The Skeletal System (Chapters 6, 7, 8, and 9)

Consider all bones on lab list a standing lecture assignment!



  1. Functions of skeleton
    1. Supports body
    2. Protects delicate parts
    3. Produces blood cells = Hemopoiesis / hematopoiesis (red marrow)
    4. Storage of minerals (mostly Ca and P) and fat (yellow marrow)
    5. Leverage for body movement
  2. Structure of bones
    1. Chemical composition (fig. 6.12)
      1. Organic (living or unique to life) components - 1/3 of body weight
        1. Bone cells (activity is hormonally influenced)
          1. Osteocytes: mature bone cells

Maintain and repair bone matrix

Repair damaged bone

Most abundant type of cell

          1. Osteoblasts: immature bone cells

Will develop into osteocytes

Make matrix around themselves

Produce new bone (osteogenesis) by working with osteoprogenitor (stem) cells in endosteum





          1. Osteoclasts: break down and remove matrix for remodeling and growth OR

When body needs calcium and phosphorus

Demolition team of bone tissue

Huge cells, derived from macrophages

        1. Collagen fibers
      1. Inorganic components - 2/3 of bone weight
        1. Mineral salts (hydroxyapatite crystals)
        2. Examples: calcium phosphate, calcium hydroxide, calcium carbonate
    1. Gross anatomy (of adult long bone) (fig. 6.2a and INTERLAB 7) )
      1. epiphysis (proximal and distal ends) - composed of spongy bone; for muscle attachment
      2. epiphyseal line - junction b/w epiphysis and diaphysis; in juvenile is cartilage and called ____________ ____________
      3. diaphysis (shaft) - composed of compact bone tissue, surrounding ...
      4. medullary (marrow) cavity - yellow marrow in adults; lightens weight
      5. endosteum - single cellular layer around medullary cavity (fig. 6.6)
        1. Thin membrane lining cavity and other internal spaces and canals
        2. Has osteoblasts and osteoclasts
      6. covered by periosteum on outside (fig. 6.6) - boundary of bone; nourishes and repairs
        1. Two layers
          1. inner layer - has osteoblasts and osteoclasts
          2. outer layer - irregular dense collagenous connective tissue
            1. very tough
            2. anchoring point for tendons and ligaments (collagen fibers merge)
            3. lots of blood vessels and nerves with penetrate and nourish the interior of bone
      7. articular cartilage at ends
        1. Hyaline cartilage
        2. Cushion
    2. Histology of bone-- porous w/ channels for blood vessels and nerves (fig. 6.3 and 6.4)
      1. Compact bone tissue
        1. Location:
          1. outside of all bones
          2. thicker in diaphysis than in epiphyses of long bone
        2. Function: very strong, resists stress
        3. Structure: composed of osteons or Haversian systems (recall photo from lab; fig. 6-3, 6-4)
          1. Central canal or Haversian canal
            1. runs longitudinally in long bone
            2. lined by ____________
            3. contains blood vessels and nerves
          2. lamellae - concentric rings of hard, calcified matrix around canal
          3. lacunae - spaces which house bone cells
          4. osteocytes - mature bone cells in lacunae
          5. canaliculi - connect lacunae to central canal
            1. little canals
            2. nutrients and waste diffuse through canliculi b/w osteocytes and blood vessels
          6. lots of osteons crowd together to form compact bone
          7. perforating (Volkmann's) canals (fig. 6.4a)
            1. run perpendicular to long axis of bone; have blood vessels
            2. connect periosteum to osteons to endosteum and marrow cavity
      2. Spongy (cancellous) bone tissue: unorganized, porous appearance
        1. Locations:
          1. interior of bones (covered by compact)
          2. in epiphyses of long bones
        2. Function: lightweight, withstands stress in different directions; may contain red marrow for __________


        3. Structure:
          1. no osteons
          2. irregular lattice of thin plates of bone = _________
          3. irregularly arranged lamellae and osteocytes connected by canaliculi
          4. spaces b/w trabeculae contain marrow and blood vessels (from which osteocytes are nourished)
            1. in infants -


            2. in adults -- red marrow remains in these locations
              1. heads of
              2. some flat bones
              3. some irregular bones
              4. marrow transplants drawn from sternum or hipbone
            3. rest of bone cavities filled w/ --




