20. Myogenesis: molecular basis of muscle development

Question 1

What is myogenesis?

Answer 1

The synthesis of new muscle

*Muscle cells are highly complex - full of metabolic & contractile proteins

Question 2

How many muscle cells in a muscle fibre?

Answer 2

Muscle cells are huge when compared to most other cells. Each muscle fibre is a single muscle cell - up to 12cm long in humans!

Question 3

What is the embryonic origin of muscle?

Answer 3

ZYGOTE
> BLASTULA 
> GASTRULA 
> MESODERM 
> SOMITES 
> MYOTOME 
> MUSCLE

Question 4

Myoblasts migrate out from the somite. What do medial and lateral somites give rise to?

Answer 4

Medial somite (near neural tube) - axial/trunk muscles
Lateral somite (distal to neural tube) - limb muscles

All skeletal muscle from the neck down is derived from somites.

Question 5

Embryonic origin of head and neck muscles

Answer 5

Some head and neck muscles are derived from the somite, such as the tongue.

Extra-ocular and jaw/facial muscles are not derived from somites (formed from head mesoderm).

Head muscles develop separately.

Question 6

What is a syncytium?

Answer 6

a large mass of cytoplasm not separated into individual cells and contains many nuclei

ie. a muscle cell

Question 7

How is a muscle fibre formed?

Answer 7

By fusion of a number of smaller precursor cells (myoblasts)

Question 8

Myoblast vs. myotube vs. mature myofibres

Answer 8

MYOBLAST:
MPCs express genes to become even more specialised (myoblast)
-committed to become muscle but retain some characteristics of stem cells, eg. still highly proliferative
-spindle shaped
-centrally nucleated

MYOTUBE:

• terminally differentiated (no longer proliferative)
• central nuclear chain

MATURE MYOFIBRILS:

• nuclei move to periphery
• specialised structures develop (eg. t-tubules)

Question 9

How is myogenesis regulated?

Answer 9

by complex network of:

• transcription factors
• growth factors
• microRNA (miRNA)

Question 10

What transcription factors regulate myogenesis?

Answer 10

Muscle progenitor cells express specific patterns of transcription factors (characteristic: Pax3 & Pax7)

Embryos lacking both fail to develop muscle

Factors controlling expression of Pax3/Pax7 very complex:
-Sonic hedgehog (Shh), Wnts, Notch, noggin, BMP4

Question 11

MRFs

Answer 11

Muscle progenitor cells begin to express myogenic regulatory factors (MRFs)
-need Pax3/Pax7

MRFs are very powerful transcription factors
-can even cause non-muscle tissue (fat, fibrotic tissue, liver) to try to become muscle

MRFs are all structurally similar
-basic helix-loop-helix (bHLH) family

MRFs to remember: MyoD, Myf5, Myogenin, MRF4

Question 12

What are the primary MRFs?

Answer 12

MyoD & Myf5

INITIATE DIFFERENTIATION
They are expressed early in muscle development
Promote expression of Myogenin & beginning of differentiation

They are vital to myogenesis but have redundancy with each other:

• knockout 1 (but not the other) = muscles develop fine
• knockout both at once = no skeletal muscle

Question 13

What are the secondary MRFs?

Answer 13

Myogenin (& later MRF4) are expressed during differentiation.

CONTROL DIFFERENTIATION
Promote further differentiation & specialisiation
*knockout either Myogenin or MRF4 and myoblast differentiation is severely affected

Promote expression of muscle-specific genes (ie. MyHC)

Question 14

MEF2 & SRF

Answer 14

Myocyte enhancement factor 2 (MEF2) & serum response factor (SRF) are other transcription factors (not MRFs) involved in myogenesis.

Can’t initiate myogenesis on their own but contribute to differentiation and maturation of muscle fibres.

Question 15

Growth factors: what do they affect?

Answer 15

Growth factors (usually soluble) bind to cell surface receptors and induce cellular effects. Involve a variety of intracellular signalling cascades.

Affects:

• myogenesis: by affecting proliferation and/or differentiation
• mature myofibres: by affecting protein metabolism
• hypertrophy (muscle fibre growth) or atrophy (muscle fibre shrinkage)

Question 16

Growth factors of interest

Answer 16

IGF-1: increased proliferation and differentiation
LIF, IL-6: increased proliferation, same differentiation
FGF, HGF: increased proliferation, decreased differentiation
Insulin, IGF-II: same prolif., increased diff.
TGF-β1 (Myostatin): decreased prolif., increased diff.

Question 17

What are the embryonic and postnatal roles of myostatin?

Answer 17

Myostatin is traditionally recognised as an inhibitor of differentiation as inhibition of myostatin increases muscle mass. But it’s not inhib of diff, it’s ENHANCEMENT OF PROLIFERATION with myostatin LoF.

H/e diff roles in embryonic vs postnatal.
Myostatin is an essential regulator of the balance between proliferation vs. differentiation of embryonic muscle progenitors.

Question 18

How does miRNA regulate myogenesis?

Answer 18

miRNA are small non-coding RNAs that play major roles in cell development & differentiation by negatively regulating gene function

Several miRNAs have been shown to be regulated by MRFs

MyomiRs = muscle specific miRNAs

• miR-1/206 family enhances myoblast differentiation
• miR-133 family drives myoblast proliferation

Question 19

Describe postnatal muscle growth

Answer 19

From birth to adulthood, muscle size increases > 20 fold

But there is no significant change in muscle fibre # after birth