Muscular Development

Question 1

What are myoblasts?

Answer 1

• derived from myogenic cells

- postmitotic

Question 2

What are myotubes?

Answer 2

• formed when myoblasts line up and adhere to one another
• requires calcium dependent CAMs
• involved in mRNA and protein synthesis
• characterized by appearance of actin, myosin, troponin, and tropomyosin
• formation of sarcomeres
• nuclei move to periphery

Question 3

All skeletal muscle originates in what?

Answer 3

Somites or somitomeres

Question 4

What are satellite cells?

Answer 4

• located between sarcolemma and basal lamina
• mitotic and myogenic
• able to fuse with muscle fiber and provide for growth
• satellite cells do not form new muscle fibers

Question 5

Differentiate between primary myotubes and secondary myotubes.

Answer 5

Primary:
-formed by fusion of earliest embryonic myoblasts

• may already be distinguished as fast or slow in some species
• differentiation occurs before innervation

Secondary:
-smaller than primary

• formed alongside primary from fetal myoblasts
• presence of motor axons may be necessary to form secondary myotubes
• contained within same basal lamina and are electrically coupled

Question 6

What two things maintain myogenic cells in labile state?

Answer 6

-FGF and TGF-beta

Question 7

What causes a loss of mitotic capability?

Answer 7

-p21

Question 8

What are the stages of differentiation in myogenic cells?

Answer 8

• originate in somites
• resemble mesenchymal cells
• restricted to muscle forming line
• mitotic cells

Question 9

What does the MyoD family do?

Answer 9

• able to convert non-muscle cells to cells capable of expressing muscle proteins
• helix loop helix transcription factors
• forms dimer and binds to E box in enhancer region of the myogenic genes
• binding is enhanced when E12 replaces one member of the dimer to form a heterodimer
• transcriptional inhibitor id can replace one member of the MyoD homodimers to form a heterodimer, resulting in poor binding ability

Question 10

What regulates MyoD?

Answer 10

Pax-3 and Myf-5 separately can activate MyoD and cause myogenic cells to become myoblasts

Question 11

What can increasing levels of MyoD result in?

Answer 11

• along with Myf-5, can result in expression of myoblast genes by myoblasts and expression of myogenin
• myogenin results in expression of myotubes genes and Myf-6
• Myf-6 leads to expression of myofiber genes

Question 12

What are the origins of the muscles of the trunk?

Answer 12

• epaxial muscles arise from dorsal lip myotome
• epaxial tendons arise from syndetome layer within somites
• hypaxial muscles arise from ventral buds of myotome
• tendons arise from lateral plate mesoderm

Question 13

What derives the muscles of the limbs?

Answer 13

• limb muscles arise from ventrolateral dermomyotome

- tendons from lateral plate mesoderm

Question 14

Where are muscles of the head and neck derived from?

Answer 14

• mostly from paraxial somitomeres
• extra ocular muscles from prechordal plate
• most of the cranial musculature from unsegmented paraxial mesoderm
• cranial musculature (lower jaw) from splanchnic mesoderm

Question 15

Where is cardiac muscle derived from?

Answer 15

• splanchnic mesoderm
• early cardiac muscle does not express MyoD
• both cardiac muscle cells and skeletal muscle cells express MADS
• cardiac muscle cells contract early
• early cardiac muscle cells maintain their ability to divide by partially disassembling their contractile apparatus prior to cell division
• cardiac muscle cells remain as mononucleated cells and attach to each other via intercalated discs