Development of skeletal muscles and limbs

Skeletal muscles

Origin: Paraxial mesoderm – myotomes

Myotomes ⇒ myoblast cells ⇒ fuse ⇒ myotubes ⇒ myofilaments develop in myotubes ⇒ myotubes become myocytes (muscle cells)

Image result for occipital myotomes

 

  • 1st occipital myotome – extra occular muscles of the eye (NB: iris and ciliary muscle – neural ectoderm)
  • Remaining 3 occipital myotomes – muscles of tongue

 

The 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 8-10 coccygeal myotomes divide into:

  • Dorsal part – Epimere – forms extensor muscles of back and neck
  • Ventral part – Hypomere:
  1. Split longitudinally eg. infrahyoid muscles
  2. Split to form layers eg. intercostal muscles
  3. Ventral tips unite to form eg. rectus abdominis muscle

Related image

NB:

  • Myotomes migrate to limb buds to form flexor and extensor muscles
  • Pharyngeal arches give muscles in head and neck

 

Limbs

  • Upper limb bud forms opposite C5-T1
  • Lower limb bud forms opposite L4-S3

Each limb bud is formed of:

  • Lateral plate mesoderm – somatic layer – bones, tendons and connective tissue
  • Migrating myotomes – muscles
  • Neural crest cells – melanocytes and schwann cells
  • Which is all covered by ectoderm – skin

 

Development:

  • Programmed apoptosis between digital rays form fingers and toes
  • Central mesoderm – cartilagenous skeleton – ossify into bone
  • 3 segments formed in each limb bud
  • Rotation of 90 degrees around long axis:
  1. Upper limb adducted, rotated laterally, thumb lateral
  2. Lower limb adducted, rotated medially, big toe is medial

 

Congenital anomalies of limbs:

  1. Amelia – failure to develop limbs
  2. Phocomelia – absence of proximal limbs, direct foot or hand
  3. Polydactyl – Extra fingers or toes
  4. Syndactyl – fusion of 2 fingers or toes
  5. Claw feet – lobster deformity
  6. Congenital digit amputation
  7. Constriction band
  8. Club foot
  9. Ectodactyl – missing middle finger

 

 

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