Skeletal muscles

Origin: Paraxial mesoderm – myotomes

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

• 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

NB:

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

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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