TE

Bones and Skeletal Tissues

Introduction

  • Skeletal system includes: cartilage, bones, ligaments, connective tissue

  • ~20% of body mass

  • Bones = dynamic organs (contain bone tissue, cartilage, nervous tissue, fibrous CT, muscle cells, epithelial cells in blood vessels)

Microscopic Anatomy of Bone

Compact Bone (Lamellar Bone)

  • Structural unit: Osteon (Haversian system)

  • Osteon = elongated cylinder, parallel to long axis

  • Lamellae: rings of matrix; collagen fibers alternate directions → resist twisting

  • Central Canal: contains blood vessels & nerves

  • Perforating (Volkmann’s) canals: connect blood supply of periosteum, medullary cavity, central canal

  • Lacunae: cavities with osteocytes

  • Canaliculi: connect lacunae → nutrient & waste exchange

  • Interstitial lamellae: remnants between osteons

  • Circumferential lamellae: extend around diaphysis, resist twisting

Spongy Bone

  • Trabeculae align along stress lines (like suspension bridge cables)

  • No osteons; nutrients from capillaries in endosteum

Bone Composition

  • Inorganic (65%): Hydroxyapatites (calcium phosphate crystals) → hardness, compression resistance

  • Organic (30%): Mostly collagen (95% of organics) → flexibility, tensile strength

Classification of Bones

  1. Long bones – longer than wide (limbs)

  2. Short bones – cube-shaped (wrist/ankle), sesamoid (patella)

  3. Flat bones – thin, curved (sternum, ribs, skull)

  4. Irregular bones – complex shapes (vertebrae, hip)

Groups

  • Axial: skull, vertebral column, rib cage

  • Appendicular: limbs, girdles

Anatomy of Bones

Long Bone

  • Diaphysis: shaft; compact bone around medullary cavity (yellow marrow in adults)

  • Epiphyses: ends; compact outside, spongy inside; covered in articular cartilage

  • Epiphyseal line: remnant of growth plate

  • Membranes:

    • Periosteum (outer fibrous + inner osteogenic layer; has nutrient foramina)

    • Endosteum (lines internal bone, contains osteogenic cells)

Short/Flat/Irregular Bones

  • Spongy bone (diploë) sandwiched between compact bone layers

  • Red marrow found in trabecular cavities

Bone Markings

  • Projections: sites of muscle/ligament/tendon attachment

  • Depressions/Openings: pathways for vessels/nerves or form joints

Bone Development

Ossification

  • Endochondral: bone replaces hyaline cartilage (most bones below skull except clavicle)

    • Steps: bone collar → cartilage calcifies → cavities → periosteal bud → spongy bone → elongation/medullary cavity → secondary centers in epiphyses → ossification complete except plates & articular cartilage

  • Intramembranous: bone develops from fibrous membrane (flat bones of skull, clavicle)

Postnatal Bone Growth

  • Length (interstitial growth) → at epiphyseal plate

    • Zones: resting → proliferation → hypertrophic → calcification → ossification

  • Width (appositional growth) → osteoblasts add bone externally, osteoclasts remove internally → stronger but not too heavy

Epiphyseal Plate Closure

  • Females ~18 yrs, males ~21 yrs

Bone Cells

  • Osteocytes: mature, maintain matrix, communicate

  • Osteoblasts: build bone, secrete osteoid (collagen + Ca-binding proteins)

  • Osteoclasts: multinucleate, resorb bone (secrete enzymes & acid)

Bone Remodeling

  • ~5–7% of bone recycled weekly

  • Spongy bone replaced every 3–4 yrs; compact every 10 yrs

  • Remodeling units: osteoblasts + osteoclasts working together

  • Deposit = osteoblasts secrete osteoid

  • Resorption = osteoclasts break down matrix

Bone Repair (Fractures)

  1. Hematoma formation – blood clot, swelling, pain

  2. Fibrocartilaginous callus – collagen + new vessels, debris cleared

  3. Bony callus – spongy bone replaces callus (~2 months)

  4. Remodeling – compact bone restores structure