Bone Physiology

Aka: Bone Physiology, Bone Anatomy, Osteoblast, Osteoclast, Cortical Bone, Trabecular Bone

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II. Types: Bone

  1. Cortical Bone (80%)
    1. Outer supporting structure (Dense)
    2. Sites
      1. Radius
      2. Skull
      3. Long bones
    3. Peak density: age 30 to 40 years
  2. Trabecular Bone (20%)
    1. Inner supporting structure (Spongy)
    2. Sites
      1. Hip
      2. Spine
      3. Femur

III. Anatomy

  1. See Bone Growth
  2. Growth Plate Layers (in the context of other bone regions)
    1. Articular cartilage of joint
    2. Epiphysis (rounded bone end)
    3. Epiphyseal Plate or Growth Plate (Hyaline Cartilage)
      1. Resting Zone (or Reserve Zone)
        1. Colloid matrix of vessels and germinal cells (chrondrocyte progenitors)
        2. Vessels (from the epiphysis) are the primary blood supply to the Growth Plate
        3. Chondrocytes differentiate from germinal cells, and remain in a resting state
      2. Proliferating Zone (or proliferative zone)
        1. Chondrocytes (formed in the reserve zone) undergo Mitosis after migrating to proliferating zone
        2. Chondrocyte organize into columns of a proteoglycan matrix
      3. Hypertrophic Zone
        1. Maturation Zone
          1. Chondrocytes further differentiate and increase in size throughout the Hypertrophic Zone
          2. Chrondrocytes also produce Collagen, glycogen, lipids and Alkaline Phosphatase
        2. Degenerative Zone
        3. Provisional Calcification Zone (or calcified zone)
          1. Least resistant to stress and greatest risk for Fractures
          2. Transition between calcified and non-calcified extracellular matrix Proteins
          3. Final layer of the Physis, in which colloid matrix calcifies and chondrocytes die
            1. Chondrocyte death allows for ostocytes to enter and osteogenesis begins
    4. Metaphysis
      1. Primary Spongiosa
      2. Secondary Spongiosa
    5. Diaphysis
  3. Images
    1. orthoBoneAnatomy.jpg

IV. Physiology: Bone Growth and Resorption

  1. Microfractures occur via everyday wear and tear
    1. Healed by ongoing bone remodeling
  2. Bone Remodeling
    1. Bone remodeling occurs in 120 day cycles
      1. Resorption by Osteoclasts over first 20 days
      2. Bone formation by Osteoblasts over last 100 days
    2. Osteoblasts
      1. Fill in bony cavity with bone matrix
      2. Triggers for activity
        1. Parathyroid Hormone
        2. Thyroxine
        3. Growth factors
          1. Interleukins (1, 6, 11)
          2. Insulin-Like Growth Factors
          3. Transforming Growth factor-B
      3. Releases Cytokines to attract Osteoclasts
    3. Osteoclasts
      1. Release proteases
        1. Dissolves bone mineral matrix9
        2. Dissolves Collagen
      2. Clears away damaged bone
      3. Releases matrix-bound growth factors
        1. Chemoattractant for Osteoblasts
  3. Metabolic Control of bone formation
    1. Calcitonin
      1. Inhibits bone resorption
      2. Inhibits intestinal Calcium, Phosphorus absorption
      3. Inhibits renal Calcium excretion
      4. Inhibits calcitriol production in Kidney
    2. Calcitriol
      1. Increases intestinal Calcium absorption
      2. Increases intestinal Phosphorus absorption
      3. Increases intestinal Magnesium absorption
      4. Decreases Parathyroid Hormone secretion
    3. Parathyroid Hormone
      1. Mobilizes Calcium and Phosphorus from bone
      2. Increases Calcitriol

V. Physiology: Bone Marrow

  1. See Bone Marrow
  2. Bone Marrow produces lymphocytic progenitor cells
    1. Lymphoblasts form in marrow, divide to Prolymphocytes
    2. Prolymphocytes develop into 2 separate cell lines
      1. B-Lymphocytes develop in Bone Marrow
      2. T-Lymphocytes develop in Thymus
  3. Bone Marrow produces three stem cell myeloid lines
    1. Erythroblasts (Initial cell known as Proerythroblast)
      1. Develop into erythrocytes (Erythropoiesis)
    2. Myeloblasts (Granuloblasts)
      1. Represent 75% of nucleated cells in marrow
      2. Develop into Promyelocyte and then Granulocyte
      3. Characteristics
        1. Evenly distributed fine chromatin
        2. Multiple nucleoli
        3. Basophilic cytoplasm (non-granular)
    3. Megakaryocyte (Megalokaryocyte)
      1. Giant cell with multi-lobulated nucleus in marrow
      2. Releases mature Blood Platelets from cytoplasm

VI. References

  1. Rosen (1998) Postgrad Med 104(4):101-14 [PubMed]

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