Bones and Skeletal Tissue

Overview

The skeletal system is a dynamic organ system essential for support, movement, protection, and various metabolic functions. This section introduces the structure, function, and classification of bones and skeletal cartilages.

Skeletal Cartilages

Types and Locations

• Hyaline cartilage: Most abundant; found in articular surfaces, costal cartilages, respiratory tract, and nasal cartilages.

• Elastic cartilage: Provides flexibility; found in the external ear and epiglottis.

• Fibrocartilage: Highly compressible; found in intervertebral discs, pubic symphysis, and menisci of the knee.

Cartilage is avascular and receives nutrients by diffusion.

Growth of Cartilage

• Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix against the external face of existing cartilage.

• Interstitial growth: Chondrocytes within the cartilage divide and secrete new matrix, expanding the cartilage from within.

Functions of Bones

Major Functions

• Support: Provides a framework that supports the body and cradles soft organs.

• Protection: Protects vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral and growth factor storage: Reservoir for minerals, especially calcium and phosphate; stores growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities of certain bones.

• Triglyceride (fat) storage: Fat is stored in yellow bone marrow.

• Hormone production: Bones produce osteocalcin, which helps regulate bone formation and protects against obesity and glucose intolerance.

Classification of Bones

By Location

• Axial skeleton: Forms the long axis of the body; includes the skull, vertebral column, and rib cage.

• Appendicular skeleton: Includes bones of the upper and lower limbs and the girdles (shoulder and pelvic) that attach them to the axial skeleton.

By Shape

• Long bones: Longer than they are wide; consist of a shaft and two ends (e.g., humerus, femur).

• Short bones: Cube-shaped (e.g., carpals, tarsals); sesamoid bones (e.g., patella) are a special type of short bone that form within tendons.

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, scapulae, ribs, most skull bones).

• Irregular bones: Complicated shapes that do not fit other categories (e.g., vertebrae, hip bones).

Summary Table: Axial vs. Appendicular Skeleton

Key Terms

• Osteology: The study of the structure and function of the skeleton and bony structures.

• Hematopoiesis: The process of blood cell formation, which occurs in the red bone marrow.

• Perichondrium: Dense irregular connective tissue membrane covering cartilage.

Example Application

When a person fractures a long bone such as the femur, the bone's ability to support weight and facilitate movement is compromised, highlighting the importance of bone structure and function in daily activities.