The Skeletal System: Osseous Tissue and Skeletal Structure

The Skeletal System

Introduction

The skeletal system is a complex organ system that provides structural support, protection, and several other vital functions in the human body. It is composed of various tissues and specialized structures that work together to maintain body integrity and facilitate movement.

• Skeletal bones: The rigid framework of the body, providing shape and support.

• Cartilage: Flexible connective tissue found in joints, ear, nose, and other areas.

• Ligaments: Bands of connective tissue that connect bones to other bones, stabilizing joints.

• Connective tissue: Additional tissues that stabilize and support the skeleton.

Bones are dynamic organs composed of several tissue types, not just mineralized matrix.

Functions of the Skeletal System

The skeletal system performs several essential functions necessary for survival and homeostasis:

• Support: Provides the framework for the attachment of organs and soft tissues.

• Storage of minerals:

  • Calcium ions: Approximately 98% of the body's calcium is stored in bones.

  • Phosphate ions: Also stored in bone tissue, important for energy metabolism and cellular function.

• Blood cell production: Bone marrow produces erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (for blood clotting).

• Leverage: Muscles attach to bones and use them as levers to produce movement.

• Protection:

  • Ribs: Protect the heart and lungs.

  • Skull: Protects the brain.

  • Vertebrae: Protect the spinal cord.

  • Pelvic bones: Protect reproductive organs.

Structure and Function of Bone (Osseous Tissue)

Bone as a Supporting Connective Tissue

Bones are a specialized form of connective tissue with a solid, mineralized matrix and several cell types.

• Specialized cells: Responsible for bone formation, maintenance, and resorption.

• Solid matrix: Provides rigidity and strength.

• Outer lining (Periosteum): A dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints.

• Inner lining (Endosteum): A thin vascular membrane lining the inner surface of the bony tissue that forms the medullary cavity.

Histological Organization of Mature Bone

The microscopic structure of bone is essential for its function and strength.

• Matrix of bone:

  • Calcium phosphate: Eventually converts to hydroxyapatite crystals, which are the primary mineral component of bone.

  • Calcium phosphate makes up about 2/3 of the bone's mass.

  • Hydroxyapatite crystals: Provide resistance to compression, making bones hard and durable.

• Collagen fibers:

  • Make up about 1/3 of the bone matrix.

  • Contribute to the tensile strength of bones, allowing them to resist stretching and twisting forces.

  • The combination of collagen and hydroxyapatite makes bone tissue extremely strong and resilient.

• Bone cells: Account for only about 2% of the bone mass but are crucial for bone maintenance and remodeling.

Key Terms

• Osseous tissue: Another term for bone tissue, a type of connective tissue with a hardened matrix.

• Periosteum: The outer fibrous layer covering bones, involved in protection, nourishment, and repair.

• Endosteum: The inner cellular layer lining the medullary cavity, important for bone growth and repair.

• Hydroxyapatite: The mineral form of calcium phosphate found in bone, with the chemical formula $Ca_{10}(PO_4)_6(OH)_2$.

Example: Bone Matrix Composition

• In a typical adult bone, about 2/3 of the weight is due to inorganic minerals (mainly hydroxyapatite), and about 1/3 is due to organic components (mainly collagen fibers).

Additional info: The combination of mineral and organic components allows bone to be both strong (resistant to compression) and flexible (resistant to tension and torsion).