Anatomy and Function of the Skeletal System

Overview of the Skeletal System

  • The skeletal system consists of bones and joints and serves several important functions.

Chapter Breakdown

  • Chapter 6: Tissue-based focus on bones and their formation and healing.

  • Chapter 7: Joints and their composition and function.

  • Chapter 8: Further exploration of joint functionality.

Functions of the Skeletal System

  • Protection:

    • Protects vital organs:

    • Cranial cavity protects the brain.

    • Thoracic cage protects vital organs in the chest (e.g., heart, lungs).

  • Support:

    • Provides structural support to the body.

    • 95% of the body’s calcium is stored in bones which is essential for maintaining blood calcium levels.

    • Blood calcium levels must be kept in a tight range to prevent fatal health consequences (e.g., heart attacks).

  • Blood Cell Production:

    • Red blood cells are produced in the bone marrow, necessary for oxygen transport.

    • White blood cells, crucial for immune response, also originate in the bone marrow.

  • Fat Storage:

    • Adult bone marrow primarily consists of fat (yellow marrow).

Bone Classification by Shape

  • Bones can be classified based on their shapes:

    • Long Bones:

    • Example: limb bones (e.g., femur).

    • Short Bones:

    • Example: carpals and tarsals.

    • Flat Bones:

    • Example: skull bones, ribs.

    • Irregular Bones:

    • Example: vertebrae.

    • Sesamoid Bones:

    • Form within tendons, with the patella (kneecap) being a primary example.

    • Develop in response to mechanical stress (e.g., ongoing pressures in a tendon).

Bone Structure & Composition

  • Parts of Long Bone:

    • Epiphysis: Ends of the bone.

    • Diaphysis: Shaft of the bone.

    • Growth Plate (Epiphyseal Plate): At the junction where growth occurs.

    • Medullary Cavity: Hollow space filled with marrow.

  • Bone Types:

    • Compact Bone:

    • Dense and forms the outer layer of the bone, important for strength.

    • Spongy Bone:

    • Found within the ends of bones, less dense, contains many small cavities.

    • Allows for weight distribution and helps in shock absorption.

  • Periosteum:

    • Dense connective tissue that surrounds the bone; not part of the bone itself.

    • Provides stability, aids in bone growth, and contains blood vessels and nerves.

    • Contains perforating fibers that anchor it to the bone.

Bone Cells

  • Osteoprogenitor Cells:

    • Stem cells that develop into osteoblasts (bone-forming cells).

  • Osteoblasts:

    • Responsible for bone synthesis; secrete the bone matrix and minerals.

  • Osteocytes:

    • Mature osteoblasts that become embedded in the bone matrix; maintain bone tissue and communicate through tiny canals called canaliculi.

  • Osteoclasts:

    • Bone resorbing cells derived from a lineage of white blood cells; involved in bone remodeling.

Bone Tissue Composition

  • Cellular Composition:

    • Approximately 2% of bone tissue consists of cells (osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts).

  • Extracellular Matrix (ECM):

    • Around 98% of bone tissue's composition is the ECM, primarily consisting of:

    • 1/3 organic matrix (e.g., collagen fibers, glycoproteins).

    • 2/3 inorganic matrix (e.g., hydroxyapatite, calcium phosphate).

Bone Remodeling and Health

  • Bone remodeling is a continuous process where old bone is removed (by osteoclasts) and new bone is formed (by osteoblasts).

  • Osteons, the structural units of compact bone, resemble tree rings and contain layers (lamellae).

  • Each osteon has:

    • A central canal for blood vessels and nerves.

  • The process of how bones adapt and strengthen in response to stress, is initiated by the interaction of mechanosensors and osteoblasts.

Other Important Bone Structures

  • Nutrient Foramina:

    • Openings in bone that allow blood vessels to enter and supply the bone tissue.

  • Endosteum:

    • A thin membrane lining the medullary cavity, comprised of dense irregular connective tissue.

  • Circumferential Lamellae:

    • Layers of bone that encircle the entire surface of the bone, contributing to bone stability and strength.

Conclusion

  • Understanding bone structure, functions, and remodeling is crucial for comprehending skeletal health and the overall biology of the human body.