Anatomy and Physiology: The Skeletal System and Bone Tissue

Introduction to the Skeletal System and Bone as an Organ

  • Bone as an Organ: Bone is classified as an organ because it is composed of multiple types of tissue working together. These include bone tissue (osseous tissue), dense irregular connective tissue, and areolar connective tissue.

  • Course Scope: The study of the skeletal system covers functions, structure, histology (microscopic appearance), blood and nerve supply, bone formation, and metabolic roles (especially calcium maintenance), as well as the effects of exercise and aging.

Principal Functions of Bone

  • Support: Bones provide a structural framework for the body and support soft tissues, such as muscle. They provide the necessary attachment points for tendons.

    • Tendons: Dense regular connective tissue that attaches skeletal muscle to bone.

    • Ligaments: Connective tissue that attaches bone to bone.

  • Protection: Bones shield delicate internal organs from injury.

    • Cranium (Skull): Protects the brain.

    • Vertebrae (Backbone): Protect the spinal cord.

    • Sternum and Ribs: Protect the heart and lungs within the thoracic cavity.

  • Assistance in Movement: Skeletal muscles attach to bones. When muscles contract, they produce force that moves the bones. Bones respond to muscle contraction.

  • Mineral Homeostasis: Bone tissue acts as a reservoir for minerals, primarily calcium and phosphorus.

    • Calcium Storage: About 99\text{%} of the body's calcium is stored in bone tissue.

  • Blood Cell Production: Red bone marrow in certain cavities produces blood cells through hemopoiesis.

  • Triglyceride Storage: Yellow bone marrow stores triglycerides as potential chemical energy reserve.

Anatomy of a Long Bone

  • Long Bone Definition: Bones that are longer than they are wide.

  • Diaphysis: The cylindrical main portion of the bone.

  • Epiphysis: The ends of the bone, proximal and distal.

    • Proximal Epiphysis: End closest to the limb attachment to the axial skeleton.

    • Distal Epiphysis: End furthest from the attachment point.

  • Metaphysis: Region between the diaphysis and epiphysis, housing the epiphyseal growth plate.

    • Epiphyseal Growth Plate: Allows the diaphysis of the bone to grow in length.

    • Epiphyseal Line: Replaces cartilage when growth stops.

  • Articular Cartilage: Covers the epiphysis where it forms a joint, reducing friction and absorbing shock.

  • Periosteum: A sheath covering the bone surface, aiding growth and repair.

    • Outer Layer: Dense irregular connective tissue.

    • Inner Layer: Contains osteogenic cells.

  • Medullary Cavity: Hollow space within the diaphysis, containing yellow bone marrow and blood vessels.

  • Endosteum: Lines the medullary cavity with areolar connective tissue.

Histology of Bone Tissue

  • Connective Tissue Composition: Bone has cellular and extracellular matrix (ECM).

  • Extracellular Matrix (ECM): Composed of 25\text{%} water, 25\text{%} collagen fibers, and 50\text{%} mineral salts.

    • Mineral Salts: Mainly calcium phosphate, providing hardness; collagen fibers offer flexibility.

The Chicken Bone Experiment

  • Acid Treatment: Soaking a bone in vinegar dissolves mineral salts, making it flexible.

  • Heat Treatment: Baking a bone destroys collagen, making it brittle.

Types of Bone Cells

  1. Osteogenic Cells: Stem cells that develop into osteoblasts.

  2. Osteoblasts: Create and secrete bone matrix and collagen.

  3. Osteocytes: Mature bone cells maintaining metabolism and regulating nutrient exchange.

  4. Osteoclasts: Break down bone tissue to release minerals and maintain calcium levels.

Compact Bone Tissue

  • Characteristics: Provides protection, support, and resists stresses.

  • Osteon: Basic functional unit containing central canal and concentric lamellae.

    • Fiber Orientation: Collagen fibers arranged for resistance against forces.

    • Lacunae: Spaces containing osteocytes.

    • Canaliculi: Canals allowing nutrient distribution.

    • Central Canal: Contains blood and lymphatic vessels.

Spongy Bone Tissue

  • Characteristics: Lattice-like structure providing lightness, found inside bones.

  • Trabeculae: Irregular lamellae arrangement.

  • Nutrient Diffusion: Nutrients diffuse from marrow spaces.

Bone Marrow Distribution

  • Infants: Primarily red bone marrow present.

  • Adults: Red marrow decreases; hemopoiesis persists in specific locations.

  • Biopsy: Usually done on iliac crest to assess blood cell production.

Blood and Nerve Supply

  • Sensitivity: Periosteum is rich in sensory nerves.

  • Vascularization: Bone is highly vascularized for nutrient supply and waste removal.