Development of Musculoskeletal System

Read up Foundation 1 Embryology & 1st aid basic sciences.

• OLIS
• http://elearning.imu.edu.my/file.php/4236/StudyGuide/Musculo/week1/xms1_7.html

Skeletal system develops from:

• Paraxial mesoderm
  • forms somites
    • a segmented series of blocks on each side of neural tube
    • somites form:
      • vertebral column
      • muscles of axial skeleton
      • body wall
      • head
• Lateral plate mesoderm (somatic layer)
  • forms:
    • pelvic
    • pectoral girdle
    • long bones (of limbs)

• 1st pair of somites appear on the 20th day
  • at cervical region
• formation of the somites
  • cranial-caudally
  • 3 somites per day
  • By the 5th week, have 44 pairs
    • Occipital – 4
      • 1st pair disappear at 4th week
    • Cervical – 8
    • Thoracic – 12
    • Lumbar – 5
    • Sacral – 7
    • Coccygeal – 8 to 10
      • Last 7 pairs disappear at 4th week
• Ventromedial: sclerotome
  • mesoderm of sclerotome
    • surrounds neural tube & notochord
      • forms vertebral column
• Dorsolateral: dermo-myotome
  • dermatome
    • forms skin
  • myotome
    • forms muscle

• Sclerotomic blocks
  • separated by intersegmental arteries
  • caudal part of each segment proliferate
    • caudal half of one sclerotome binds to cranial half of next one (see image below)
    • therefore body of vertebrae is of intersegmental origin (segmented)

• Messenchymal cells between cranial and caudal half of sclerotome
  • form intervertebral disc
    • annulus fibrosus (purple)
• The remains of notochord
  • forms nucleuss pulposus

Developmental abnormalities

Neural tube defect

• diagnostic:
  • maternal alpha-fetoprotein
• prevention
  • folic acid supplementation

• Neural crest cells
  • forms mesoderm of head
    • which forms bones of face & skull
• Occipital somites
  • forms cranial vault & base of skull

Types of ossification (skull)

Membranous (messenchyme –> bone)

• Neurocranium (inner part of skull)
  • forms cranial cavity
    • houses the brain
  • membranous part of neurocranium
    • fontanelles
      • flat ones of cranial vault sutures
      • enables baby’s skull to enlarge to accommodate growing brain
      • failure of formation
        • brain exposed to amnion causing degeneration
          • anencephaly
        • with herniation of brain
          • cranial meningocele
• Viscerocranium (outer part of skull)
  • facial skeleton
    • from 1st & 2nd pharyngeal arches
• Chondrocranium
  • base of skull

Cells of somites migrate to form precursors of:

• limb bud
• body wall musculature

but retains nerves from segment of origin (?).

By the end of 4th week,

• limb buds out pocket
  • from ventral body wall
  • start from a mesoderm core
    • derived from somatic layer of lateral plate mesoderm
  • upper limb buds appear 1st
  • lower limb buds appear 2 days later
  • mesenchyme in bud condenses
    • 6th week – 1st cartilage model
    • 7th week – 1st limb muscles at base of limb bud
      • pattern of muscle depends on connective tissue into which myoblasts migrate
        • head region
          • C/T from neural crest cells
        • axial, body wall, limbs
          • C/T from somatic mesoderm
• thickened ectoderm at distal border of limb bud (at the tip)
  • Apical ectodermal ridge (AER)
    • induces the pattern of the limb
      • differentiation of
        • limb bone
        • cartilage
        • muscle
    • if defected
      • fingers/toes may not form properly

At the 7th week,

• limbs rotate in the opposite direction
  • upper limb: 90% lateral
    • extensor muscles
      • lateral & posterior surface
    • thumb
      • lateral side (anatomical position)
  • lower limb: 90% medial
    • extensor muscles
      • anterior surface
    • great toe
      • medial side (anatomical position)

Bone growth with age

Muscle development

Prospective muscle cells

• Epimere
  • extensors of vertebral column
  • innervated by: dorsal rami of spinal nerve
• Hypomere
  • limb and body muscle wall
  • innervated by: ventral rami of spinal nerve
  • thoracic hypomere
    • splits into 3 layers
      • external intercostal
      • internal intercostal
      • innermost intercostal
  • abdominal hypomere
    • splits into 3 layers
      • external oblique
      • internal oblique
      • transversus abdominis

Anomalies

Dwarfism

• 9 members of fibroblast growth factors & fibroblast growth factor receptors (FGFR)
  • regulate cellular events in proliferation and differentiation
• FGFR-3
  • expressed in cartilage growth plates in long bones
  • mutation in p-arm of chromosome 4
    • autosomal dominant – hereditary
    • amino acid substitution
    • proliferation on chondrocytes in epiphyseal plate is disturbed
      • achondroplasia
        • most common causes of dwarfism
        • large head, small face, limbs shorted than trunk, bowed

• Amelia
  • complete absense of limbs
• Meromelia
  • partial absense of limbs
• Phocomelia
  • long bones absense
  • rudimentary hands & feet

Causes:

• hereditary
• drug induced
• mothers on thalidomide
  • teratogen damage
    • mostly 3rd – 8th week

• Polydactyly
  • extra digits
• Ectrodactyly
  • absence of digits
• Syndactyly
  • abnormal fusion
    • caused by: anti-convulsant phenytoin
• Lobster claw deformity
  • cleft hand & foot
• Congenital hip dislocation
  • due to underdevelopment of acetabulum & head of femur
  • mostly female