In-depth Notes on Bones and Bone Tissue

Chapter 6: Bones and Bone Tissue

Functions of Bone

• Protection: The skeleton protects vital organs (e.g., the brain).
• Mineral Storage: Bones store minerals like calcium ($Ca^{2+}$) and phosphate ($PO_4^{3-}$) necessary for electrolyte balance.
• Blood Cell Formation: Red bone marrow is crucial for producing blood cells.
• Fat Storage: Yellow bone marrow stores triglycerides.
• Movement: Bones act as levers for muscles.
• Support: The skeleton provides structural support to the body.

Classification of Bones by Shape

• Long Bones: Longer than they are wide (e.g., humerus).
• Short Bones: About as long as they are wide (e.g., patella).
• Flat Bones: Broad, flat, and thin (e.g., sternum).
• Irregular Bones: Unique shapes (e.g., vertebrae).
• Sesamoid Bones: Round and flat, located within tendons (e.g., trapezium).

Structure of Long Bones

• Key Components:
  • Hyaline cartilage (articular)
  • Epiphysis (ends of long bone)
  • Diaphysis (shaft of long bone)
  • Medullary cavity (hollow center filled with yellow marrow)
  • Nutrient foramen (opening for blood vessels)
  • Periosteum (outer layer containing osteoblasts)

Structure of Short, Flat, Irregular, & Sesamoid Bones

• Components: Compact bone surrounding spongy bone or diploë (with red bone marrow), covered by periosteum.

Bone Matrix

• Composition:
  • Organic: ~1/3 collagen fibers for flexibility.
  • Inorganic: ~2/3 calcium phosphate (hydroxyapatite) and calcium carbonate for hardness.

Types of Bone Cells

• Osteoblasts: Build bone matrix by depositing collagen and minerals.
• Osteocytes: Maintain the bone matrix and communicate changes in the bone environment.
• Osteoclasts: Resorb (break down) bone matrix via enzymes and acids.

Structure of Compact Bone

• Composed of osteons (Haversian systems):
  • Central canals with blood vessels.
  • Lamellae (layers of mineralized matrix).
  • Lacunae (spaces housing osteocytes).
  • Canaliculi (tiny canals for communication).

Structure of Spongy Bone

• Contains trabeculae (thin plates) that provide structural support while reducing weight.
• Filled with red bone marrow and blood vessels.

Normal vs. Osteoporotic Bone

• Healthy bone has denser structure compared to osteoporotic bone which appears more fragile and porous.

Bone Development: Intramembranous and Endochondral Ossification

• Intramembranous Ossification:
  • Occurs in flat bones (e.g., frontal bone of the skull).
  • Osteoblasts develop from mesenchymal cells and secrete matrix to form bone.
• Endochondral Ossification:
  • Occurs in long bones (e.g., femur).
  • Hyaline cartilage serves as a precursor that is gradually replaced by bone tissue.

Zones of the Epiphyseal Plate

• Zone of Proliferation: Rapid chondrocyte division.
• Zone of Hypertrophy and Maturation: Chondrocytes mature and enlarge.
• Zone of Calcification: Matrix calcifies.
• Zone of Ossification: Newly formed bone replaces cartilage.

Bone Remodeling: Deposition vs. Resorption

• Deposition: Osteoblasts deposit new bone.
• Resorption: Osteoclasts break down old bone.
• Factors influencing remodeling include mechanical stress, calcium intake, and hormone levels.

Hormonal Regulation of Bone Remodeling

• Parathyroid Hormone (PTH): Increases blood calcium levels by stimulating osteoclast activity.
• Calcitonin: Lowers blood calcium levels by inhibiting osteoclast activity.
• Vitamin D: Essential for calcium absorption from the diet.

Bone Fractures

• Types:
  • Spiral: Caused by twisting forces.
  • Compression: Bone is crushed (common in elderly).
  • Comminuted: Bone shatters into multiple fragments.
  • Avulsion: Tendon/ligament pulls off bone fragment.
  • Greenstick: Bone bends on one side without breaking entirely (common in children).

Bone Fracture Repair Sequence

1. Hematoma formation following fracture.
2. Soft callus formation by fibroblasts and chondroblasts.
3. Hard callus formation through osteoblast activity.
4. Bone remodeling to restore original structure.