  1. Ossification - bone formation
    1. Skeleton of embryo (fig. 6.8)
      1. Composed of : hyaline cartilage and fibrous membranes -- shaped like future bones
      2. Ossification = replacing other tissues with bone; begins at 6th week and never truly stops
    2. Types of ossification (same end result)
      1. Intramembraneous (fig. 6.7)
        1. Occurs - in flat bones
        2. Begins w/ -- fibrous connective tissue membrane
        3. Steps in intramembraneous ossification:
          1. Formation of bone matrix
            1. Mesenchymal cells in membrane differentiate into osteoblasts - form clusters
            2. Osteoblasts make bone matrix (collagen fibers)
            3. Calcium salts deposit around fibers = calcification
          2. Organization into spongy bone & development of periosteum
            1. Thin plates of bone (trabeculae) form
            2. Neighboring trabeculae fuse to form spongy bone
            3. Spaces fill w/ red marrow
          3. Periosteum forms from more mesenchyme
          4. Compact bone formed
            1. Surfaces of spongy bone remodeled into compact bone by periosteum
            2. Middle may be remodeled into marrow cavity
        4. Intramembranous ossification takes place from center of a bone toward edges, in flat bones
          1. at birth, corners are still soft = fontanelles
          2. usually fuse by 2nd year
      2. Endochondral (fig. 6-8)
        1. Occurs - in almost all bones
        2. Begins with -- cartilage (hyaline), the model or pattern present in embryo
        3. Cartilage model covered by perichondrium
        4. Steps in endochondral ossification of a long bone (recall juvenile skeleton from lab)
          1. Perichondrium becomes converted into periosteum (more vascular)
          2. Bone collar formed outside:

Osteoblasts in periosteum (which layer?) begin to form spongy bone 'collar' around model

          1. Bone core (primary ossification center) formed inside:
            1. Cartilage in center of bone calcifies, chondrocytes die, matrix deteriorates, forming cavities w/ thin pieces of cartilage matrix, but no cells
            2. Blood vessels invade center, bringing osteoblasts; then cavities fill w/ spongy bone
          2. Remodeling:
            1. results in compact bone on outside, spongy bone in center (ends of bone are still cartilage)
            2. Osteclasts break down spongy bone in center, producing medullary cavity [starts about the time of birth in most bones]
            3. Cartilage model continues to grow on the ends and bone gets longer (interstitial growth)
          3. Epiphyses ossify:
            1. During 1st two years of life, blood vessels (w/ osteoblasts) invade epiphyses
            2. called secondary ossification centers
            3. spongy first, remodeled at surface into compact
          4. By year 5, cartilage only remains in two regions
            1. articular cartilage
            2. epiphyseal plate; will become ______ _______ in adult
    1. Bone growth (length) after age 5 (in long bone, fig. 6.9)
      1. New cartilage formed at epiphyseal plates
      2. Old cartilage is converted to bone
      3. Ossification 'chases' cartilage growth
      4. Thickness of epiphyseal plate constant
      5. Bone will lengthen: 2-3 inches/year
      6. Stops about 18 (females) or 21 (males)
      7. Requires:
        1. Growth hormone, thyroid hormone, others (table 6.1)
        2. Adequate nutrition - calcium, protein, vitamin D (calcitriol), vitamin C, zinc
        3. Sex hormones speed process up
        4. Sex hormones will eventually cause epiphyseal plate to become epiphyseal line
    2. Growth in diameter -- occurs as osteoblasts form new bone in periostem (outside) while osteoclasts erode bone in endosteum (inside) bone and marrow cavity both increase in diameter (appositional growth) fig. 6.10
  1. Bone remodeling
    1. In adult
      1. Osteoclasts continuously destroy old bone
      2. Osteoblasts make new
      3. Process balanced; bone mass constant; no obvious change
    2. Control
      1. Hormones- neg. feedback, will sacrifice skeleton to maintain blood level of calcium ions
        1. Calcium used for nerve impulse transmission, muscle contraction, blood clotting, etc.
        2. If Ca decreases below normal (poor nutrition, no vit D) fig. 6.13a
          1. parathyroid glands secrete parathyroid hormone (PTH)
          2. stimulates bone resorption by osteoclasts
          3. releases Ca into blood
          4. as blood Ca increases, parathyroid stops secreting PTH
        3. If blood Ca increases above normal (fig. 6.13b)
          1. thyroid gland secretes calcitonin
          2. inhibits bone resorption (osteoclasts) and causes calcium to be deposited in matrix
          3. as blood Ca decreases, thyroid stops secreting calcitonin
      2. Mechanical stress and gravity
        1. Bone responds to stress by growing and remodeling to become thicker and heavier
        2. Bony projections for muscle attachment are large in weight lifters
        3. Bones in casts, of bedridden, in zero-gravity, will atrophy
        4. How?
          1. stressing bones bends them a little
          2. produces electrical impulses
          3. inhibit osteoclasts, stimulate osteoblasts
          4. can use electrical stimulation to speed healing of fractures


