Lecture 15: Musculoskeletal Development II

Question 1

Be able to describe each stage in the differentiation of skeletal muscle tissue

Answer 1

myogenic cells –> myoblasts –> myotubes –> muscle fibers

Question 2

Differentiate between primary and secondary myotubes

Answer 2

Primary myotubes: formed by fusion of earliest (embryonic) myoblasts, may already be distinguished as fast or slow in some species, differentiation occurs before innervation

Secondary myotubes: smaller than primary, formed alongside primary from late (fetal) myoblasts, presence of motor axons may be necessary to form secondary myotubes, contained within same basal lamina and are electrically coupled

Question 3

Describe the role of satellite cells

Answer 3

• Attach to myotubes before basal lamina is laid down
• Located between sarcolemma and basal lamina of myofiber
• Mitotic and myogenic
• Able to fuse with muscle fiber and provide for growth
• Satellite cells do not form new muscle fibers
• Generally quiescent
• Function as stem cells
• Give rise to myogenic precursor cells

Question 4

What is the role of Myf-5, Pax-3, MyoD, myogenin, and Myf-6?

Answer 4

During muscle development, the MyoD family are expressed.

Muscle formation begins in the somite, where Pax-3 and Myf-5, through separate pathways, activate MyoD… causing cells of the dermomyotome to become committed to forming muscle.

With increased levels of MyoD, the mononuclear cells withdraw from the mitotic cycle and begin to fuse into myo- tubes. At this stage, myogenin is expressed. Finally, in matur- ing myotubes, Myf-6 is expressed.

Question 5

What is the role of Pax-3 and Pax-7?

Answer 5

Stimulate myogenic progenitor cells of the trunk to form myoblasts, expressed dorsally

Inhibited by Shh

Question 6

What is the role of FGF and TGF-B?

Answer 6

Overall, three phases are involved in tendon formation: (1) induction by FGFs, (2) early organization through the action of TGF-β, (3) expression of scleraxis

Question 7

What is the role of MyoD?

Answer 7

capable of converting non-muscle cells into cells expressing the full range of muscle proteins

form dimers with muscle-specific genes

Question 8

Describe the origin and characteristics of trunk muscles

Answer 8

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

Question 9

Describe the origin and characteristics of limb muscles

Answer 9

• Limb muscles arise from ventrolateral dermomyotome
• Tendons of limb muscles arise from lateral plate mesoderm

Question 10

Describe the origin and characteristics of head and trunk muscles

Answer 10

• Mostly derived from paraxial somitomeres
• Extraocular muscles arise from prechordal plate
• Most of the cranial musculature is derived from the unsegmented paraxial mesoderm
• Some cranial musculature (lower jaw) is derived from splanchnic mesoderm

Question 11

Describe the origin and characteristics of cardiac muscle

Answer 11

• Derived from splanchnic mesoderm
• Early cardiac musculature does not express MyoD
• Both cardiac muscle cells and skeletal muscle cells express MADS
• Cardiac muscle cells begin to 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 cells

Question 12

How can a molecular deficiency result in prune belly syndrome?

Answer 12

• the absence of abdominal muscles
• caused by the deficiency of these myogenic cells, probably caused by a defect in Pax3 or Lbx1 genes