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The Skeletal System

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					The Skeletal System
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
  Components of the skeletal system

• Bones, cartilage and ligaments
   – Bones (obvious)
   – Cartilage (at each joint)

   – Ligaments
      • Join bones together at a joint
          – “tendons” actually hold muscle
             to bone
          – Tendons are similar to ligaments
             and hold muscle to bone.
       4 Major Shapes of Bone
1. Long bone
   • Longer than wide (like a straw)
   • Serve as levers for mobility/action
        • Humerus of the arm (brachium), femur of the thigh,
          radius/ulna of the forearm (antebrachium),
          tibia/fibula of the shin (crus/leg), and the tarsus and
          manus bones of foot and hand
2. Short bone
   • Nearly equal in length and width (think: cube or square)
        • Have limited motion, usually glide over one
          another
        • Usually in multiples (many short bones adjacent to
          each other) to provide a wide range of motion
       4 Major Shapes of Bone
3. Flat bone
   •   Has flat surface (like a plate or sheet of paper)
        • Act as shield for soft organs, and as a broad
            surface for muscle attachment
        • Cranium, ribcage, sternum, scapula, hip/pelvic
            bones
4. Irregular bones
   •   Weird-shaped bones that don’t fit into any of the other
       3 categories
         • Vertebrae, some skull bones (sphenoid and
            ethmoid bones)
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
 Functions of the skeletal system
1. Support
  •   Legs, pelvis, vertebrae (spine)
      hold the body UP
  •   Support muscle and other tissue
      types
      •   Jaw (mandible & maxilla) serve to
          support teeth
2. Protection
  •   Encloses and protects vital
      organs
      •   Brain (enclosed by cranium), spinal
          cord (enclosed by vertebrae),
          chest/thorax encloses heart and
          lungs, bone marrow
 Functions of the skeletal system
3. Movement
  • Movement is produced by muscles
    acting on bone (using bones as levers)
4. Electrolyte homeostasis
  • Serves as the major storage site for
    calcium and phosphate
    • Releases both on specific needs/commands
5. Acid-base balance
  • Buffers blood pH by absorbing and
    releasing alkaline salts
Functions of the skeletal system
6. Blood formation
  • Red bone marrow is the major source of
    blood cells
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
                      Bone tissue
•   Bone = “osseous tissue”
    –   connective tissue
        •   The extracellular matrix is “hardened” by
            depositing calcium phosphate (CaPO4) 
            “mineralization” or “calcification”
        •   Note the bone is NOT the hardest material in your
            body (tooth enamel is the hardest material)
            – If bone were as hard as tooth enamel, it would also be
              quite brittle


    –   Osteon in compact bone or trabeculae in spongy bone
               Cells of the bone
1. Osteogenic cells
        •   A stem cell type that develops from fibroblasts
            – Divides to give rise to osteoblasts
        •   Located in the periosteum and endosteum
            (connective tissue that lines the bones)


2. Osteoblasts (osteoblast cells)
   •   They deposit bone matrix
Cells of the bone
       3.   Osteocytes
            •   Osteoblasts that have become
                “trapped” in their matrix (lacunae)
            •   Has cell processes that reach into
                the canalicula to join with other
                osteocytes by gap junctions
                  •     allows for transfer of
                       nutrients and waste to and from
                       the central/Haversian canal
            •    Re-absorb or deposit minerals into
                 the matrix
                  •    Important in bone density,
                       blood calcium and phosphate
                       homeostasis
            •    Act as “strain sensors” as well
                  •    Stress on bone acts as a signal
                       for osteocytes to alter bone
                       shape
                   Cells of the bone
4. Osteoclasts
•   Derived from bone marrow stem cells
    (not from osteogenic cells or the
    endosteum)

•   “bone dissolving” cells
       • Surface “touching” the inner surface of
         bone = “ruffled border” to increase
         absorptive surface area
       • Etch the bone surface by synthesizing
         HCl (local pH=4)
       • Also secrete “acid phosphatase” :
         Enzyme that works in an acid
         environment to digest collagen

