Skeletal Cartilages

Types and Properties of Cartilage

Cartilage is a resilient and flexible connective tissue found in various parts of the skeletal system. It provides support, flexibility, and cushioning in joints and other structures.

• General Properties:

  • Ability to compress and spring back

  • Surrounded by perichondrium (dense irregular connective tissue)

  • Resists expansion when compressed

  • Blood vessels limit thickness of cartilage

  • Remains in areas where flexibility is needed

• Types of Cartilage:

  • Hyaline cartilage: Most abundant type; fine collagen fibers not visible; found in articular, costal, respiratory, and nasal cartilages.

  • Elastic cartilage: Contains more elastic fibers; found in the external ear and epiglottis.

  • Fibrocartilage: Strongest type; found in areas with high pressure and stretch (intervertebral discs, menisci of the knee, pubic symphysis).

Cartilage Growth

Cartilage grows in two main ways, allowing for development and repair.

• Appositional growth: Cells in perichondrium add to cartilage surface.

• Interstitial growth: Chondrocytes divide within cartilage, expanding from within.

• Cartilage growth usually ends during adolescence as cartilage can harden.

Classification of Bones

Axial and Appendicular Skeleton

The human skeleton is divided into two main regions: axial and appendicular.

• Axial skeleton: Skull, vertebral column, rib cage; protects vital organs.

• Appendicular skeleton: Upper and lower limbs, pectoral and pelvic girdles; allows movement.

Types of Bones (Shape)

Bones are classified by their shapes, which relate to their functions.

• Long bones: Longer than wide (e.g., femur, humerus).

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

• Flat bones: Thin, flattened, usually curved (e.g., sternum, scapula).

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

Functions of Bone

Main Functions

Bones serve several critical functions in the body:

• Support: Framework for the body

• Protection: Shields internal organs

• Movement: Acts as levers for muscles

• Mineral and growth factor storage: Stores calcium, phosphorus

• Blood cell formation: Occurs in bone marrow (hematopoiesis)

• Triglyceride storage: Energy reserve in yellow marrow

• Hormone production: Osteocalcin regulates bone formation and energy metabolism

Gross Anatomy of Bone

Compact and Spongy Bone

Bones consist of two types of tissue: compact and spongy bone.

• Compact bone: Dense, forms outer layer

• Spongy bone: Internal, honeycomb-like structure (trabeculae)

Structure of Long Bones

• Diaphysis: Shaft

• Epiphysis: Ends of bone

• Medullary cavity: Central cavity containing marrow

• Epiphyseal plate/line: Growth area

• Articular cartilage: Covers joint surfaces

• Periosteum: Outer membrane

• Endosteum: Inner membrane

• Red marrow: Site of hematopoiesis

• Yellow marrow: Fat storage

Short, Irregular, and Flat Bones

• Thin plates of spongy bone (diploë) surrounded by compact bone

• Have periosteum, endosteum, red bone marrow, hyaline cartilage on joint surfaces

• No diaphysis or epiphysis

Bone Markings (Landmarks)

Bones have various projections, depressions, and openings for muscle attachment, joint formation, and passage of blood vessels and nerves.

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic cells: Stem cells in periosteum and endosteum; become osteoblasts or bone-lining cells

• Osteoblasts: Bone-forming cells; secrete bone matrix

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix

• Bone lining cells: On bone surfaces; help maintain bone

• Osteoclasts: Resorb bone matrix

Microscopic Anatomy of Compact Bone

• Haversian system (Osteon): Structural unit; consists of lamellae, central canal, osteocytes in lacunae, canaliculi

• Perforating (Volkmann's) canals: Connect blood vessels and nerves

• Lamellae: Concentric, circumferential, and interstitial layers

Microscopic Anatomy of Spongy Bone

• No osteons

• Trabeculae align along stress lines

Chemical Composition of Bone Matrix

• Organic: Osteoid (collagen fibers and proteins)

• Inorganic: Crystallized mineral salts (hydroxyapatite: calcium phosphate)

Functions

• Hardness, flexibility, tensile strength

Bone Development and Formation

Ossification

Ossification is the process of bone formation. It occurs in two main ways:

• Endochondral ossification: Bone develops by replacing hyaline cartilage; forms most bones below the skull.

• Intramembranous ossification: Bone forms in a fibrous membrane; forms flat bones (e.g., skull, clavicle).

Endochondral Ossification Steps

1. Osteoblasts secrete osteoid around hyaline cartilage.

2. Cartilage matrix calcifies; chondrocytes die; cavities form.

3. Periosteal bud invades; blood vessels, nerves, osteogenic cells enter.

4. Diaphysis elongates; medullary cavity forms.

5. Epiphyses ossify; spongy bone forms.

Intramembranous Ossification Steps

• Ossification centers form in fibrous connective tissue membranes.

• Osteoblasts secrete osteoid; osteoid calcifies; trapped osteoblasts become osteocytes.

• Spongy bone forms around blood vessels; red bone marrow develops.

• Compact bone forms along outside.

Postnatal Growth

• Length: Occurs at epiphyseal plates (interstitial growth)

• Width: Appositional growth (osteoblasts in periosteum and endosteum)

Bone Remodeling

Bone is continuously renewed and repaired through remodeling, balancing deposition and resorption.

• Replace spongy bone every 3-4 years; compact bone every 5-6 months

• Triggered by mineral and hormonal controls

• Hormonal controls: Parathyroid hormone (PTH), calcitonin

• Mechanical stress: Wolff's law: bone grows/remodels in response to stress

Bone Disorders

Osteomalacia and Rickets

• Osteomalacia: Soft bones due to insufficient mineralization

• Rickets: Occurs in children; bones are soft and deformed

Osteoporosis

• Bone resorption exceeds deposition; bone mass declines

• Estrogen and androgens maintain normal density

Paget's Disease

• Excessive and haphazard bone deposit and resorption

• Spongy bone replaces compact bone

Fractures

Classification

• Position of bone ends: nondisplaced, displaced

• Completeness: complete, incomplete

• Penetration of skin: open (compound), closed (simple)

Repair of Bone Fracture

1. Hematoma forms: blood, inflammation

2. Fibrocartilaginous callus forms: collagen and cartilage bridge the break

3. Bony callus forms: spongy bone replaces callus

4. Bone remodeling: osteoclasts remove excess, compact bone forms

Key Table: Types of Cartilage

Key Table: Bone Cells

Key Equations

• Hydroxyapatite formula:

Additional info: Academic context and terminology have been expanded for clarity and completeness. Tables have been inferred and organized for study purposes.