Cartilage

Composition and Structure

Cartilage is a specialized connective tissue that provides support and flexibility to various parts of the body. It is primarily composed of:

• Bone salts (not present in cartilage, but in bone)

• Water – major component, providing resilience

• Blood vessels – cartilage is avascular (lacks blood vessels)

• Nerves – cartilage is aneural (lacks nerves)

Additional info: Cartilage matrix consists mainly of water, collagen fibers, and proteoglycans.

Perichondrium

The perichondrium is a dense layer of connective tissue that surrounds most cartilage types (except articular cartilage). Its functions include:

• Nourishing cartilage and resisting expansion

• Forming lacunae (spaces housing chondrocytes)

• Producing collagen

• Replacing osteons (not applicable to cartilage)

Chondrocytes

Chondrocytes are the primary cells in cartilage, responsible for maintaining the cartilage matrix.

• Derived from bone stem cells

• Reside within lacunae

• Other cell types: Fibroblasts (produce collagen), Osteoclasts (bone resorption)

Types of Cartilage

Cartilage types are classified based on their fiber composition:

• Hyaline cartilage: Contains only collagen fibers; most abundant type

• Elastic cartilage: Contains elastic fibers

• Fibrocartilage: Contains thick collagen fibers

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional growth: Addition of new matrix by perichondrium cells at the surface

• Interstitial growth: Division of chondrocytes within lacunae, expanding cartilage from within

Bones

Functions of Bones

Bones serve several vital functions in the human body:

• Support: Framework for the body

• Protection: Shields vital organs

• Hormone production: Osteocalcin secretion

• Digestion: Not a function of bones

Bone Matrix and Cells

The bone matrix is composed of:

• Hydroxyapatite: Mineral component ()

• Osteoid: Organic component (collagen and ground substance)

• Periosteum: Outer fibrous layer covering bone

Bone cells include:

• Osteoblasts: Bone-forming cells

• Osteocytes: Mature bone cells located in lacunae

• Osteoclasts: Bone-resorbing cells

• Chondrocytes: Cartilage cells

Classification of Bones

Bones are classified by shape and location:

• Long bones: e.g., femur, humerus

• Short bones: e.g., carpals, tarsals

• Flat bones: e.g., sternum, skull

• Irregular bones: e.g., vertebrae

• Sesamoid bones: e.g., patella

The human skeleton contains 206 named bones.

Axial vs. Appendicular Skeleton

The skeleton is divided into two main parts:

• Axial skeleton: Skull, vertebral column, rib cage

• Appendicular skeleton: Limbs, girdles

Limbs are not part of the axial skeleton.

Bone Development and Growth

Ossification

Ossification is the process of bone tissue formation. Two main types:

• Intramembranous ossification: Forms flat bones (e.g., skull, clavicle)

• Endochondral ossification: Forms most bones, including long bones

Epiphyseal Plate Zones

The epiphyseal plate (growth plate) has several zones:

• Resting zone: Inactive cartilage

• Proliferation zone: Rapid cell division

• Hypertrophic zone: Enlarged chondrocytes

• Calcification zone: Matrix becomes calcified

Hormonal Regulation of Bone Growth

Bone growth and remodeling are regulated by hormones:

• Growth hormone: Stimulates epiphyseal plate activity in childhood

• Thyroid hormone: Modulates growth hormone effects

• Testosterone and estrogen: Promote growth spurts and epiphyseal plate closure

• Parathyroid hormone (PTH): Stimulates osteoclasts to raise blood calcium

• Calcitonin: Inhibits osteoclast activity, lowering blood calcium

Bone Remodeling and Repair

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption and formation:

• Osteoclasts: Resorb bone tissue

• Osteoblasts: Form new bone tissue

Remodeling is influenced by mechanical stress, hormones, and nutritional status.

Bone Repair

Bone repair after fracture occurs in stages:

• Hematoma formation: Blood clot forms at fracture site

• Fibrocartilaginous callus formation: Soft callus bridges the fracture

• Bony callus formation: Hard callus replaces soft callus

• Bone remodeling: Restores bone to original shape

A fracture in which bone ends retain normal position is called nondisplaced.

Summary Table: Bone Cells and Functions

Key Equations

• Hydroxyapatite formula:

Examples and Applications

• Sesamoid bone example: Patella (kneecap)

• Flat bone example: Sternum

• Intramembranous ossification: Formation of skull bones

• Endochondral ossification: Formation of femur, humerus