•   Bone “remodeling” is the combined
    efforts of osteoblasts and osteoclasts
            Bone matrix (ECM)
•   Extracellular matrix (ECM)
    • 1/3 organic
       • Collagen, made by osteoblasts
    • 2/3 inorganic minerals
       • About 85% = crystallized calcium phosphate
         (hydroxyapatite = Ca10[PO4]6(OH)2
       • 10% calcium carbonate (CaCO2)
          – Remainder = magnesium, sodium, fluoride, sulfate,
            carbonate and hydroxyl ions
       • Note: other “chemicals” can be deposited into
         the bone
                        Bone matrix
• Collagen alone is quite rubbery
  (not good for support)

• Calcium phosphate crystals are
  quite brittle

• Rickets = childhood disease
  where bones are softened and
  deformed from the patient’s
  weight (most often due to Vit. D
  deficiency)
Compact bone: the Osteon
• Basic structure: Osteon
• Collagen fibers wrap
  around the lamellae like a
  spiral
   • Spirals are anti-parallel
     (each concentric ring
     spirals in the opposite
     direction)
      • Spirals enhance the strength
        of the bone
      Compact bone
• Blood supply and neural supply runs through
  the central/Haversian canals
   • Skeleton receives  ½ Liter blood/min
        • Enters via nutrient foramina on the
           surface, and then branches into
           “perforating canals” (Volkmann
           canals), which then branch off to
           each respective central/Haversian
           canal
   • Remember that osteocytes in the
      outermost rings of lamellae receive their
      nutrients and pass their waste into the
      central canal through the gap junctions
      that connect them to the osteocytes in the
      next lamellae in…and so on
 Compact bone
• The outermost
  layer of compact
  bone is formed by
  1 or 2 large rings
  that surrounds the
  entire bone)

  • “giant lamellae”
    that holds the
    osteons together
Spongy bone
• A meshwork of slender rods,
  plates and spines all called
  “trabeculae”
   • Trabeculae is hard and calcified
     (looks like a sponge, but is solid)
   • Spaces are filled with bone marrow
• The “matrix” of spongy bone is
  arranged like lamellae in
  compact bone
• Strength without the weight
• Trabeculae structure appear
  random, but the arch shape
  gives it strength against stress
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
 Features of long bone
• The shaft of a long bone =
  diaphysis
  - lever (the longer the lever, the
   more power the muscles can
    “exert”)

• The two extremities = epiphysis
  - strengthen the joint area, serves
  as a surface for ligament and
  tendon attachment
 Features of long bone
• The joint is where 1 bone meets another
   – Covered by hyaline cartilage called “articular
     cartilage”
      • Helps the bones move easily
      • Cartilage also secretes lubricant fluid to aid
        in movement

• Blood vessels enter the bone through specific
  “holes” called “nutrient foramina”
   – Foramen = hole or entrance

   – Do not confuse: canaliculi – perforating canals
     – nutrient foramen
 Periosteum
The bone is covered by a specialized tissue
     type called “periosteum”

     - Outer layer = “outer fibrous layer”
     (Made primarily of collagen fibers). In
     some areas of bone, the outer fibrous
     layer is continuous with a tendon

     - Inner layer (closest to the bone itself) =
     “osteogenic layer” (where the bone-
     forming cells are located.
     - Penetrating fibers binds this layer to the
     bone

     - Osteoblasts present in the periosteum
      deposit bone matter (once surrounded
     by bone matter  Osteocytes: maintain
     bone matter)
                      Endosteum
•   The inner layer of the
    compact bone (facing the
    central/medullary canal

