6.1 Structure and Types of Cartilage

Composition and Characteristics of Cartilage

The skeleton is initially composed of cartilage and fibrous membranes, which are gradually replaced by bone during development. Cartilage is a specialized connective tissue that provides support and flexibility in various parts of the body.

• Cartilage consists primarily of: water and collagen fibers, which allow it to withstand compression and tension.

• Cartilage contains no: blood vessels or nerves, making it avascular and aneural.

• Perichondrium: Dense irregular connective tissue that surrounds cartilage, providing nutrients and support to cartilage cells.

• Chondrocytes: The cells found in all types of cartilage, responsible for maintaining the cartilage matrix.

Types and Locations of Cartilage

• Three types of cartilage:

  1. Hyaline cartilage

  2. Elastic cartilage

  3. Fibrocartilage

• Locations of hyaline cartilage: Nose, articular surfaces of joints, costal cartilages of ribs, and respiratory tract (trachea, larynx).

• Locations of elastic cartilage: External ear (auricle), epiglottis.

• Locations of fibrocartilage: Intervertebral discs, pubic symphysis.

Additional info: Hyaline cartilage is the most abundant type and provides smooth surfaces for joint movement. Elastic cartilage offers flexibility, while fibrocartilage resists compression and tension.

6.2 Function of Bone

Major Functions of Bone

Bones serve several critical functions in the human body, contributing to both structure and physiology.

• Support: Provides a framework for the body and supports soft tissues.

• Protection: Shields 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 storage: Reservoir for minerals, especially calcium and phosphorus.

• Blood cell formation: Hematopoiesis occurs in red bone marrow.

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

• Hormone production: Osteocalcin is produced by bones and helps regulate insulin and glucose metabolism.

Example: The femur supports body weight and enables walking, while the sternum protects the heart.

6.3 Classification of Bones by Location and Shape

Bone Types and Skeletal Divisions

Bones are classified based on their location and shape, which reflects their function and structure.

• Axial skeleton: Skull, vertebral column, and rib cage; provides central support and protection.

• Appendicular skeleton: Upper and lower limbs, including girdles; enables movement and manipulation.

• Long bones: Longer than they are wide (e.g., femur, phalanges).

• Short bones: Cube-shaped (e.g., carpals, tarsals).

• Flat bones: Thin and often curved (e.g., cranial bones, sternum).

• Irregular bones: Complex shapes (e.g., vertebrae, hip bones).

Additional info: Phalanges are considered long bones due to their elongated shape, despite their small size.

6.4 Gross Structure of Bone

Bone as an Organ and Its Components

Bones are considered organs because they contain multiple tissue types that work together to perform complex functions.

• Bone tissue (osseous tissue)

• Other tissues found in bone:

  1. Nervous tissue

  2. Cartilage

  3. Fibrous connective tissue

  4. Muscle tissue (associated with tendons)

  5. Blood vessels

• Compact bone: Dense outer layer providing strength.

• Spongy bone: Inner layer with trabeculae (small needle-like pieces).

• Periosteum: Connective tissue covering the outside of bone.

• Endosteum: Connective tissue lining the inside of bone.

Typical Long Bone Structure

• Diaphysis: Shaft of the bone containing the medullary cavity.

• Epiphysis: Ends of the bone, often containing spongy bone.

• Epiphyseal line/plate: Site of bone growth in length.

• Periosteum: Outer fibrous layer and inner osteogenic layer (contains osteoprogenitor cells).

• Main blood vessels: Nutrient artery and vein enter through the nutrient foramen.

Additional info: Red marrow is found in the trabecular cavities of spongy bone in adults, especially in the hip bones and sternum.

6.5 Development of Bone (Ossification)

Bone Formation Processes

Bone tissue formation, or ossification, occurs through two main processes: intramembranous and endochondral ossification.

• Endochondral ossification: Bone develops by replacing hyaline cartilage (most bones).

• Intramembranous ossification: Bone develops from fibrous connective tissue (cranial bones, clavicles).

• Primary ossification center: Located in the diaphysis of long bones.

• Secondary ossification centers: Develop in the epiphyses before birth.

• Epiphyseal plate: Site of longitudinal bone growth until closure in adolescence.

Example: During fetal development, chondrocytes in the cartilage model die and are replaced by bone-forming cells (osteoblasts).

6.6 Bone Remodeling

Dynamic Nature of Bone Tissue

Bone is a dynamic and living tissue that undergoes continuous remodeling throughout life.

• Remodeling cycle: Entire skeleton replaced every 10 years; spongy bone every 3-4 years.

• Osteoclasts: Cells that resorb bone by secreting acids and enzymes.

• Osteoblasts: Cells that form new bone by secreting osteoid.

• Hormonal regulation: Parathyroid hormone maintains calcium levels; calcitonin and vitamin D also play roles.

• Mechanical stress: Wolff's Law states that bone grows in response to the forces placed upon it.

Additional info: Remodeling helps repair microdamage and adapt bone structure to changing mechanical demands.

6.7 Bone Repair

Fracture Healing Process

Bone repair involves a series of steps to restore integrity after a fracture.

• Fracture: Any break in bone continuity.

• Types of fractures:

  • Comminuted: Bone fragments into three or more pieces.

  • Spiral: Twisting fracture.

  • Depressed: Bone pressed inward.

  • Compression: Bone crushed.

  • Epiphyseal: Growth plate fracture.

  • Greenstick: Incomplete break.

  • Nondisplaced: Bone ends in normal position.

  • Displaced: Bone ends out of normal alignment.

  • Open (compound): Break penetrates skin.

  • Closed (simple): Break does not penetrate skin.

• Repair steps:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

Additional info: Realignment (reduction) is crucial for proper healing; closed reduction is non-surgical, open reduction involves surgery.

6.8 Bone Disorders

Common Pathologies Affecting Bone

Several disorders can affect bone health and structure.

• Osteomalacia: Softening of bones due to inadequate mineralization.

• Rickets: Childhood disorder similar to osteomalacia, often due to vitamin D deficiency.

• Osteoporosis: Reduced bone mass and increased fragility, common in older adults.

• Paget's Disease: Disorganized bone remodeling leading to abnormal bone structure.

Example: Osteoporosis increases fracture risk, especially in the hip, spine, and wrist.

Key Table: Types of Bone Fractures

Key Equations and Terms

• Ossification:

• Wolff's Law:

• Calcium Homeostasis: