Bones and Bone Tissue

The Skeletal System

  • Includes bones, joints, and supporting tissues.
  • Bones are the main organs; adults typically have 206.
  • Each bone includes osseous tissue, dense regular collagenous tissue, dense irregular connective tissue, and bone marrow.

Functions of the Skeletal System

  • Protection: Protects underlying organs (e.g., skull, sternum, ribs).
  • Mineral Storage and Acid-Base Homeostasis: Stores minerals like calcium, phosphorus, and magnesium salts, which are critical for electrolyte and acid-base maintenance.
  • Blood Cell Formation: Red bone marrow is the site of hematopoiesis.
  • Fat Storage: Yellow bone marrow contains adipocytes with stored triglycerides.
  • Movement: Bones are attachment sites for skeletal muscles.
  • Support: Supports body weight and provides structural framework.

Classification of Bone by Shape

  • Long Bones: Longer than wide (e.g., limb bones).
  • Short Bones: About as long as wide (e.g., wrist and ankle bones).
  • Flat Bones: Thin and broad (e.g., skull and pelvis bones).
  • Irregular Bones: Irregular shapes (e.g., vertebrae).
  • Sesamoid Bones: Small, flat, oval-shaped within tendons (e.g., kneecap).

Bone Structure of a Long Bone

  • Periosteum: Outer dense irregular connective tissue membrane with blood vessels and nerves.
  • Perforating Fibers: Collagen anchors attaching periosteum to bone matrix.
  • Diaphysis: Shaft of the bone with a medullary cavity lined by the endosteum and filled with marrow.
  • Epiphyses: Ends of a long bone (filled with red marrow) covered with articular cartilage (hyaline cartilage).
  • Compact Bone: Hard, dense outer bone resisting linear compression and twisting.
  • Spongy (Cancellous) Bone: Inner, honeycomb-like framework resisting forces from multiple directions; houses bone marrow.
  • Epiphyseal Lines: Remnants of epiphyseal (growth) plate.

Bone Marrow

  • Yellow Bone Marrow: Mostly blood vessels and adipocytes.
  • Red Bone Marrow: Network of reticular fibers supporting hematopoietic cells.

Bone Marrow Transplantation

  • Used for patients with leukemia, sickle-cell anemia, aplastic anemia.
  • Bone Marrow Harvest: Red marrow withdrawn from pelvic bone of a donor.
  • Recipient's marrow is destroyed and replaced with donor marrow.

Extracellular Matrix of Bone

  • Inorganic Matrix: ~65% of bone weight; consists of calcium salts (hydroxyapatite crystals) for strength and compression resistance.
  • Organic Matrix (Osteoid): ~35% of bone weight; consists of collagen fibers, proteoglycans, etc. for resisting torsion and tensile forces.

Bone Cells

  • Osteoblasts: Build bone (bone deposition); mature into osteocytes.
  • Osteocytes: Maintain bone.
  • Osteoclasts: Break down bone (bone resorption).

Histology of Bone

  • Compact Bone: Hard, dense outer shell resisting stress.
    • Units are osteons or Haversian systems.
  • Spongy Bone: Resists forces from many directions.
    • Organized into trabeculae.

Compact Bone Structure

  • Lamellae: Rings of thin layers of bone.
  • Central (Haversian) Canal: Contains blood vessels and nerves.
  • Lacunae: Cavities between lamellae, contain osteocytes.
  • Canaliculi: Tiny canals connecting lacunae.
  • Perforating (Volkmann) Canals: Connect central canals.

Spongy Bone Structure

  • Trabeculae: Covered with endosteum; contain concentric lamellae with lacunae and canaliculi housing osteocytes.

Bone Formation: Ossification

  • Ossification (Osteogenesis): Process of bone formation.
  • Primary (Woven) Bone: Immature bone with irregular collagen bundles.
  • Secondary (Lamellar) Bone: Mature bone with organized collagen bundles.

Intramembranous Ossification

  • Bones built on mesenchymal membrane.
  • Includes flat bones (skull) and clavicles.

Endochondral Ossification

  • Bones built on hyaline cartilage model.
  • Includes bones below the head (except clavicles).

Osteoporosis

  • Bone disease caused by inadequate inorganic matrix in the ECM, leading to brittle bones.

Achondroplasia

  • Most common cause of dwarfism due to abnormal growth factor receptor on cartilage.

Bone Growth in Length

  • Longitudinal growth occurs at the epiphyseal plate, which contains five zones:
    • Zone of reserve cartilage
    • Zone of proliferation
    • Zone of hypertrophy and maturation
    • Zone of calcification
    • Zone of ossification

Bone Growth in Width

  • Appositional Growth: Growth of bones in width by osteoblasts laying down new bone.

Hormones in Bone Growth

  • Growth Hormone: Increases mitosis of chondrocytes and activity of osteogenic cells.
  • Testosterone: Increases appositional growth and mitosis at the epiphyseal plate.
  • Estrogen: Similar effects to testosterone, closes epiphyseal plates.

Bone Remodeling

  • Continual process of bone formation and loss for maintenance, repair, and adaptation.

Bone Deposition

  • Osteoblasts make components of the organic matrix and facilitate formation of inorganic matrix

Bone Resorption

  • Osteoclasts secrete H+H^+ ions and enzymes to break down bone ECM

Bone Remodeling in Response to Tension and Stress

  • Compression stimulates bone deposition.
  • Tension stimulates bone deposition.
  • Pressure stimulates bone resorption.

Factors Influencing Bone Remodeling

  • Hormones: Testosterone, estrogen, growth hormone.
  • Age: Decline in hormone levels.
  • Nutrient Intake: Calcium, vitamin D, vitamin K, vitamin C, protein.

Bone Remodeling and Calcium Ion Homeostasis

  • Maintained by negative feedback loops using parathyroid hormone (PTH) and calcitonin.

Bone Repair

  • Steps of Fracture Healing:
    • Hematoma formation
    • Soft callus formation
    • Bone callus formation
    • Bone remodeling

Classes of Fractures

  • Simple (Closed): Skin intact.
  • Compound (Open): Damage to surrounding tissue.