•   Rich in osteoclasts:
    specialized cells able to
    dissolve bone matrix

•   Promote an increase in the
    size of the medullary canal
    Epiphyseal plate
•   In infants through to the
    early 20’s, there is a “growth
    plate” or “epiphyseal plate”
    of hyaline cartilage that
    separates the epiphysis and
    diaphysis (the head and shaft
    of long bone)
    –   On an X-ray, the long bones
        actually look like they’re
        separated into a shaft and a
        short “head” region
    –   Called “growth plate” because
        this is where bone will grow
        in length
    –   In adults, this plate disappears
        (calcifies), but you can often
        still see a remnant “line” of
        calcification where it used to
        be
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
           Flat bone structure: Cranium
Sandwich of outer compact bone,
    spongy bone and inner compact
    bone
•   Middle spongy bone =
    “diploe”
          •   involved in growth of the
              cranium (also helps with
              repairing damage to the
              cranium)
          •   acts to cushion an impact

               –   An impact/blow that
                   cracks the outer
                   compact bone would
                   be cushioned by the
                   diploe to minimize
                   the risk of cracking
                   the inner compact
                   bone
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
                     Bone marrow
• A “general term” to cover the soft tissue that is inside a
  long bone cavity
• 2 types of bone marrow:
     1. Red bone marrow: myeloid tissue
          • The hematopoietic or blood cell producing tissue
          • Looks like blood (hence the name red bone marrow), but more
            thick consistency
          • Made of reticular connective tissue and immature blood cells
          • Seen in skull, vertebrae, ribs, sternum, regions of the pelvis, and
            the ball/epiphysis of femur and humerus
     2. Yellow bone marrow: in adults
          – No longer produces blood cells, BUT, can be stimulated to
            “revert” back to red bone marrow if pathological anemia occurs
            (severe lack of blood cells)
Note how the long
 bones of your arm
and legs are not the
bones that have red
bone marrow! The
   red marrow is
  actually in your
    pelvis, ribs,
   vertebrae and
     cranium.
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
  Bone development and growth

• Bone development
  - Fetal development
       - intramembranous
  bone growth
       - endochondral bone
  growth
• Bone growth
       - growth in length
       - growth in width
  Bone development and growth
• Bone development
  - Fetal development
      - intramembranous bone growth
      - endochondrial bone growth
• Bone growth
      - growth in length
      - growth in width
          Bone development
• Formation of bone = ossification /
  osteogenesis

• Fetal development of bone occurs via 2
  mechanisms:
  1. Intramembranous ossification (“dermal
     bones”)  flat, irregular bones
  2. Endochrondral ossification (“chondral bones”)
      long bones
          Bone development
• Intramembrane ossification
  – Starts around 8th week of development
  – Flat bones in the skull and collarbone
    (clavicle), hips
  – As the embryo develops, fibroblasts form into
    membranesfrom which the perioteum will
    form. The fibroblasts differentiate into
    osteoblasts which will first form spongy bone
    then compact bone
  – Compact bone is deposited on the external and
    internal surface
Can you guess what this represent?




       Intramembranous bone growth !
  Bone development and growth
• Bone development
  - Fetal development
      - intramembranous bone growth
      - endochondral bone growth
• Bone growth
      - growth in length
      - growth in width
          Endochondral ossification
the majority of the bones in your body

   – Around 8th week of embryonic
     development: the mesenchyme (connective
     tissue derived from the original mesoderm)
     forms chondroblasts which deposit hyaline
     cartilage chondrocytes  perichondrium

   – About 4-5 months of development, the
     perichondrium stops making chrondroblasts
     and switches to producing osteoblasts 
     periosteum
           Endochondral ossification
• Periosteum producing
  osteoblasts/osteocytes

• The osteoblasts deposit new bone
  matter which replaces cartilage 
  primary ossification center

   -   Chondrocytes are destined to die 
       formation of medullary canal in the
       center

   - New bone matter replaces cartilage -
       cartilage is not transformed into
       bone
              Endochondral ossification
• Around birth: the bone shaft is formed
  and include the medullary cavity

  - The epiphysis are still formed by hyaline
  cartilage.

