Bone Histology Overview

Bone Histology

• Bone is a highly vascular specialized connective tissue
• Matrix is calcified, making it hard and supporting body weight
• Composed of three main components: bone cells, fibers, and hard matrix
• Anatomical classifications: long (e.g., humerus, femur), short (e.g., carpal, tarsal), flat (e.g., skull, ribs), and irregular (e.g., vertebrae)
• Functions: support, protection, movement, mineral storage, and blood cell formation

Learning Objectives

• Classify bones anatomically and histologically
• Locate compact and spongy bones in the human body
• Understand bone microstructure

Histological Classification

• Compact bone: dense, solid lamellae without cavities
  • Located in the shaft surrounding the medullary canal and covering flat, short, and irregular bones
• Cancellous (spongy) bone: irregular branching, anastomosing bony trabeculae
  • Located in the center of flat bones, short bones, irregular bones, and the epiphysis of long bones

Bone Structure

• Bone cells:

  • Osteogenic cells (osteoprogenitor cells):
  • Osteoblasts (bone-forming cells), producing osteoid tissue (organic part)
  • Osteocytes (bone-maintaining cells): mature bone cells maintaining bone hardness, residing in lacunae
  • Osteoclasts: large, multinucleated cells
    • Resorb bone by releasing H+ ions and lysosomal enzymes
    • Located in Howship's lacunae, shallow depressions near resorbing surfaces

• Fibers (mainly collagen type I): organic part (35%) of the matrix

• Ground substance: mostly GAGs and glycoproteins (without water). It's where hydroxyapatite is deposited

• Hydroxyapatite crystals: inorganic part (65%)

  • Calcium salts (Ca phosphate and hydroxide) deposited on collagen fibers
  • Calcify the matrix

Histology of Compact Bone

• Layers from outward to inward:
  • Periosteum
  • External circumferential lamellae
  • Haversian system (osteon)
  • Interstitial lamellae
  • Internal circumferential lamellae
  • Endosteum

Histology of Spongy Bone

• Irregular branching, anastomosing bony trabeculae
• Trabeculae contain bone lamellae, osteocytes, but no osteons
• Enclose bone marrow cavities