Osteology: Bone Structure Overview

Bone Structure

• Bone is a complex organ consisting of bone tissue, cartilage, connective tissue, epithelial tissue, adipose tissue, and nervous tissue.
• Osteology is the study of bone structure.
• Skeletal System Functions:
  • Support: Provides framework for muscles and tissues.
  • Protection: Safeguards internal organs.
  • Movement: Enables movement through muscle contractions.
  • Mineral Homeostasis: Stores and releases calcium and phosphorus.
  • Blood Cell Production: Red blood cells are produced in specific bones.
  • Triglyceride Storage: Yellow bone marrow stores triglycerides.

Long Bone Structure

• Diaphysis: The shaft of a long bone.
• Epiphyses: The knobby ends of a long bone.
• Metaphyses: Regions connecting the diaphysis to the epiphyses.
• Articular Cartilage: Reduces friction at joints.
• Periosteum: The outer covering of bones, where tendons and ligaments attach.
• Medullary Cavity: The hollow part of the diaphysis, containing yellow bone marrow and blood vessels.
• Endosteum: The membrane lining the medullary cavity, composed of osteocytes and connective tissue.

Bone Tissue Composition

• ECM Components: Water (15%), collagen fibers (30%), and mineral salts (55%).
• Calcification: Mineral salts deposit on collagen fibers, making the tissue hard.
• Mineral Salts:
  • Hydroxyapatite (Ca10(PO4)6(OH)2)
  • Calcium phosphate (Ca3(PO4)2)
  • Calcium hydroxide (Ca(OH)2)
  • Calcium carbonate (CaCO3)

Bone Cells

• Osteoprogenitor Cells: Stem cells of bone, located deep to the periosteum.
• Osteoblasts: Secrete collagen and other ECM components, do not divide, and become osteocytes.
• Osteocytes: Mature bone cells, do not divide or secrete, acquire nutrients and eliminate wastes.
• Osteoclasts: Catabolize bone, formed by fusion of monocytes, secrete lysosomal enzymes and acids for bone resorption.

Bone Tissue Types

• Compact Bone: Strongest bone tissue, densely packed ECM, structural unit is the osteon.
• Osteon: Arranged along lines of stress, contains blood vessels in the osteonic canal, surrounded by concentric lamellae.
  • Interstitial lamellae: Old concentric lamellae from old osteons.
  • Circumferential lamellae: Surround long bone (along circumference), joined to the periosteum via the perforating fibres.
• Osteocytes: Occupy lacunae within the ECM.
• Canaliculi: Permit communication between osteocytes, filled with extracellular fluid.
• Interosteonic Canals: Provide passage for blood vessels and nerves.
• Spongy Bone: Lighter than compact bone, structural unit is trabeculae, surrounded by red bone marrow and blood vessels.
  • Functions: Provides support, protects bone marrow, handles stress along multiple axes.

Bone Vascularization and Innervation

• Periosteal Arteries: Nourish the periosteum.
• Nutrient Arteries: Penetrate the diaphysis and branch into bone marrow, enter through the nutrient foramen.
• Epiphyseal and Metaphyseal Arteries: Nourish the internal tissues of the epiphyses, branches of the nutrient artery.
• Veins: Exit through the same entrance point as the cognate arteries (e.g., nutrient vein exits via the nutrient foramen).
• Nerves: Run similar paths to blood vessels through bones.

Bone Remodeling

• Bone Remodeling: Continuous process of bone resorption and deposition (5% of bone is remodeled at any given time).
• Functions:
  • Bone growth.
  • Healing after injuries.
  • Adaptation to exercise and diet changes.
• Process: Osteoclasts resorb bone releasing minerals into the blood, osteoblasts deposit new bone along lines of mechanical stress.
• Factors affecting bone remodeling: Weight-bearing exercise, sedentary lifestyle, injury, space travel (lack of stress leads to bone loss).