   –    the metaphysis are located at either end,
        between the shaft and epiphysis


• After birth: epiphysis starts to
  ossify =“secondary ossification
  center”
            Endochondral ossification
• During infancy and childhood: epiphysis fills with
  spongy bone
        •   Cartilage is then “limited” or “left” at the joint


• “band” of cartilage at the metaphysis (called the
  epiphyseal plate or growth plate)
 Endochondral ossification
– During late teens / early 20’s, the epiphyseal
  plate is consumed and calcified, and the 2
  marrow cavities unite (epiphysis and diaphysis)
  Bone development and growth
• Bone development
  - Fetal development
      - intramembranous bone growth
      - endochondrial bone growth
• Bone growth
      - growth in length
      - growth in width
               Bone growth in length
• Bones only grow in
  LENGTH from infancy
  through to the early 20’s
        •   After calcification of the
            epiphyseal plate (early
            20’s), bones can only grow
            in width


• The “length” growth depends
  on the epiphyseal plate
        •   Mature chondrocytes die off
        •   Osteoblasts, below, synthesize
            bone matter which replaces
            cartilage
           Bone growth in width
• Bone widening occurs throughout life
  – Occurs by deposition on the outer surface of bone

  – Periosteum contains osteogenic cells that can
    develop into osteocytes

     • Lay down matrix on the outer surface of compact bone
       in a large circumferential layer

     • As bone widens due to “outer shell widening”, the
       osteoclasts digest the inner marrow cavity to move the
       compact bone “wall” outwards
        Bone growth in width
• One aspect of bone growth in width is the
  widening of the medullary cavity
         Bone development
• Did you know?
  – At birth, you normally have about 270 bones.
  – By maturity, you will only have about 206 bones
      • Why is this?
Abnormal bone growth

• Gigantism
• Dwarfism
  – - pituitary
  – - achondroplasia

  – Due to hormonal
    disturbances
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
            Bone remodeling
• Bone is in a constant state of flux
  – Constantly being modeled and remodeled
     • 10% skeletal tissue is broken down and replaced
       over 1 year
  – “Wolf’s law of bone” = architecture of bone is
    determined by the mechanical stresses it
    endures
     • The shape of a bone is determined by “functional
       experience”
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
                             Bone repair
• Fractures:
   – If the bone is poking out of the
     skin = “open”
   – If the bone is still inside the skin
     = “closed”

   – “displaced” = bone is broken
     and out of place/position
        • Very common = once broken,
          muscles contract (partly
          because of the intense pain)
          and pull the bone out of
          position
   – “comminuted” = shattered bone
   – “impacted” = bone fragment
     driven into the marrow or
     spongy bone
   Colles and Pott fractures are
common sports injuries, and refer to
   specific radius/ulnar fracture
(Colles) or tibia/fibia fracture (Pott)
                   Bone repair
• Closed reduction 
  manipulating the bones
  back into place without
  surgery

• Open reduction 
  surgical placement of
  bone

• Uses of cast, pins, plates
  and/or traction
Bone fracture: healing stages
                  Outline
•   General features of bone
•   Functions
•   Bone tissue
•   Long bone structure
•   Flat bone structure
•   Bone marrow
•   Bone development and growth
•   Bone remodeling
•   Bone repair
•   Bone degeneration
                 Bone degeneration
• Bone degenerates when
  demineralization outpaces
  mineralization
   – Osteoporosis =
     demineralization of bone
     to the point where pores or
     pits form
             – Hip fractures in
               elderly are
               common because
               the hip is a high-
               load area
       • “Widow’s hump” =
         vertebrae weaken and
         compress
    Osteoporosis:
demineralization to the
point of pore formation
  (weakens the bone)




           Widow’s hump:
          osteoporosis of the
         vertebrae leading to
            compression of
         individual vertebrae
             Bone degeneration
• Why females more than males?
   – Estrogen inhibits osteoclasts
   – Males produce estrogen in the testes, and continue to
     produce “enough” to inhibit osteoclasts until much
     later in life
• Athletes can also suffer osteoporosis
   – Too little body fat = reduced estrogen secretion

