Anatomy and Physiology of Cartilage and Bone: Types, Locations, and Growth

Skeletal Cartilage

Types of Cartilage

Hyaline Cartilage

Most abundant; glassy, smooth appearance

Locations of Hyaline Cartilage

Articular cartilages (joint surfaces), costal cartilages (ribs), respiratory cartilages (larynx, trachea, bronchi), nasal cartilages, epiphyseal plates

Elastic Cartilage

Contains elastic fibers; more flexible

Locations of Elastic Cartilage

External ear (auricle), epiglottis

Fibrocartilage

Contains thick collagen fibers; highly compressible

Locations of Fibrocartilage

Intervertebral discs, menisci of knee, pubic symphysis

Why Cartilage is Resilient

Avascular: No blood supply; receives nutrients via diffusion from perichondrium

Chondrocytes

Cartilage cells housed in lacunae that maintain the matrix

Matrix composition of Cartilage

High water content (60-80%) combined with resilient collagen and elastic fibers allows for shock absorption and flexibility

GAGs (glycosaminoglycans)

Attract water, creating a gel-like cushion that resists compression

Slow healing of Cartilage

Due to avascularity; relies on perichondrium for repair

Diaphysis

Shaft composed of compact bone that provides structural support and leverage

Epiphysis

Ends composed of spongy bone with thin compact bone covering; site of articulation (joints)

Epiphyseal Plate/Line

Hyaline cartilage in growing bones (plate); site of longitudinal growth; becomes epiphyseal line when growth ceases

Medullary Cavity

Hollow chamber in diaphysis containing yellow marrow (fat storage) in adults

Periosteum

Outer fibrous membrane secured by Sharpey's (perforating) fibers; richly supplied with nerves and blood vessels

Endosteum

Thin membrane lining medullary cavity and trabeculae; contains osteogenic cells

Articular Cartilage

Hyaline cartilage covering epiphyses at joints; reduces friction and absorbs shock

Compact Bone

Dense, appears solid; organized into osteons (Haversian systems)

Osteon components

Central (Haversian) canal, concentric lamellae, lacunae, canaliculi, perforating (Volkmann's) canals, interstitial lamellae, circumferential lamellae

Function of Compact Bone

Provides protection, support, and resistance to stress

Spongy (Cancellous) Bone

Honeycomb appearance with trabeculae; no osteons; lightweight but strong

Function of Spongy Bone

Supports and protects red marrow; lightweight structure reduces bone mass while maintaining strength

Wolff's Law

Bone forms according to the stresses placed upon it

Bone remodeling

Bones remodel in response to mechanical stress

Bone deposition

Occurs at sites of stress/tension

Bone resorption

Occurs at sites of low stress

Bone architecture

Matches functional demands

Maximum compression

Bony deposits strengthen areas where weight bears down

Maximum tension

Bone thickens along stress lines, areas being pulled/stretched

No stress

Bone resorption (atrophy), neutral zone

Load sites

Develop large tubercles and processes for muscle/tendon attachment (weight transfer)

Clinical relevance of bone density

Athletes develop denser bones; bedridden/paralyzed patients experience bone loss; astronauts lose bone mass in zero gravity

Intramembranous Ossification

Bone develops from fibrous connective tissue membrane

Ossification center

Develops when mesenchymal cells cluster and differentiate into osteoblasts

Osteoid

Unmineralized bone matrix secreted by osteoblasts

Calcification

Alkaline phosphatase facilitates calcium salt deposition

Woven bone

Trabeculae of woven bone develop with trapped osteocytes in lacunae

Endochondral Ossification

Bone develops by replacing hyaline cartilage model

Cartilage model

Hyaline cartilage shaped like future bone develops

Primary ossification center

Forms when periosteal bud invades deteriorating cartilage shaft; medullary cavity forms

Secondary ossification centers

Form in epiphyses after birth; spongy bone develops

Epiphyseal plates

Hyaline cartilage persists between diaphysis and epiphyses

Epiphyseal lines

Form when epiphyseal plates ossify at the end of adolescence; longitudinal growth ceases

Epiphyseal Plate Zones

Sequence of zones from epiphysis to diaphysis during bone growth

Zone of Reserve (Resting) Cartilage

Small, scattered chondrocytes; no cell division; anchors plate to epiphysis

Zone of Proliferation (Growth)

Rapidly dividing chondrocytes stacked like coins; interstitial growth via mitosis

Zone of Hypertrophy (Maturation)

Large, mature chondrocytes; cells enlarge and prepare for calcification

Zone of Calcification

Thin zone with calcified matrix; matrix becomes calcified; chondrocytes die

Zone of Ossification (Bone Deposition)

New bone replacing calcified cartilage; osteoblasts deposit bone matrix on calcified cartilage spicules

Prenatal development

Begins weeks 8-12

Childhood/adolescence

Active longitudinal growth

Growth Patterns

Interstitial growth: Growth from within (cartilage only); occurs at epiphyseal plates

