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Bone & Cartilage Histology — Exam Guide

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PART 1 — CARTILAGE

Overview

• Chondrocytes — the only cell type
• Extracellular matrix (ECM) — makes up >95% of cartilage volume

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The 3 Types of Cartilage

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Hyaline Cartilage — Close-Up

• Chondroblasts — active cells on the surface/perichondrium; secrete matrix
• Chondrocytes — mature cells within lacunae; maintain matrix
• Isogenous groups — clusters of 2–8 chondrocytes derived from one progenitor; a marker of interstitial growth

1. Capsular (pericellular) matrix — immediately surrounds each chondrocyte; highest concentration of sulfated proteoglycans + type VI collagen; intensely basophilic/metachromatic
2. Territorial matrix — surrounds the isogenous group; less intensely stained
3. Interterritorial matrix — between groups; least stained; bulk of the matrix

• Matrix stains basophilic with H&E (sulfated proteoglycans attract hematoxylin)
• Matrix stains metachromatic with toluidine blue (sulfated GAGs)

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Elastic Cartilage

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Fibrocartilage

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Cartilage Growth

• Appositional growth — new chondrocytes arise from the inner chondrogenic layer of the perichondrium and add to the surface → increases width
• Interstitial growth — chondrocytes within the matrix divide mitotically → expands cartilage from within → seen as isogenous groups; occurs mainly during fetal/early life

Cartilage Repair

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PART 2 — BONE

General Classification

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Bone Cells (Must Know All 4)

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Bone ECM (Osteoid)

• Organic (35%): ~90% type I collagen; non-collagenous proteins (osteocalcin, osteopontin, bone sialoprotein)
• Inorganic (65%): Hydroxyapatite — Ca₁₀(PO₄)₆(OH)₂ — deposited along collagen fibrils

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The Osteon (Haversian System)

Central Haversian canal (blood vessels + nerves)
    ↑ surrounded by
Concentric lamellae (mineralized bone matrix in rings)
    ↑ between lamellae
Lacunae (osteocytes) → connected by → Canaliculi
    ↑ outer boundary
Cement line (marks boundary of one osteon)

• Volkmann (perforating) canals — run perpendicular to the long axis; connect Haversian canals to each other and to the periosteum/endosteum
• Interstitial lamellae — remnants of old, remodeled osteons between current osteons
• Circumferential lamellae — outer and inner rings of lamellar bone just beneath periosteum/endosteum

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Bone Formation — Two Methods

1. Intramembranous Ossification

• No cartilage precursor — bone forms directly in mesenchyme
• Bones formed: Flat bones of skull, mandible, clavicle
• Sequence: mesenchymal condensation → osteoprogenitors → osteoblasts secrete osteoid → mineralization → woven bone → remodeled to lamellar bone

2. Endochondral Ossification

• Uses a hyaline cartilage model as scaffold
• Most bones in the body form this way

1. Cartilage model forms (hyaline)
2. Chondrocytes at center hypertrophy → matrix calcifies
3. Periosteal bony collar forms around diaphysis (via intramembranous ossification of periosteum)
4. Blood vessels invade → primary ossification center in diaphysis
5. Secondary ossification centers form in epiphyses later (most postnatal)
6. Epiphyseal growth plate (physis) persists between primary and secondary centers until skeletal maturity

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Epiphyseal Growth Plate — Zones (Proximal → Distal)

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Periosteum & Endosteum

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Bone Remodeling & Calcium Regulation

• BMU (bone remodeling unit): Coupled osteoclasts + osteoblasts working together
• PTH: ↑ osteoclast activity (via RANKL on osteoblasts) → bone resorption → ↑ blood Ca²⁺
• Calcitonin: Inhibits osteoclasts → ↓ bone resorption → ↓ blood Ca²⁺
• Vitamin D (1,25-OH₂D₃ / calcitriol): ↑ Ca²⁺ absorption from gut + ↑ renal reabsorption

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Fracture Repair (Secondary / Indirect)

1. Hematoma formation
2. Periosteal cells activated → form soft callus (fibrocartilage)
3. Soft callus replaced by hard (bony) callus via endochondral ossification
4. Callus remodeled to lamellar bone

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Quick High-Yield Summary Table

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Bone & Cartilage Histology — MBBS First Year Guide

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PART 1 — CARTILAGE

What is cartilage?