• Some new anti-osteoarthritis medications actually
  target the osteoclasts (inhibit them, some actually
  kill them)
     Various “hardcore” bones
• Ocular orbit (where your eye sits) is made of
  7 bones:
   – Roof of the orbit = Frontal & sphenoid bones
   – Floor of the orbit = Maxilla, zygomatic &
     palatine bones
   – Lateral wall & margin of the orbit = Zygomatic
     bone
   – Posterior wall of the orbit = Sphenoid bone
   – Medial wall = Maxilla, lacrimal & ethmoid bone
   – Medial margin = Maxilla bone
                     Axial skeleton
• Skull
• Vertebral column
• Rib cage
  – Ribs
  – Sternum
             Skull: cranium + face
•   Cranial bones:
•   Frontal
•   Parietal
•   Temporal
•   Occipital
•   Sphenoid
•   Ethmoid
Temporal bone
Sphenoid - Ethmoid
Facial bones
Bones the form the orbit of the eye
     Various “hardcore” bones
• Nasal cavity (where your eye sits) is made of
  7 bones:
   – Roof (superior cavity) = Ethmoid & frontal
     bones
   – Floor (inferior cavity) = Maxilla & palatine
     bones
   – Lateral = Maxilla & palatine bones
   – Nasal “septum” = Ethmoid, vomer & nasal
     bones
   – Bridge = Nasal bone
   – Conchae = Ethmoid bone & inferior nasal
     conchae
   Bones of the nasal cavity




Which nasal cavity bone is missing from this picture?
     Various “hardcore” bones
• Remember that many of the cranial & facial
  bones are “paired”
   – Paired = left & right
      •   Left & right parietal bones
      •   Left & right temporal bones
      •   Left & right lacrimal bones
      •   Nasal bones
      •   Zygomatic bones
      •   Maxilla bones
      •   Palatine bones
      •   Inferior nasal concha
     Various “hardcore” bones
• Remember that many of the cranial & facial
  bones are “paired”
   – UN-paired (single)
      •   Frontal bone
      •   Occipital bone
      •   Sphenoid bone
      •   Ethmoid bone
      •   Vomer bone
      •   Mandible
      •   Hyoid bone
          Various “hardcore” bones
• Auditory ossicles
   – Three of the smallest bones
     in your body
   – Middle ear cavity of the
     petrous part of the
     temporal bone (paired or
     unpaired?)
   – From lateral – medial
      • Malleus, incus & stapes
Appendicular skeleton: upper extremity
 • Pectoral girdle:
    – Scapula
    – Clavicle
 • Upper arm:
    – Humerus
 • Lower arm:
    – Ulna
    – Radius
 • Wrist:
    – Matacarpals bones
 • Hand
    – Metacarpals
    – Phalanges
                     Pectoral girdle
• Scapula
  – Left or right?
  – The smooth side
    (subscapular fossa) is against
    the rib cage
  – The glenoid fossa is lateral
    (it is where the humerus
    attaches)
• Clavicle
  – Left or right:
  – Sternal end (square end)
    against sternum, acromial
    end (flatter end) against
    acromiom
• Clavicle
  – The curvature near the sternal end sticks out
  – See previous drawing




• Humerus
  – Left or right?
  – The head of the humerus
       must face medially
  – The olecranon fossa
    (the largest fossa at the elbow)
     is posterior
             Wrist and Hand

• See book
   Appendicular skeleton: lower extremity
• Pelvic girdle
   – Coxal bones
• Thigh
   – Femur
• Lower leg:
   – Tibia
   – Fibula
• Ankle
   – Tarsal bones
• Foot
   – Metatarsal
   – Phalanges
                       Coxal bone
• Left or right?
   – The acetabulum must face
     externally
   – The symphysis pubis is
     anterior
                          Femur
• Left or right?
   – The head of the femur
     must face internally
   – The smooth surface in the
     lower femur is anterior
     while the deep groove is
     posterior
                           Tibia - Fibula
• Tibia:
   – Left or right?
   – The styloid process, at the base is
     the exernal malleolus or internal
     ankle
   – The tibial tuberosity is anterior
• Fibula:
   – The hed of the fibula is against the
     lateral condyle
   – The styloid process is the lateral
     malleolus or external ankle
   – Slented side of styloid process is
     posterior
Ankle - Foot

				
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