Bone Histology and Classification

Bone Histology

• Bone is a highly specialized connective tissue. Its matrix is calcified, making it hard and strong.
• It is composed of bone cells, fibers, and a hard matrix.
• Anatomically, bone is classified as long, short, flat, and irregular, according to its shape and location in the body.
  • Examples of long bones: humerus, femur
  • Examples of short bones: carpal, tarsal
  • Examples of flat bones: skull, ribs
  • Examples of irregular bones: 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 the microstructure of bone

Histological Classification

• Compact bone: Dense, solid lamellae, without cavities, located in the shaft surrounding the medullary canal and on the surface of flat, short, and irregular bones.
• Cancellous (spongy) bone: Irregular, branching, anastomosing bony trabeculae enclose bone marrow cavities. Found in the center of flat bones, short bones, irregular bones, and the epiphysis of long bones

Bone Structure

• Bone cells:

  • Osteogenic cells (osteoprogenitor cells): precursor cells for osteoblasts.
  • Osteoblasts: bone-forming cells
  • Osteocytes: mature bone cells, maintaining bone hardness. They reside within lacunae of the matrix. They function in maintaining bone health and assisting in calcium homeostasis.
  • Osteoclasts: large, multinucleated bone-resorbing cells. They resorb bone by releasing hydrogen ions and lysosomal enzymes. They reside in Howship's lacunae.

• Fibers: Collagen fiber type I, which makes up 35% of the organic portion of bone.

• Ground Substance: GAGs and glycoproteins

• Hydroxyapatite Crystals: Inorganic part (65%) comprises calcium salts. They are deposited on collagen fibers and within the ground substance, hardens the matrix.

Histology of Compact Bone

• Periosteum: Outermost layer of fibrous connective tissue.
• External circumferential lamellae: Layers immediately beneath the periosteum
• Haversian system (osteon): Fundamental structural unit of compact bone, concentric lamellae surrounding a central Haversian canal. The central canal contains blood vessels, nerves, and loose connective tissue
• Interstitial lamellae: Irregularly arranged lamellae between osteons.
• Internal circumferential lamellae: Layers parallel to the endosteum
• Endosteum: Innermost single layer of osteoblasts lining the marrow cavity

Histology of Spongy Bone

• Formed by irregular branching, anastomosing bony trabeculae (T) that enclose bone marrow (BM) cavities in between.
• Each trabeculum is formed of irregularly arranged bone lamellae enclosing osteocytes but lacks osteons.

References

• Junqueira's Basic Histology (2013) by Anthony L. Mescher (Text and atlas, 13th Edition)

This quiz covers the complex structure and classification of bones in human anatomy. It focuses on the histological differences between compact and spongy bone, along with anatomical classifications. Test your knowledge on bone functions and microstructure.