Big Idea

Overview

• Bone as a Living Organ

  • Contains all four types of tissues:

    • Connective Tissue: Predominantly bone (osseous) tissue along with cartilage and dense connective tissue covering the bone’s exterior.

    • Nervous Tissue: Present in the nerves associated with bones.

    • Epithelial Tissue: Found in blood vessels that supply nourishment to bones.

    • Muscle Tissue: Skeletal muscle tissue is attached to bones.

Bone Facts

• Total Count: 206 bones constitute nearly 1/5th of a healthy adult's body weight, including bones, cartilage, ligaments, and joints.

• Skeleton Regeneration: Entire skeleton is rebuilt every 7-10 years.

• Importance of Skeleton:

  • Provides essential support to other body parts.

  • Protects delicate organs like the brain.

  • Capable of self-repair after injury.

Functions of the Skeletal System

• Support: Acts as a framework to support the body's structure.

• Protection: Safeguards vital organs; e.g., skull protects the brain, rib cage guards the heart.

• Movement: Skeletal muscles are connected to bones via tendons, leveraging bones as levers for movement.

Storage and Production

• Mineral Storage: Stores minerals such as calcium and phosphate, which can be released into the bloodstream when necessary.

• Energy Storage: Stores fat in yellow bone marrow.

• Blood Cell Formation (Hematopoiesis): Occurs in red bone marrow of certain bones.

• Hormone Production: Vital for maintaining homeostasis.

  • Produces osteocalcin, a hormone important for insulin secretion, glucose regulation, and energy usage.

  • Helps regulate blood calcium levels.

Classification of Bones

• Criteria: Bones categorized by location (axial vs. appendicular skeletons) and shape, which dictates their function.

• Four Main Types:

  1. Long Bones:

     • Longer than wide.

     • Mostly in limbs, acting as levers for movement.

     • Examples: Humerus, femur, metacarpals.

  2. Short Bones:

     • Cube-shaped, equal width and length.

     • Provide stability with minimal movement.

     • Examples: Carpals, tarsals, patella (sesamoid bone).

  3. Flat Bones:

     • Thin and flat, sometimes curved.

     • Large surface area for muscle attachment.

     • Examples: Sternum, scapula, ribs.

  4. Irregular Bones:

     • Abnormal shape, highly specialized.

     • Examples: Hip bones, vertebrae.

Bone Structure

• General Structure:

  • Consists of a dense outer layer of compact (cortical) bone and a porous inner layer of spongy bone.

  • Basic internal structure is consistent across bones.

Compact Bone Structure

• Osteons: The structural unit; long cylinders that function as weight-bearing pillars.

  • Composed of lamellae: Hollow tubes filled with salts and collagen to resist torsion stress.

  • Contains Haversian canals with blood vessels and nerve fibers.

Spongy Bone Structure

• Less organized, lacking osteons.

  • Contains trabeculae, tiny struts that resist stress and house bone marrow.

  • Types of Bone Marrow:

    • Red Bone Marrow: Responsible for blood cell production.

    • Yellow Bone Marrow: Stores energy in fat.

Bone Surface Markings

• Often rough with distinct markings due to muscle and ligament activity.

• Types of Markings:

  • Projections: Where muscles and ligaments attach.

  • Surfaces: Form joints.

  • Depressions/Openings: Allow blood vessels and nerves to run through.

Bone Cells

• Osteocytes: Maintain bone structure, housed in lacunae; function like foremen creating a response to stimuli.

• Osteoblasts: Responsible for building and constructing bones through calcification.

• Osteoclasts: Regenerate bone by absorbing tissue in areas that are not required.

Bone Formation (Ossification)

• Ossification: Process of forming bone tissue, crucial for embryo formation, growth, and remodeling.

  • Types:

    • Intramembranous Ossification: Development from fibrous tissue, e.g., clavicle.

    • Endochondral Ossification: Replacement of cartilage with bone, applicable to most bones.

  • Cartilage remains at certain locations, such as articular surfaces and epiphyseal plates for growth.

Bone Remodeling

• Continuous process essential to prevent brittleness due to crystallization of calcium.

• Process:

  • Osteocytes signal osteoclasts to address damage.

  • Osteoclasts resorb bone, returning calcium to blood.

  • Macrophages facilitate tissue remodeling.

  • Osteoblasts build new bone.

Bone Damage and Repair

• Fracture: Break in bone.

• Treatment: Needs reduction (realignment) and immobilization for healing.

• Repair Steps:

  • Hematoma forms at the break site.

  • Fibrocartilaginous callus connects broken ends.

  • Osteoblasts generate spongy bone to replace the callus.

  • Remodeling consolidates bone structure.