Bone Histology and Classification

Bone Histology

• Bone is a highly vascularized connective tissue where the matrix is calcified, making it hard and strong enough to support the body.
• Bone is composed of bone cells, fibers, and a hard matrix.
• Bone is classified anatomically as long (e.g., humerus, femur), short (e.g., carpal, tarsal), flat (e.g., skull, ribs), or irregular (e.g., vertebrae).
• Bone functions include support, protection, movement, mineral storage, and blood cell formation.

Learning Objectives

• Classify bone anatomically and histologically.
• Identify the location of compact and spongy bone 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 the plates of flat, short, and irregular bones.
• Cancellous (spongy) bone: Formed of irregular branching, anastomosing bony trabeculae enclosing bone marrow cavities. Found in the center of flat bones, short bones, irregular bones, and the epiphyses of long bones.

Bone Structure

• Bone cells:
  • Osteogenic cells (osteoprogenitor cells): precursor cells
  • Osteoblasts: bone-forming cells that synthesize osteoid tissue (the organic part of bone).
  • Osteocytes: mature bone cells, branched in shape, enclosed within the matrix, maintaining bone hardness.
  • Osteoclasts: large, multinucleated cells with foamy acidophilic cytoplasm located in Howship's lacunae, responsible for bone resorption by releasing H ions and enzymes.
• Fibers: Primarily collagen type I, composing about 35% of the organic part of bone.
• Ground substance: GAGs and glycoproteins (without water).
• Hydroxyapatite crystals: Inorganic part of bone (65%), calcium salts, deposited on collagen fibers and within the ground substance, causing calcification.

Histology of Compact Bone

• The layers, from outside to inside:
  • Periosteum: outer layer of fibrous connective tissue.
  • External circumferential lamellae: regular parallel lamellae immediately beneath the periosteum.
  • Haversian system (osteon): structural unit; composed of concentric lamellae surrounding a central (Haversian) canal containing blood vessels, nerves, and connective tissue. Haversian canals communicate with marrow cavity and periosteum via Volkmann's canals.
  • Interstitial lamellae: irregularly arranged lamellae between Haversian systems.
  • Internal circumferential lamellae: regular parallel lamellae adjacent to the endosteum.
  • Endosteum: single layer of osteoblasts lining the marrow cavity.

Histology of Spongy Bone

• Irregular branching, anastomosing bony trabeculae (T) enclose bone marrow (BM) cavities.
• Each trabeculum is composed of irregularly arranged lamellae that encase osteocytes but does not contain osteons.