Bone Histology

Structure of a Long Bone

• Diaphysis - the shaft, composed mainly of compact bone surrounding the marrow cavity
• Epiphysis - the expanded ends, mostly spongy bone with a thin compact outer shell; covered by articular cartilage (hyaline)
• Metaphysis - the flared zone between diaphysis and epiphysis; contains the epiphyseal line (remnant of the growth plate)

Microscopic Organization

Compact (Cortical) Bone

Spongy (Cancellous / Trabecular) Bone

Bone Matrix

• Type I collagen = 90% of all bone protein; provides tensile strength
• Non-collagenous proteins: osteocalcin, osteonectin, osteopontin (BSP-1), bone sialoprotein-2 (BSP-2), thrombospondins, proteoglycans

• Hydroxyapatite crystals deposited along collagen fibrils; provides compressive strength and rigidity
• Also stores calcium, phosphate, carbonate, magnesium, and fluoride

Bone Cells (4 Types)

1. Osteoprogenitor Cells

• Derived from mesenchymal stem cells in bone marrow
• Flat/squamous appearance, pale elongated nucleus, scanty cytoplasm
• Found on periosteal and endosteal surfaces
• Key transcription factor: CBFA1/RUNX2 triggers differentiation into osteoblasts
• Stimulated by IGF-1, IGF-2, and BMPs

2. Osteoblasts

• The bone-forming cell; cuboidal to polygonal, basophilic cytoplasm (rich rER and Golgi), mononuclear
• Located on bone surfaces (forming a single layer)
• Secretes type I collagen and bone matrix proteins (osteocalcin, osteonectin, BSP)
• Initiates mineralization via matrix vesicles
• Expresses alkaline phosphatase (TNAP) - a clinical marker of osteoblast activity
• Also secretes RANKL (stimulates osteoclasts) and osteoprotegerin (OPG) (inhibits osteoclasts - acts as a decoy RANKL receptor)
• Life span: ~12 days
• When trapped in matrix → becomes an osteocyte

3. Osteocytes

• The mature bone cell, the most numerous bone cell (~95% of all bone cells)
• Trapped within lacunae; communicate via ~50 cell processes in canaliculi
• Connect to neighboring cells via gap junctions (connexins) at process tips
• Transformation from osteoblast to osteocyte takes ~3 days; cell volume shrinks ~70%
• In H&E sections: small, dark nucleus, scant cytoplasm, often shrunken by decalcification artifact
• Functions: maintain bone matrix, sense mechanical loading, regulate calcium homeostasis, secrete sclerostin (inhibits bone formation) and FGF-23 (regulates phosphate)
• Life span: 10-20 years

4. Osteoclasts

• The bone-resorbing cell; derived from monocyte/macrophage precursors (CFU-GM) in bone marrow - hematopoietic origin, NOT mesenchymal
• Large, multinucleated (up to 50 nuclei), acidophilic cytoplasm
• Located in Howship's lacunae (resorption pits on bone surface)
• Have a ruffled border (amplified membrane facing bone) and a sealing zone (actin ring that isolates the resorption compartment)
• Mechanism of resorption:
  1. Secrete H⁺ (via carbonic anhydrase II + proton pump TCIRG1) → acidifies sealed compartment → dissolves hydroxyapatite
  2. Secrete lysosomal enzymes including cathepsin K → digests collagen and organic matrix
• Regulated by RANK-RANKL signaling (RANKL on osteoblasts/T-cells binds RANK on osteoclasts → activates resorption); OPG blocks this
• PTH indirectly increases osteoclast activity (osteoclasts have no PTH receptors; PTH acts on osteoblasts/T-cells which upregulate RANKL)
• Calcitonin directly inhibits osteoclasts (they do have calcitonin receptors)

Types of Bone Tissue

Lamellar Bone (Mature / Secondary Bone)

• Adult bone - highly organized, collagen fibers run in alternating directions between adjacent lamellae
• Includes both compact and spongy bone in the adult
• Remodeled continuously throughout life

Woven Bone (Immature / Primary Bone)

• First bone formed: during fetal development, fracture repair, or pathological states (e.g., Paget's disease, osteosarcoma)
• Collagen fibers run randomly (haphazardly) in all directions - hence also called fiber bone
• Weaker than lamellar bone; normally replaced by lamellar bone

Periosteum and Endosteum

• Periosteum: dense fibrous connective tissue covering all external bone surfaces (except articular cartilage). Has two layers:
  • Outer fibrous layer: dense CT with fibroblasts; anchored by Sharpey's (perforating) fibers into bone matrix
  • Inner cellular (cambium) layer: osteoprogenitor cells and osteoblasts; responsible for appositional bone growth and fracture repair
• Endosteum: thin layer of cells (osteoprogenitor + bone-lining cells) lining all internal cavities - marrow cavity, Haversian canals, Volkmann canals, and trabecular surfaces

Summary Table: Bone Cells