Macro and microscopic features of adult cortical bone

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Macro and Microscopic Features of Adult Cortical Bone

Macroscopic Features

General Definition and Distribution

Cortical bone (also called compact or lamellar bone) represents approximately 80% of the total skeletal mass. It forms the outer shell (cortex) of all bones and constitutes the bulk of the diaphysis (shaft) of long bones. Its proportion varies by site: the radial diaphysis cortex is ~95% cortical bone; the femoral head cortex ~50%; and vertebral bodies are only ~33% cortical. - Imaging Anatomy, Vol. 3

Gross Anatomy of a Long Bone

The long bone is organized into three regions:

Region	Description
Diaphysis	Tubular shaft; thick cortex surrounding a central medullary (marrow) cavity
Metaphysis	Flared transition zone; thin cortical shell surrounding cancellous bone
Epiphysis	Rounded end; thin cortical shell with cancellous interior; capped by articular cartilage

In adults, the epiphyseal plate is fused (seen as the epiphyseal line). The medullary cavity contains yellow (fatty) marrow in adult diaphyses; red marrow persists in the epiphyses and metaphyses of flat bones (sternum, iliac crest).

Macroscopic bone regions - Rheumatology textbook histological section showing cortical bone (diaphysis), cancellous bone, and epiphyseal plate

Cross-section showing the epiphysis, metaphysis, and diaphysis with cortical and cancellous bone regions (McNeal tetrachrome stain). - Rheumatology, 2-Volume Set

Surfaces and Covering Layers

Periosteum - dense fibrous connective tissue covering the outer surface of all cortical bone except articular surfaces. Contains osteoblasts, osteoclasts, blood vessels, and nerve fibers. Attached tightly to cortex via thick collagenous Sharpey's fibers. Does not extend over joint surfaces.
Endosteum - thin single-cell layer lining the inner (medullary) surface of cortical bone, all trabecular surfaces, Haversian canals, and Volkmann's canals. Contains osteoprogenitor cells and bone-lining cells. Has higher bone formation activity than the periosteal surface. - Imaging Anatomy, Vol. 3

Microscopic Features

The Osteon (Haversian System) - Fundamental Unit

The fundamental structural unit of cortical bone is the osteon (Haversian system): a cylindrical column approximately 4 mm long and running along the long axis of the bone, with its components including:

Structure of cortical bone showing osteons with Haversian canals, lacunae, canaliculi, and interstitial lamellae - from Guyton & Hall

Structure of cortical bone. - Guyton and Hall Textbook of Medical Physiology

3D diagram of compact bone showing osteons, haversian canals, Volkmann canals, circumferential and interstitial lamellae, periosteum, and endosteum

Diagram of compact bone structure showing all lamellar types and vascular channels. - Histology: A Text and Atlas (Ross)

Components of the Osteon

Structure	Description
Haversian canal	Central canal, 50-90 µm in diameter. Contains a pair of fenestrated capillaries (no smooth muscle), lymphatics absent; accompanied by 2-7 unmyelinated/poorly myelinated sensory and sympathetic nerve fibers ~60% of the time
Concentric lamellae	4-20 rings of mineralized collagen matrix surrounding the canal; collagen fibrils in adjacent lamellae are oriented in alternating directions (like plywood), maximizing mechanical strength
Lacunae	Small lens-shaped spaces between lamellae containing osteocytes
Canaliculi	Tiny radiating channels connecting lacunae to each other and to the Haversian canal; transmit cellular processes of osteocytes; facilitate Ca²⁺ transfer (osteocytic osteolysis)
Cement line	Peripheral boundary of the osteon; composed largely of sulfated glycosaminoglycans; arrests crack propagation and prevents micro-fractures from growing to critical size

Rheumatology (Firestein & Kelley); Histology: A Text and Atlas; Medical Physiology (Boron & Boulpaep)

The Vascular Supply - Canals

Haversian canals - run longitudinally along the long axis of the bone, supplying each osteon
Volkmann's (perforating) canals - run transversely/obliquely, connecting Haversian canals to each other and to the periosteal and endosteal surfaces; carry vessels from the periosteum into the cortex. Unlike Haversian canals, Volkmann canals are NOT surrounded by concentric lamellae.

Types of Lamellae in Cortical Bone

Detailed cross-section diagram of an osteon showing osteoblasts, osteocytes, canaliculi, Haversian canal, calcified matrix, osteoid, and lining cells

Osteon cross-section detailing cell types and matrix layers. - Medical Physiology (Boron & Boulpaep)

Three distinct lamellar systems are present in adult cortical bone:

Osteonal (Haversian) lamellae - concentric rings within each secondary osteon; product of bone remodeling (secondary bone)
Circumferential lamellae
- Outer circumferential lamellae - below the periosteum, encircling the entire bone shaft; laid down by periosteal osteoblasts
- Inner circumferential lamellae - lining the endosteal surface; laid down by endosteal osteoblasts
Interstitial lamellae - irregular angular fragments filling spaces between osteons; these are remnants of previously remodeled (partially resorbed) older osteons

Bone Cells

Cell	Location	Function
Osteoblasts	Bone surfaces, Haversian canals	Secrete osteoid (unmineralized matrix); stimulated by PTH, mechanical load
Osteocytes	Lacunae within matrix	Derived from osteoblasts; mechanosensing via canalicular processes; regulate remodeling via sclerostin, Wnt ligands, osteocalcin
Lining cells	Haversian canal lining	Quiescent osteoblasts covering surfaces
Osteoclasts	Resorption surfaces	Multinucleated; ruffled border; resorb bone via acid + proteases; controlled by RANKL/OPG

