Bone

Definition:

Bone is a specialised Connective tissue1.

Bone includes cells and extracellular matrix. Most common type of collagen is seen bone is Type 1 collagen 2

Table 1: Composition of Bone

Cells (10%) ECM (90%)

  • Osteoblasts

  • Osteoclasts

  • Osteocytes

  • Osteo-progenitor cells

  • Lining cells

Proteins (40%)

-Collagenous Protein (90%)

-Non-collagenous protein (10%)

eg: Proteoglycan

Minerals (60%)

-Calcium, Phosphorous

-Hydroxyapatite (Ca10PO4(OH)2

Bone Functions

  1. Primary Calcium Reservoir
  2. Haemopoesis
  3. Mechanical Support for Soft Tissues and Organs

Bone Structure

  1. Woven (Immature)
  2. Lamellar (Mature)
    • Cortical Bone
    • Cancellous Bone
  3. Periosteum

Woven Bone (Immature)

Features
- Disorganized haphazard collagen arrangement
- Weaker
- Contains more cells than lamellar bone per unit volume
- Isotropic: Properties do not vary despite the type of loading
- High turnover

Examples
- Embryonic and neonatal skeleton
- Paediatric callus is mainly woven
- Metaphysis up to 4 years old
- Early hard callus in adults
- Pathologic high turnover bone (e.g. tumor, Paget’s, osteogenesis)

Lamellar Bone (Cortical & Cancellous)

General Features
- Stronger
- Collagen fibers stress-orientated, arranged in parallel sheets of lamellae
- Herring bone structure
- Adjacent lamellae oblique to each other
- Cement lines exist between lamellae, which are potential weak points
- Anisotropic: Properties vary depending on mechanical stress
- Fewer cells per unit volume
- Composed mainly of matrix laid down by osteoblasts

Cortical Bone

  • Makes up 80% of bone density in the body
  • 20x denser than cancellous (Young’s Modulus 20 GPa)
  • Arranged into multiple longitudinal osteons
  • Resists torsion and bending better than cancellous bone

Osteon (Harvesian System)
- The fundamental unit of lamellar bone
- Comprises 5 or 6 concentric lamellar rings running longitudinally around a central Haversian canal
- Each osteon is 50 μm in diameter
- Cement lines between osteons are potential sites of weakness

Haversian Canal
- Formed by 5 or 6 lamellar rings
- Contains blood vessels, nerves, and lymphatics

Volkmann’s Canals
- Perpendicular to the Haversian canals
- Contain blood vessels and allow communication between periosteal and intramedullary blood vessels

Cancellous Bone

  • Found in metaphyses and epiphyses
  • Forms a 3D trabecular lattice
  • Still arranged in lamellar fashion
  • No Haversian systems
  • Lower tensile strength, more elastic (Young’s Modulus 1 GPa)
  • 8x higher turnover due to larger surface area
  • More subject to Wolff’s Law (remodeling according to mechanical stress)

Periosteum

  • Inner Osteogenic Layer: Vascular
  • Outer Structural Layer: Fibrous
  • Responsible for bone growth in diameter

Cells of Bone

Osteoblasts
- Formed by osteoprogenitor cells (mesenchymal stem cells)
- Secrete osteoid (organic bone matrix)
- Type 1 collagen, RANKL, OPG, osteocalcin, alkaline phosphatase, bone sialoprotein
- Have receptors for:
- Parathyroid hormone, Vitamin D3, glucocorticoids, estrogen, prostaglandins
- Life span: 100 days
- Afterward, they either undergo apoptosis, become bone lining cells, or become osteocytes

Osteocytes
- Inactive osteoblasts trapped in lacunae & canaliculi
- Maintain bone and alter matrix in their vicinity
- High nucleus-to-cytoplasm ratio
- Play a role in calcium and PTH homeostasis

Bone Lining Cells
- Flat, inactive osteoblasts
- Reactivate in times of increased bone turnover

Osteoclasts
- Function: Bone resorption
- Derived from monocytes
- Activated by RANKL secreted by osteoblasts
- Secrete acid (carbonic anhydrase) to dissolve hydroxyapatite crystals

Bone Matrix

  • Inorganic (60%)
    • Calcium hydroxyapatite [Ca10(PO4)6(OH)2]
    • Responsible for compressive strength of bone
    • 99% of total body calcium is stored in bone
  • Organic (40%)
    • 90% Type 1 collagen, 10% BMPs, cytokines, growth factors
    • Provides tensile strength

Blood Supply of Bone

  • 5-10% of cardiac output goes to bone
  • 3 main sources:
    1. Nutrient Artery System: Dominant in adults
    2. Periosteal System: Dominant in children
    3. Metaphyseal/Epiphyseal System

Bone Metabolism

Calcium
- 99% stored in bone, main source from diet
- Functions in bone mineralization, nerve, muscle, and clotting function
- Calcium absorption actively in the duodenum, passively in the jejunum

Phosphate
- 85% stored in bone within hydroxyapatite crystals
- Absorbed mainly in the duodenum

Calcium and Phosphate Regulation

Main Hormones
- Vitamin D, Parathyroid Hormone (PTH), Calcitonin

Vitamin D
- Increases calcium and phosphate absorption
- Stimulates osteoclastic resorption indirectly
- Inhibits PTH production

PTH
- Produced by the chief cells of the parathyroid gland
- Increases serum calcium and decreases serum phosphate

Calcitonin
- Directly inhibits osteoclasts
- Produced by parafollicular cells of the thyroid

Markers of Bone Activity

  1. Alkaline Phosphatase: Elevated during bone formation
  2. Urinary Hydroxyproline & Hydroxypyridoline: Elevated during bone resorption
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Footnotes

  1. This is the standard answer for all questions about the definition of any living tissue we deal with in Orthopaedics: it is a specialised connective tissue.↩︎

  2. b” ONE ”↩︎