MH 045 Bone

Osteon (or Haversian System)

Bones contain a high concentration of calcium salts, which makes them too hard to cut into thin tissue sections. It must be decalcified using acids or chelating agents to remove the mineral content, leaving behind the organic matrix, which can be sectioned and stained.

The osteon, or Haversian system, is the primary structural unit of compact bone. Each osteon is a cylindrical structure of concentric layers, called lamellae, arranged around a central Haversian canal. These cylinders run parallel to the long axis of the bone, contributing to its strength and nutrient delivery.

To properly analyze the structure of osteons, it is important to examine both cross-sectional and longitudinal sections.

• Cross-Section
  • Haversian Canals - circular openings in the center of each osteon
    • Haversian Vessels - remnants of blood vessels and nerve fibers may be visible in some canals
    • Osteoblasts - bone forming cells, sometimes visible lining the outer surface of the canals
  • Haversian Lamellae - concentric rings of bone matrix surrounding each canal
  • Cement Line - thin, darker staining (basophilic) boundary that marks the outer edge of an osteon
  • Osteocytes - found inside small spaces called lacunae between the lamellae (may appear shrunken or missing)
• Longitudinal Section - this section was taken along the edge of the shaft, rather than across it, making it look thicker than the cross-section
  • Haversian Canals - elongated channels running parallel to the length of the bone
    • Haversian Vessels - remnants of blood vessels and nerve fibers may be visible within some canals
    • Osteoblasts - line the outer surface of the canals
  • Osteocytes - located in lacunae between the lamellae

An alternative method for studying bone is the preparation of ground bone sections. The bone is not demineralized; instead, it is sawed into thin wafers and polished using abrasives. This preserves the mineral content, improving the visualization of structures such as lamellae, canaliculi, and lacunae, which are less clear in decalcified sections (see MH 044 Ground Bone).