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.

This quiz covers the histological classification of bone, including the characteristics of compact and cancellous bone. You will learn to classify bone anatomically and identify its microstructure within the human body. Test your understanding of bone's functions and its critical role in the skeletal system.