Chapter 6 Study Guide: Skeletal Cartilages, Bone Structure, and Bone Homeostasis

Skeletal Cartilages

Cartilage is a specialized connective tissue that provides support, flexibility, and resilience in various parts of the skeleton. There are three main types of cartilage found in the human body, each with distinct properties and functions.

• Functional Properties: Cartilage tissue is avascular, flexible, and capable of withstanding compression and tension. It is composed of chondrocytes embedded in an extracellular matrix rich in collagen and proteoglycans.

• Major Cartilages of the Adult Skeleton: The three types are hyaline cartilage (found in articular surfaces, costal cartilages, respiratory tract), elastic cartilage (external ear, epiglottis), and fibrocartilage (intervertebral discs, pubic symphysis).

• Cartilage Growth: Cartilage grows by two mechanisms: appositional growth (addition of new layers on the surface) and interstitial growth (expansion from within).

Classification of Bones

Bones are classified based on their shape and function, which relate to their roles in the skeleton.

• Major Regions of the Skeleton: The skeleton is divided into the axial skeleton (skull, vertebral column, rib cage) and appendicular skeleton (limbs and girdles).

• Bone Classes: Bones are classified as long bones (e.g., femur, humerus), short bones (e.g., carpals, tarsals), flat bones (e.g., sternum, skull), and irregular bones (e.g., vertebrae, hip bones).

• Functions: Each class of bone serves specific functions such as support, protection, movement, mineral storage, and blood cell formation.

Functions of Bones

Bones perform several vital functions necessary for the maintenance and protection of the body.

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

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

• Movement: Bones act as levers for muscles to produce movement.

• Mineral Storage: Bones store minerals, especially calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Formation: Hematopoiesis occurs in the red marrow of certain bones.

• Triglyceride Storage: Yellow marrow stores fat as an energy reserve.

Bone Structure

The structure of bones varies depending on their type and function. Long bones and flat bones have distinct anatomical features.

• Gross Anatomy: A typical long bone consists of the diaphysis (shaft), epiphyses (ends), metaphysis (region between diaphysis and epiphysis), articular cartilage (covers joint surfaces), periosteum (outer membrane), and endosteum (lining of the medullary cavity).

• Bone Marrow: Red marrow is involved in hematopoiesis, while yellow marrow stores fat.

• Bone Markings: Surface features such as projections, depressions, and openings serve as attachment sites for muscles, passageways for nerves and blood vessels.

• Compact vs. Spongy Bone: Compact bone is dense and forms the outer layer; spongy bone (trabecular bone) is porous and found at the ends of long bones and inside flat bones.

• Chemical Composition: Bone is composed of organic components (collagen fibers, cells) and inorganic components (hydroxyapatite crystals).

Bone Development

Bones develop through two main processes: intramembranous and endochondral ossification.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue, forming flat bones like the skull.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage, responsible for the formation of long bones.

• Epiphyseal Plate Growth: Longitudinal bone growth occurs at the epiphyseal plate through proliferation, hypertrophy, calcification, and ossification of cartilage cells.

Bone Homeostasis: Remodeling and Repair

Bone is a dynamic tissue that undergoes continuous remodeling and repair, regulated by cellular and hormonal mechanisms.

• Cells Involved: Osteoblasts (bone-forming cells), osteocytes (mature bone cells), and osteoclasts (bone-resorbing cells) coordinate bone remodeling.

• Remodeling Process: Bone remodeling involves resorption by osteoclasts and formation by osteoblasts, maintaining bone strength and mineral homeostasis.

• Regulation: Hormones (parathyroid hormone, calcitonin, sex hormones) and physical stress (mechanical loading) influence bone remodeling.

• Fracture Repair: Bone repair occurs in stages: hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Homeostatic Imbalances of Bone

Disorders of bone remodeling can lead to various diseases affecting bone density and structure.

Additional info: Bone remodeling is essential for adapting to mechanical stress, repairing microdamage, and regulating calcium levels in the body. Disorders can result from hormonal imbalances, nutritional deficiencies, or genetic factors.