Appositional growth

Growth from outside; increases diameter

Osteoblasts

Under periosteum deposit bone matrix

Osteoclasts

In medullary cavity resorb bone

Calcium Functions

Muscle contraction, Blood clotting, Neurotransmitter release, Nerve impulse transmission, Cell division, Enzyme activity, Bone/tooth structure

Calcium Storage

98% stored in bone as calcium phosphate salts (hydroxyapatite crystals)

Hypocalcemia

When Blood Ca²⁺ Drops

Parathyroid Hormone (PTH)

Released when parathyroid glands detect low calcium

Blood Ca²⁺ normal range

9-11 mg/dL

Hypercalcemia

When Blood Ca²⁺ Rises

Calcitonin

Released when thyroid gland (C cells) detect high calcium

Negative feedback

Response reverses the initial stimulus to maintain homeostasis

Osteocytes

Mature bone cells that maintain bone in viable state

Periosteal surface

Location where osteoblasts are more active (increasing diameter)

Types of Fractures

Common Fracture Types include Closed (Simple), Open (Compound), Comminuted, Spiral, Depressed, Compression, Greenstick, Impacted, Avulsion, Epiphyseal, Colles', and Pott's

Closed (Simple) Fracture

Bone broken but skin intact

Open (Compound) Fracture

Broken bone penetrates skin; high infection risk

Comminuted Fracture

Bone fragments into three or more pieces

Spiral Fracture

Ragged break due to twisting force

Depressed Fracture

Broken bone portion pressed inward (common in skull)

Compression Fracture

Bone crushed (common in vertebrae)

Greenstick Fracture

Incomplete fracture; one side breaks, other bends (children)

Impacted Fracture

Broken bone ends forced into each other

Avulsion Fracture

Bone fragment pulled off by tendon/ligament

Epiphyseal Fracture

Separation at epiphyseal plate (growth plate injury)

Colles' Fracture

Fracture of distal radius with posterior displacement

Pott's Fracture

Fracture of distal fibula, often with tibial involvement

Phase 1: Hematoma Formation

Blood vessels rupture at fracture site, blood clot (hematoma) forms, inflammatory response begins, bone cells deprived of nutrition die, swelling and pain occur.

Phase 2: Fibrocartilaginous Callus Formation

Granulation tissue forms: Capillaries grow into hematoma, phagocytes remove debris: Clean dead tissue and blood clot, fibroblasts and osteoblasts migrate from periosteum and endosteum, fibrocartilaginous callus: Collagen fibers and cartilage span break, callus acts as splint (not yet strong).

Phase 3: Bony Callus Formation

Osteoblasts produce trabeculae of spongy bone, fibrocartilaginous callus replaced by bony callus, callus composed of woven (immature) bone, bone ends united but not fully remodeled, 'bulge' visible on X-ray.

Phase 4: Bone Remodeling

Compact bone replaces spongy bone, excess material on shaft exterior and within medullary cavity removed, bone returns to original shape, remodeling follows Wolff's Law based on stress patterns.

Osteoporosis

Bone resorption exceeds deposit; decreased bone density. Causes include aging, hormonal changes (esp. estrogen loss in postmenopausal women), inadequate calcium/Vitamin D, lack of weight-bearing exercise. Effects include compression fractures (vertebrae), hip fractures, increased fracture risk. Treatment includes calcium/Vitamin D supplements, weight-bearing exercise, medications (bisphosphonates), hormone therapy.

Paget's Disease (Osteitis Deformans)

Excessive, abnormal bone remodeling. Characteristics include bones becoming enlarged, misshapen, structurally weak despite increased thickness. Cause is unknown; possibly viral. Effects include skull enlargement, bone pain, hearing loss, bowed long bones.

Osteomyelitis

Bone infection, usually bacterial. Risk includes spreading to bone marrow and periosteum. Example: Cho's infected finger requiring X-ray to check for bone involvement. Treatment includes antibiotics, possible surgical debridement.

Ostealgia

Bone pain.

Diploe

Spongy bone layer sandwiched between compact bone layers in flat skull bones.

Central (Haversian) Canal

Longitudinal canal in osteon center containing blood vessels and nerves.

Perforating (Volkmann's) Canal

Transverse/oblique canals connecting Haversian canals.

Lacunae

Small cavities in bone matrix housing osteocytes.

Canaliculi

Tiny canals connecting lacunae, allowing osteocyte communication.

Trabeculae

Thin plates/bars of bone in spongy bone that align along stress lines.

Lamellar Bone

Mature bone with organized lamellae (vs. immature woven bone).

Interstitial Growth

Growth in length from within (cartilage only).

Hematopoiesis

Blood cell formation; occurs in red marrow.

Periosteal Bud

Invading structure containing blood vessels, nerves, osteoblasts, and osteoclasts during endochondral ossification.

Epiphyseal Plate

Hyaline cartilage allowing longitudinal growth in children/adolescents.

Epiphyseal Line

Remnant of epiphyseal plate after growth ceases.