cast or paralysis due to eg. Polio), age

Question 33

What regulates the proliferation and fusion of satellite cells?

Answer 33

A group of basic helix loop helix transcription factors

Question 34

List the main four myogenic regulatory factors

Answer 34

MyoD, Myf5. MRF4, Myogenin

Question 35

Which regulatory factors are involved in satellite cell proliferation?

Answer 35

MyoD and Myf5

Question 36

Which regulatory factors are involved in fusion of committed myogenic precursor cells into the myofibre for repair?

Answer 36

MRF4 and Myogenin