Calcium Homeostasis

• Calcium Storage: Bone stores 99% of the body's calcium.
• Calcium Functions: Muscle and nerve function, blood clotting, enzyme cofactor.
• Calcium Regulation:
  • Parathyroid Hormone (PTH): Stimulates osteoclast activity and increases calcium reabsorption in the kidneys.
  • Calcitonin: Inhibits osteoclast activity and promotes calcium deposition in bone.

Ossification: Bone Formation

• Ossification: Process of bone formation.
• Phases:
  • Development of the embryonic skeleton.
  • Development during childhood and adolescence.
  • Bone remodeling.
  • Repair of fractures.
• Types:
  • Intramembranous ossification: Bone forms directly from mesenchymal tissue (e.g., skull bones, facial bones).
  • Endochondral ossification: Bone forms from a hyaline cartilage intermediate (e.g., long bones).

Intramembranous Ossification

• Steps:
  • Ossification Center Formation: Mesenchymal cells become osteoblasts.
  • Calcification: Mineral salts deposit within the ECM.
  • Trabeculae Formation: Spongy bone tissue formation.
  • Periosteum Formation: Mesenchymal tissue surrounding trabeculae compacts.

Endochondral Ossification

• Steps:
  • Hyaline Cartilage Model: Mesenchymal cells form chondroblasts.
  • Chondrocyte Proliferation: Interstitial growth, chondrocytes die and become lacunae.
  • Vascular Invasion: Nutrient artery penetrates the perichondrium.
  • Primary Ossification Center: Osteoprogenitor cells become osteoblasts, forming trabeculae in the diaphysis.
  • Secondary Ossification Center: Forms at birth.
  • Articular Cartilage: Hyaline cartilage becomes articular cartilage.
  • Epiphyseal Plate Closure: Cartilage remains at the metaphysis until adolescence and ossifies eventually.

Bone Growth: Interstitial and Appositional

• Interstitial Growth: Lengthwise growth,occurs at the epiphyseal plate.
  • Zones:
    • Resting Zone: Anchors epiphyseal plate.
    • Proliferating Zone: Actively dividing chondrocytes.
    • Hypertrophic Zone: Mature chondrocytes.
    • Calcified Zone: Dead chondrocytes that are replaced by bone.
• Epiphyseal Plate Closure: Chondrocytes stop dividing, the plate calcifies (around 18 years for females, 21 years for males).
• Appositional Growth: Circumferential growth, occurs due to periosteal cells becoming osteoblasts.
  • Steps:
    • Periosteal Osteoblast Activity: Periosteal cells become osteoblasts.
    • Ridge Fusion: Ridges of ECM fuse, periosteum becomes endosteum.
    • Endosteal Osteoblast Activity: Endosteal osteoblasts secrete ECM.
    • Concentric Lamellae Formation: Increases bone diameter.

Factors Affecting Bone Growth

• Nutrition: Calcium, vitamin D, and other nutrients are essential for bone growth.
• Hormones:
  • Growth Hormone: Stimulates osteoblasts.
  • Thyroid Hormones: Stimulate osteoblasts.
  • Estrogen: Inhibits bone resorption, promoting apoptosis of osteoclasts.
  • Testosterone: Alters bone growth rates.
• Exercise: Weight-bearing exercise stimulates bone remodelling.

Bone Disorders

• Osteoporosis: Loss of bone density, increased bone resorption, increased fracture risk, affects elderly women.
• Rickets: Vitamin D deficiency in children leading to bone softening and deformation.
• Osteomalacia: Vitamin D deficiency in adults.

Fractures and Bone Repair

• Fractures: Breaks in bones.
  • Stress Fractures: Microscopic breaks, do not damage surrounding tissues.
  • Compound Fractures: Large breaks, damage surrounding tissues.
• Treatment:
  • Reduction: Realignment of bone fragments.
  • Closed Reduction: Non-surgical, "setting" the bones.
  • Open Reduction: Surgical intervention, bone fixation with screws, plates.
• Healing Phases:
  • Reactive Phase: Hematoma formation, inflammation.
  • Reparative Phase: Fibrocartilaginous callus formation, bony callus formation.
  • Remodelling Phase: Osteoclasts remove dead bone, compact bone replaces spongy bone.