Study Notes on Bones and Bone Structure

Chapter Six: Bones and Bone Structure

Bone Functions

  • Major Functions of Bone:

    1. Support: Provides a framework for the body.

    2. Protection: Shields vital organs (e.g., skull protects the brain).

    3. Movement: Muscles attach to bones and contract to facilitate movement.

    4. Mineral and Growth Factor Storage: Stores essential minerals and growth factors.

    5. Hematopoiesis: Responsible for the formation of blood cells.

Bone Classification

  • Total Number of Bones: Approximately 206 bones (varies by individual).

  • Two Main Groups:

    • Axial Skeleton: Includes the skull, vertebral column, and rib cage.

    • Appendicular Skeleton: Comprises the upper and lower limbs, including shoulder and pelvic girdles (commonly confused with axial).

  • Classification by Shape:

    1. Long Bones: Longer than wide, consists of a shaft and two ends.

    • Examples: Limb bones, excluding patella and wrist & ankle bones.

    • Definition: A long bone must possess a shaft (diaphysis) with two ends (epiphyses).

    1. Short Bones: Cube-shaped bones.

    • Examples: Wrist and ankle bones (carpals and tarsals).

    1. Sesamoid Bones: Specialized short bones embedded within tendons, exemplified by the patella.

    2. Flat Bones: Generally thin and sometimes curved.

    • Examples: Ribs, skull, sternum, scapula.

    1. Irregular Bones: Complex shapes that don’t fit into other categories.

    • Examples: Vertebrae, hip bones.

    1. Sutural (Wormian) Bones: Found within the sutures of the skull and may vary in number among individuals.

Bone Markings

  • Importance of Understanding Bone Surface Features:

    • Key for lab activities and memorization.

  • Key Terms:

    • Process: Projection or bump on a bone.

    • Foramen: Round passageway for blood vessels or nerves; commonly referred to as a "hole".

    • Bone features examples: foramen (hole), fissure (groove), process (projection), tuberosity (large bump), head (rounded end), crest (prominent ridge), condyle (rounded process).

Bone Structure

  • Structure of Long Bones:

    • Diaphysis: Shaft of the long bone.

    • Epiphyses: Ends of the long bone.

    • Metaphysis: Region between diaphysis and epiphysis.

    • Articulations: Where bones meet; covered by hyaline cartilage.

  • Flat Bones Structure:

    • Consist of compact bone exterior and spongy (diploë) interior.

    • Spongy bone contains trabeculae (irregular projections).

  • Bone Matrix Composition:

    • Comprised of organic (living) and inorganic (non-living) materials:

    • Organic Matrix: Composed of cells and matrix material.

    • Inorganic Matrix: Mainly mineral salts, predominantly calcium phosphates, providing hardness and longevity.

  • Major Cell Types in Bone:

    1. Osteogenic (Osteoprogenitor) Cells: Stem cells found in periosteum and endosteum that differentiate into osteoblasts.

    2. Osteoblasts: Bone-forming cells that participate in osteogenesis (bone formation).

    3. Osteocytes: Mature bone cells situated in lacunae.

    4. Osteoclasts: Large multinucleated cells that resorb (break down) bone for calcium regulation.

Compact Bone Structure

  • Osteon (Haversian System): Main structural unit of compact bone; resembles elongated cylinders.

    • Central Canal: Contains blood vessels and nerves.

    • Concentric Lamellae: Growth rings in compact bone around central canal, with alternating collagen fibers providing strength.

    • Interstitial Lamellae: Gaps between osteons filled with bone matrix.

    • Lacunae: Small cavities housing osteocytes.

  • Compact bone is hard and dense, constituting most of the cortex of long bones.

Spongy Bone Structure

  • Difference from Compact Bone:

    • Composed of irregularly arranged lamellae and does not contain osteons.

    • Nutrients reach osteocytes via diffusion through canaliculi (small channels).

    • Contains trabeculae, contributing to structural integrity without weight.

Membranes Associated with Bone

  • Periosteum: Double-layered membrane covering the outer surface of bones;

    • Rich in osteoblasts and osteoclasts; anchored to bone through Sharpey’s fibers.

    • Nutrient foramina: Openings that allow blood vessels and nerves to enter bone.

  • Endosteum: Covers trabeculae of spongy bone and lines medullary canals, containing osteoblasts and osteoclasts.

Growth and Development of Bone

  • Postnatal Bone Growth:

    • Length Growth: Interstitial Growth occurs at epiphyseal plates (growth plates).

    • Width Growth: Appositional Growth involves both osteoblasts and osteoclasts in remodeling.

  • Mechanisms of Growth:

    • Interstitial Growth: Cartilage at epiphyseal plate converts to bone, elongating the bone.

    • Appositional Growth: Osteoclasts resorb bone from the inner section, while osteoblasts add bone to the periphery, increasing width.

Blood and Nerve Supply

  • Nutrient Arteries and Veins: Supply blood to diaphysis; enter via nutrient foramina.

  • Metaphyseal Vessels: Supply epiphyses.

  • Periosteal Vessels: Supply superficial osteons and assist in healing and development.

Bone Homeostasis

  • Regulation of Bone Health:

    • Bones continuously remodel throughout life based on activity levels and nutritional intake.

    • Osteoclasts and osteoblasts work in tandem for calcium regulation in the blood.

  • Hormonal Regulation:

    1. Parathyroid Hormone (PTH): Elevated when blood calcium is low; stimulates osteoclast activity to release calcium from bone.

    2. Calcitonin: Released when blood calcium is high to inhibit osteoclasts and promote calcium storage in bone.

  • Optimal Blood Calcium Levels: Approximately 9 to 11 mg/100 mL essential for physiological functions incl. muscle contraction and nerve impulse transmission.

Factors Affecting Bone Density

  • Hormones:

    • Sex hormones (particularly estrogen) impact bone density; deficiency leads to conditions like osteoporosis more prevalent in women post-menopause.

    • Thyroxine (T4), growth hormone, and calcitriol (vitamin D) also play roles in bone health.

  • Composition of Bone:

    • 70% inorganic material (mainly calcium phosphates) contributes to hardness; 33% organic matter (cells and collagen).

Bone Repair Mechanisms

  • Fracture Types: Classified based on:

    1. Position of Bone Ends: Nondisplaced (aligned) or displaced (misaligned).

    2. Completeness: Complete (broken through) or incomplete (partial break).

    3. Orientation: Linear (parallel) or transverse (perpendicular to long axis).

    4. Skin Penetration: Compound (penetrates skin) or simple (remains internal).

  • Treatment of Fractures:

    • Requires reduction (realignment) and immobilization (casts/traction).

  • Stages of Bone Repair:

    1. Hematoma Formation: Blood vessel rupture creates clot.

    2. Callus Formation: Fibrocartilage callus forms.

    3. Bony Callus Formation: Replacement with bony tissue.

    4. Remodeling: Reorganizing the bone structure.

Common Pathological Conditions

  • Inadequate Bone Development: Osteopenia, Osteomalacia (vitamin D deficiency), Rickets (failure to calcify epiphyseal plates).

  • Osteoporosis: Bone resorption greater than deposits, especially in postmenopausal women due to estrogen loss.

  • Paget’s Disease: Excessive but weak bone formation; increases spongy bone tissue.

  • Congenital Conditions: Achondroplasia (disproportionate dwarfism due to defective endochondral ossification).

  • Bone Tumors: Osteosarcoma, difficult to treat due to extensive vascularization and risk of metastasis.

Conclusion

  • Bone structure and function are crucial to overall health; proper nutrition, exercise, and hormonal balance are key to maintaining skeletal integrity.