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

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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)

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  • 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

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  • 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)

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

Developmental abnormalities

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Neural tube defect

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

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  • 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

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Cells of somites migrate to form precursors of:

  • limb bud
  • body wall musculature

but retains nerves from segment of origin (?).

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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

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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

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Muscle development

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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

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Anomalies

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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

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  • 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

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  • 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

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