  1. Repair of fractures (fig. 6.14)
    1. Hematoma formation - blood clot fills space b/w broken ends
    2. Fibrocartilage callus formation
      1. Granulation tissue forms - made of capillaries and fibroblasts
      2. Fibroblasts -- secrete collagen fibers to span space
      3. Some fibroblasts differentiate -- into chondroblasts; secrete cartilage matrix around fibers
      4. Callus protrudes and bulges beyond outer bone surface (3 to 4 weeks)
    3. Bony callus formation
      1. Osteoblasts and osteoclasts -- differentiate & migrate into callus and replace it with spongy bone (ossification)
      2. Still protrudes
      3. Usually complete in 6-8 weeks -- if bones are immobilized and circulation (and nutrition) is good
    4. Remodeling - takes many months
      1. In places, spongy bone is converted to compact bone
      2. Excess bone on outside is removed
      3. Medullary cavity restored (if appropriate)
      4. May be little or no remaining sign of injury; may be same strength or slightly stronger than surrounding area
  2. Osteoporosis - bone resorbed faster than rebuilt (fig. 6.16)
    1. Symptoms:
      1. Fragile, easily fractured bones
      2. Lost height, lost teeth
      3. Humpbacked appearance
    2. Who is affected?
      1. Everyone (male and female) in old age; bone mass loss begins @ age 30-40
      2. Women more than men - 8% /decade vs. 3% / decade
        1. Why? Lose estrogen abruptly at menopause; men taper off gradually on testosterone
        2. Women have 30% less bone than men at start
        3. Women lose 1% of bone mass/year after menopause
      3. White women more than black women
      4. Little women more than large women
        1. Weight of big body stresses and builds bone
        2. Fat tissue is a source of estrogen after menopause
        3. Very thin women may stop producing estrogen - 20 year old w/ bones of 70 year old
    3. Prevention
      1. Exercise to stress bones - walking for spine and hips; weight-lifting for upper body (swimming doesn't help)
      2. Nutrition -- Adequate Ca and vitamin D
      3. Treatment: Hormone replacement therapy
      4. Treatment: Na fluoride; calcitonin; alendronate (Fosamax)
  3. Articulations
    1. Classification - see text and INTERLAB 7; use study guide
      1. Fibrous joint-- eg: sutures; immovable
      2. Cartilaginous joint - eg: costal cartilage; intervertebral joints; somewhat movable
      3. Synovial joints - freely movable
        1. Have capsule filled with synovial fluid
        2. Have cartilage (meniscus at knee)
        3. Types of synovial joints
          1. ball and socket
          2. hinge
    2. Structure of synovial joint (fig. 9.1) - found at ends of long bones
      1. Articular cartilage (has no perichondrium)
      2. Joint cavity - fluid-filled space
      3. Articular capsule also called synovial membrane - tube-like; double-layered
        1. Fibrous (articular) capsule - outer
          1. tough, fibrous c.t.
          2. continuous w/ periosteum
        2. Synovial membrane - inner
          1. loose c.t.
          2. covers internal surfaces not covered by cartilage
          3. secretes fluid; some cells act as phagocytes
      4. Synovial fluid
        1. Fills cavity and bears weight; keeps articular cartilages from rubbing
        2. Lubricant, minimizes friction
        3. Nourishes and protects articular cartilage
      5. Ligaments
        1. Dense regular collagenous c.t.
        2. Spans joint and reinforces it
      6. Joints also strengthened by tendons attached to strong muscles
      7. In addition to the above, knee has 2 extra discs of fibrocartilage separating articular surfaces of bone = lateral and medial menisci (meniscus = singular); also has fat pads
  4. Arthritis - inflammation or degeneration of joints
    1. Acute arthritis
      1. Cause: usually due to infection; bacterial invasion of joints cause inflammation
      2. Examples: Lyme disease, TB, rheumatic fever
    2. Chronic arthritis
      1. Osteoarthritis - wear and tear
        1. Degeneration of articular cartilage - probably due to aging and lack of circulation (non-inflammatory); may be a genetic factor involving collagen production
        2. Exposed bone thickens and forms bony spurs, restricting movement
        3. Who gets it? Both sexes in old age
        4. Where? Weight-bearing joints first (hips and knees); also distal phalanges
        5. Treatment: treat pain associated (aspirin)




      2. Rheumatoid arthritis
        1. Chronic inflammatory disorder - autoimmune disease; immune system attacks synovial membrane
        2. Inflammation causes fluid to accumulate in joint; pain from pressure
        3. Over time - granulation tissue (pannus) and scarring replace normal joint tissue, deformity causes loss of joint function
        4. Who? Mostly women aged 30-40
        5. Where? Usually small joints first - fingers, wrists, ankles, feet
        6. Symmetry - right and left sides at same time
        7. Treatment: relieve pain and reduce inflammation (aspirin and, sometimes, corticosteroids)
      3. Gouty arthritis
        1. Cause: uric acid produced from breakdown of nucleic acids
        2. Some people produce too much; others can't eliminate it properly (or both)
        3. When uric acid increases in blood, it deposits as sodium urate crystals in soft tissues
        4. Crystals cause pain, inflammation, and swelling
        5. Who? Mostly men; middle aged or older
        6. Treatment: suppress production of, or increase secretion of uric acid (colchicine)