Osseous Tissue and Bone Structure: The Skeletal System

Ch.6 Osseous Tissue and Bone Structure

Overview of the Skeletal System

The skeletal system is a complex organ system that provides the framework for the human body. It consists of bones, cartilage, tendons, and ligaments, each contributing to the structure, movement, and protection of the body.

• Bones: Approximately 206 bones in the adult human body.

• Cartilage: Flexible connective tissue found in joints, ear, nose, and respiratory tract.

• Tendons: Connect muscle to bone, enabling movement.

• Ligaments: Connect bone to bone, stabilizing joints.

Divisions of the Skeletal System

The skeletal system is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton (80 bones):

  • Includes the bones of the skull, thorax (rib cage), and vertebral column.

  • Forms the longitudinal axis of the body.

• Appendicular Skeleton (126 bones):

  • Includes the bones of the limbs (arms and legs) and the girdles (shoulder and pelvic) that attach them to the axial skeleton.

Functions of Bones

Bones perform several vital functions necessary for survival and movement.

• Support: Provide structural support and a framework for the attachment of soft tissues and organs.

• Storage:

  • Minerals: Reservoir for calcium and phosphorus, essential for various physiological processes.

  • Lipids: Triglyceride storage in yellow bone marrow.

• Blood Cell Production: Hematopoiesis occurs within the marrow cavities of bones, producing red blood cells, white blood cells, and platelets.

• Protection: Bones encase and protect vital organs such as the brain (skull), spinal cord (vertebrae), and thoracic organs (rib cage).

• Leverage: Bones act as levers to change the magnitude and direction of skeletal muscle forces, enabling a wide range of movements.

Classification of Bones

Bones are classified based on their shape and structure, which relate to their function.

• Long Bones: Longer than they are wide; primarily found in the limbs (e.g., humerus, femur).

• Short Bones: Approximately equal in length and width; found in the wrist (carpals) and ankle (tarsals).

• Flat Bones: Thin, flattened, and usually curved; found in the skull (parietal bone), ribs, and sternum.

• Irregular Bones: Complex shapes that do not fit other categories; found in the vertebrae and certain facial bones.

• Sesamoid Bones: Small, round bones embedded within tendons; example is the patella (kneecap).

• Sutural (Wormian) Bones: Small bones located within the sutures of the skull.

Bone Type	Example	Location
Long Bone	Humerus	Arm
Short Bone	Carpal bones	Wrist
Flat Bone	Parietal bone	Skull
Irregular Bone	Vertebra	Spine
Sesamoid Bone	Patella	Knee
Sutural Bone	Sutural bones	Skull sutures

Microscopic Structure of Bone

Bones are composed of specialized cells and a matrix of organic and inorganic materials. The two main types of bone tissue are compact bone and spongy bone.

• Compact Bone: Dense outer layer; contains osteons (Haversian systems) with concentric lamellae surrounding a central canal.

• Spongy Bone (Cancellous Bone): Internal layer with a honeycomb network of trabeculae, filled with red or yellow bone marrow.

Key Structures in Compact Bone:

• Osteon: The structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes (bone cells).

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

• Lamellae: Layers of bone matrix within the osteon.

• Cement Line: Outer boundary of the osteon.

Example:

The humerus is a long bone in the arm, composed of a dense outer layer of compact bone and an inner region of spongy bone, which contains red bone marrow for blood cell production.

Additional info: The axial skeleton provides protection for the central nervous system and vital organs, while the appendicular skeleton is primarily involved in movement and manipulation of the environment.