Bone

Definition:

Bone is a specialised connective tissue¹.

Bone includes cells and extracellular matrix. The most common type of collagen in bone is Type I collagen.²

Table 1: Composition of Bone

Bone Functions

1. Primary Calcium Reservoir
2. Haematopoiesis
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 (Haversian 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

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. Mnemonic: “bONE” → Type I collagen.