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Bone & Cartilage Histology — Exam Guide

Source: Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Pawlina & Ross)

PART 1 — CARTILAGE

Overview

Cartilage is an avascular connective tissue composed of:

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

The ECM contains type II collagen fibrils (tension-resisting) + heavily hydrated proteoglycan aggregates (compression-bearing). This combination gives cartilage its characteristic resilience and pliability. Because it is avascular, nutrients diffuse from surrounding perichondrium through the matrix to reach the chondrocytes.

The 3 Types of Cartilage

Feature	Hyaline	Elastic	Fibrocartilage
Dominant ECM	Type II collagen + proteoglycans	Type II collagen + elastic fibers/lamellae	Type I collagen (abundant) + minimal matrix
Perichondrium	Yes (except articular + growth plate)	Yes	No
Calcifies?	Yes (aging, endochondral ossification)	No	No
Locations	Articular surfaces, costal cartilages, tracheal rings, larynx, fetal skeleton, epiphyseal plate	Auricle of ear, epiglottis, Eustachian tube, vocal folds	IVD (annulus fibrosus), pubic symphysis, menisci, TMJ disc, sternal-clavicular joint
Function	Cushioning, smooth joint surface, structural support	Flexibility/elasticity	Resists compression and shearing
Stain for elastic	—	Orcein (elastic fibers stain brown)	—

Hyaline Cartilage — Close-Up

Hyaline cartilage H&E ×450 — chondrocytes in lacunae within abundant homogeneous matrix

H&E ×450: Chondrocytes in lacunae dispersed within the extensive, lightly basophilic matrix. Pairs of chondrocytes = isogenous groups (daughter cells from mitotic division still sharing a lacuna).

Key cellular terms:

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

Matrix zones (from cell outward):

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

Staining properties:

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

Elastic Cartilage

Orcein stain ×180: Dense network of brown elastic fibers among chondrocytes. Perichondrium visible at top.

Looks like hyaline cartilage on H&E but has abundant elastic fibers visible only with orcein or Weigert stains. Does not calcify with aging — clinically important distinction.

Fibrocartilage

Gomori trichrome ×60: Dense green type I collagen bundles. Chondrocytes (round nuclei) arranged in rows and isogenous groups among fibroblasts (elongated, flattened nuclei). Inset shows an isogenous group at high magnification.

Key exam point: Fibrocartilage has no perichondrium, contains fibroblasts alongside chondrocytes, and its ECM is dominated by type I collagen (unlike other cartilage types).

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

Hyaline cartilage has very limited repair capacity — avascular, chondrocytes cannot easily migrate. Damage typically heals as fibrocartilage scar.

PART 2 — BONE

General Classification

Type	Description
Compact (cortical)	Dense outer shell; consists primarily of osteons (Haversian systems)
Spongy (cancellous)	Internal lattice of trabeculae; spaces filled with marrow
Woven (immature)	Developing bone; collagen fibers randomly oriented; temporary
Lamellar (mature)	Organized concentric lamellae; collagen fibers in alternating parallel arrays

Bone Cells (Must Know All 4)

Cell	Origin	Function	Key Feature
Osteoprogenitor	Mesenchymal stem cells (bone marrow)	Precursor; divides to replenish osteoblasts	Activated by CBFA1/RUNX2 transcription factor
Osteoblast	Osteoprogenitor cells	Secretes osteoid (unmineralized matrix); mineralization triggered by matrix vesicles	Cuboidal cells lining bone surface; abundant rER + Golgi
Osteocyte	Trapped osteoblasts	Maintain bone matrix; mechanosensing	Enclosed in lacunae; communicate via canaliculi (cell processes); network = canalicular system
Osteoclast	Hematopoietic progenitors (monocyte lineage)	Resorb bone	Large multinucleated cells; sit in Howship's lacunae (resorption pits); ruffled border faces bone

RANK–RANKL pathway: Osteoblasts express RANKL → binds RANK on osteoclast precursors → osteoclast differentiation. PTH acts on osteoblasts (not osteoclasts directly) to upregulate RANKL. OPG (osteoprotegerin) from osteoblasts blocks RANKL → inhibits osteoclastogenesis.

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

The Osteon (Haversian System)

The structural unit of compact bone:

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

Ground section of compact bone showing osteons (Haversian systems), Haversian canals, and interstitial lamellae

Ground section ×80: Circular osteons with central Haversian canals (dark = air or India ink). Interstitial lamellae fill spaces between osteons. Circumferential lamellae visible at periphery.

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

Endochondral ossification — fetal finger showing hypertrophic chondrocytes, calcified cartilage, periosteal bony collar

Fetal digit H&E ×210: Epiphyses (E) = cartilage. Hypertrophic chondrocytes (HC) in center. Calcified cartilage matrix (CCM) darker staining. Periosteal bony collar (BC) forming around shaft.

Stages:

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

Epiphyseal Growth Plate — Zones (Proximal → Distal)

Zone	What's Happening	Key H&E Feature
Reserve (resting) zone	Small chondrocytes, storing lipid/glycogen	Cells scattered, matrix pale
Proliferative zone	Rapid mitosis → columns (stacks) of flat chondrocytes	Columns ("coins stacked") = interstitial growth
Hypertrophic zone	Cells enlarge; matrix calcification begins	Large clear cytoplasm
Calcified cartilage zone	Chondrocytes die; matrix mineralized	Dark-staining matrix
Resorption (ossification) zone	Osteoclasts resorb cartilage; osteoblasts lay down bone	Bone trabeculae on calcified cartilage spicules

Mnemonic: "Really Pretty Healthy Children" → Reserve, Proliferative, Hypertrophic, Calcified

Periosteum & Endosteum

	Periosteum	Endosteum
Location	Outer surface of bone	Inner surface (marrow cavity, trabeculae, canals)
Layers	Outer fibrous + inner osteogenic (cambium) layer	Single layer of osteoprogenitors + osteoblasts
Function	Appositional growth, fracture repair	Remodeling, fracture repair

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

Fracture Repair (Secondary / Indirect)

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

Quick High-Yield Summary Table

Feature	Cartilage	Bone
Vascularity	Avascular	Vascular (Haversian + Volkmann canals)
Main collagen	Type II (hyaline/elastic), Type I (fibrocartilage)	Type I
Cell in matrix	Chondrocyte (in lacuna)	Osteocyte (in lacuna + canaliculi)
Nutrition	Diffusion through matrix	Canalicular system + blood vessels
Mineralization	Hyaline only (aging/ossification)	Always — hydroxyapatite
Repair capacity	Very limited → fibrocartilage scar	Good — callus formation
Growth	Interstitial + appositional	Appositional only (adult)

Source: Histology: A Text and Atlas with Correlated Cell and Molecular Biology, 9th ed. (Pawlina & Ross)

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