Bone Matrix Composition

Bone matrix is composed of:

Mineral - ~65-67% by weight; hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂] nanocrystals deposited in the hole zones between collagen fibrils (D-periodic 67 nm spacing)
Organic matrix - ~22-25%; type I collagen (~90% of organic fraction) composed of two α1 chains + one α2 chain; also minor collagens (types III, V) and noncollagenous proteins (osteocalcin, osteopontin, bone sialoprotein)
Water - ~10%

The N-telopeptide and C-telopeptide at each end of the collagen molecule are cleaved during resorption and serve as biochemical markers of bone turnover. - Rheumatology (Firestein & Kelley)

Primary vs. Secondary Bone

Feature	Primary Osteon	Secondary Osteon (Haversian System)
Origin	Vascular channels that "filled in" with lamellae	Product of remodeling - existing bone resorbed then replaced
Cement line	Absent (no definable cement line)	Present - clearly demarcated
Size	Smaller, fewer concentric lamellae	Larger; Haversian canal 50-90 µm
Clinical relevance	Cannot be reconstructed after loss (contributes to irreversible trabecular perforation in osteoporosis)	Normal turnover in adult cortex

Woven vs. Lamellar Bone

Feature	Lamellar Bone (normal adult cortex)	Woven Bone (abnormal in adults)
Collagen arrangement	Alternating parallel orientations	Random/disorganized
Strength	High (plywood-like)	Low
Seen in	All normal adult cortex	Fracture callus, Paget's disease, hyperparathyroidism, primary bone formation

Summary

Adult cortical bone is a dense, organized tissue (~80% of skeletal mass) providing mechanical strength for weight-bearing. Macroscopically it forms the outer shell of all bones, covered externally by periosteum (with Sharpey's fibers) and internally by endosteum. Its microscopic hallmark is the secondary osteon (Haversian system): concentric lamellae of alternating-direction collagen around a central vascular canal, bounded by a crack-arresting cement line, and interconnected via Volkmann's canals. Three lamellar systems - osteonal, circumferential, and interstitial - fill the cortex and reflect cycles of lifelong remodeling. - Imaging Anatomy Vol. 3; Junqueira's Basic Histology; Histology: A Text and Atlas (Ross); Guyton & Hall Medical Physiology; Rheumatology (Firestein & Kelley)

Tell fast

Adult Cortical Bone - Quick Summary

MACROSCOPIC

= Compact bone, outer shell of all bones, 80% of skeletal mass
Diaphysis = thick cortex + central medullary cavity (yellow marrow in adults)
Covered by periosteum (outer) - attached by Sharpey's fibers
Lined by endosteum (inner, facing marrow)
Cut surface appears dense, ivory-white, smooth - no visible spaces to naked eye

MICROSCOPIC

Fundamental unit = Osteon (Haversian system)

Haversian canal (centre)
    ↑ blood vessels + nerves
Concentric lamellae (4-20 rings)
    ↑ collagen alternates direction each ring → plywood effect → strength
Lacunae → contain osteocytes
Canaliculi → connect lacunae → nutrient/signal highway
Cement line → outer boundary → arrests crack spread

3 types of lamellae:

Osteonal - concentric, within each osteon
Circumferential - outer (periosteal) + inner (endosteal) rings around whole shaft
Interstitial - angular remnants of old resorbed osteons, fill gaps between osteons

Canals:

Haversian - longitudinal, carry vessels along osteon
Volkmann's - transverse, connect Haversian canals to each other + to periosteum

Cells:

Cell	Where
Osteoblasts	Surface - form bone
Osteocytes	Inside lacunae - mechanosensors
Osteoclasts	Resorption surfaces
Lining cells	Haversian canal lining

Matrix: 65% mineral (hydroxyapatite), 25% organic (90% type I collagen), 10% water

Key point: Adult cortical bone = secondary lamellar bone - the alternating collagen orientation in lamellae + cement lines give it high strength and fracture resistance.

Create a labeled diagram poster of cortical bone microstructure

Generating Image

Scientific educational poster of cortical bone microstructure, detailed anatomical diagram with clear labels. Show a 3D cutaway cross-section of compact cortical bone with all key structures labeled with arrows: Osteon (Haversian system), Haversian canal (central canal with blood vessel and nerve), concentric lamellae rings around the canal, lacunae (small oval spaces containing osteocytes), canaliculi (tiny radiating channels connecting lacunae), cement line (outer boundary of osteon), interstitial lamellae (angular fragments between osteons), outer circumferential lamellae (below periosteum), inner circumferential lamellae (above endosteum), Volkmann canal (perforating canal running transverse), periosteum (outer fibrous layer), endosteum (inner lining), osteoblasts on surface, osteocyte within lacuna. Include a small inset of a whole long bone (diaphysis) showing where the cortex is. Use a clean medical illustration style with beige/tan bone colors, red for blood vessels, blue for nerves, white label lines on dark background or light background with dark text. Title at top: CORTICAL BONE MICROSTRUCTURE. Professional medical textbook quality.

Here is your labeled diagram poster of cortical bone microstructure. It shows:

Osteon (Haversian system) - the cylindrical structural unit
Haversian canal - central channel with blood vessel and nerve
Concentric lamellae - rings of alternating collagen orientation
Lacunae + osteocytes - cells embedded within the matrix
Canaliculi - radiating channels for cell communication
Cement line - outer boundary of each osteon
Interstitial lamellae - remnants of old osteons between current ones
Circumferential lamellae - outer (periosteal) and inner (endosteal) rings
Volkmann's canals - transverse perforating canals linking Haversian canals
Periosteum and endosteum - surface coverings

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