1. Chondrocytes — the only cells in cartilage, sitting inside little spaces called lacunae (singular: lacuna = "little lake")
2. Extracellular matrix (ECM) — makes up over 95% of the tissue; contains type II collagen (for tension) + proteoglycans/GAGs (for compression and water-holding)

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The 3 Types of Cartilage — The Big Picture

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1. Hyaline Cartilage (Most Important)

• Named "hyaline" from Greek hyalos = glassy, because the matrix looks glassy/translucent in life
• Matrix stains basophilic (blue-purple) with H&E because sulfated proteoglycans attract hematoxylin
• Matrix stains metachromatic with toluidine blue (another common exam point)

1. Capsular/pericellular matrix — immediately around the chondrocyte; darkest staining; richest in sulfated proteoglycans and type VI collagen
2. Territorial matrix — surrounds the isogenous group; less intensely stained
3. Interterritorial matrix — between groups; least stained; bulk of the matrix

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• Outer fibrous layer — collagen fibers, fibroblasts; mechanical protection
• Inner chondrogenic layer — contains chondroblasts (immature cells); responsible for appositional growth

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2. Elastic Cartilage

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3. Fibrocartilage

1. No perichondrium
2. Contains both chondrocytes and fibroblasts
3. Collagen is type I (not type II like other cartilage)

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Cartilage Growth — Two Ways

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Why cartilage can't repair well

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PART 2 — BONE

What makes bone different from cartilage?

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Types of Bone Tissue

• Compact (cortical) bone — solid, dense outer shell; organized into osteons
• Spongy (cancellous) bone — internal lattice of trabeculae; spaces filled with marrow

• Woven (immature) bone — collagen fibers randomly arranged, more cells per area, stains more with hematoxylin; seen in embryo and fracture healing (temporary)
• Lamellar (mature) bone — collagen fibers in organized parallel layers (lamellae); stains more with eosin; this is what adult bone looks like

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The 4 Bone Cells (Know all of them!)

• Origin: mesenchymal stem cells in bone marrow
• Appearance: flat/squamous cells with pale elongated nucleus, little cytoplasm
• Location: inner periosteum (cambium layer) and endosteum
• Activated by transcription factor RUNX2/CBFA1 → differentiate into osteoblasts

• Cuboidal cells lining bone surfaces
• Full of rough ER + large Golgi → actively synthesize protein
• Secrete osteoid (unmineralized bone matrix = type I collagen + glycoproteins)
• Mineralization triggered by matrix vesicles they release
• Once surrounded by matrix, they become osteocytes

• Trapped former osteoblasts sitting in lacunae within mineralized matrix
• Send long thin processes through channels called canaliculi
• These processes touch each other and touch osteoblasts → forms the canalicular network
• Function: maintain bone matrix, sense mechanical stress

• Origin: hematopoietic progenitors (monocyte/macrophage lineage — completely different from the others!)
• Appearance: very large, multinucleated (6-50 nuclei), acidophilic cytoplasm
• Have a ruffled border (finger-like projections facing the bone surface) — this is where acid and enzymes are secreted to dissolve bone
• Sit in Howship's lacunae (shallow resorption pits they carve out)
• Controlled by RANK-RANKL pathway: osteoblasts express RANKL → binds RANK on osteoclast precursors → activates osteoclasts

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The Osteon (Haversian System) — The Structural Unit of Compact Bone

Central Haversian canal (blood vessels + nerves + loose CT)
    ↑  surrounded by
Concentric lamellae (rings of mineralized bone matrix)
    ↑  between lamellae
Lacunae (containing osteocytes) ←→ connected by → Canaliculi
    ↑  outer boundary
Cement line (basophilic line separating one osteon from another)

• Volkmann (perforating) canals — run at RIGHT ANGLES to the Haversian canals; connect Haversian canals to each other and to the periosteum/endosteum; do NOT have concentric lamellae around them (key distinction from Haversian canals)

• Circumferential lamellae — rings running all the way around the bone, just inside the periosteum (outer) and endosteum (inner)
• Interstitial lamellae — irregular fragments between osteons; remnants of old osteons that got partially resorbed during remodeling

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Periosteum and Endosteum

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Bone Formation — Two Methods

Method 1: Intramembranous Ossification (simpler)

• Bone forms directly in mesenchyme — no cartilage template
• Bones formed: flat bones of skull, mandible, clavicle
• Steps: mesenchymal condensation → mesenchymal cells → osteoblasts → secrete osteoid → mineralize → woven bone → remodel to lamellar bone

Method 2: Endochondral Ossification (forms most of the skeleton)

• Bone forms on top of a hyaline cartilage model
• All long bones, short bones, vertebrae, ribs form this way
• Steps in brief:
  1. Hyaline cartilage model forms (shape of future bone)
  2. Cartilage in the center (diaphysis) hypertrophies and calcifies
  3. Periosteal bony collar forms around the diaphysis shaft
  4. Blood vessels invade → primary ossification center forms in diaphysis
  5. Secondary ossification centers form later in the epiphyses
  6. Epiphyseal growth plate (physis) remains between primary and secondary centers → source of bone elongation in children

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The Epiphyseal Growth Plate — 5 Zones (Proximal to Distal)

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Calcium Regulation in Bone

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Fracture Repair (Secondary/Indirect Healing)

1. Hematoma — blood clot fills fracture gap
2. Soft callus — periosteal cells proliferate → form fibrocartilage bridging the gap
3. Hard (bony) callus — fibrocartilage replaced by woven bone via endochondral ossification
4. Remodeling — woven bone remodeled to lamellar bone; excess callus removed by osteoclasts

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Quick Comparison — The Most Important Exam Points

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