Bone Histology and Function

Objectives of Bone Histology

• Understand the important features of bone
• Learn about the composition of bone
• Identify the types of bone and their covering
• Recognize the cells of bone and their functions

Bone Function

• Supports the body
• Protects internal organs
• Acts as the body's Ca2+ reservoir

Osteoblasts

• Originates from mesenchymal stem cells
• Produces the organic components of bone matrix, including:
  • Type I collagen fibers
  • Proteoglycans
  • Glycoproteins such as osteonectin
• Deposition of inorganic components of bone depends on osteoblast activity
• Active osteoblasts are located exclusively at the surfaces of bone matrix, bound by integrins, forming a single layer of cuboidal cells joined by adherent and gap junctions

Osteocytes

• Are the cells of mature bone
• Lie in the lacunae of bone, representing osteoblasts that have become 'imprisoned' in the matrix during bone formation
• Have eosinophilic or lightly basophilic cytoplasm, correlated with negligible secretory activity and the presence of only a small amount of endoplasmic reticulum in the cytoplasm
• Present in greatest numbers in young bone, decreasing with age
• Functions:
  • Maintain the integrity of lacunae and canaliculi, keeping open channels for diffusion of nutrition through bone
  • Play a role in removal or deposition of matrix and calcium when required

Osteoclasts

• Are bone-removing cells
• Found in relation to surfaces where bone removal is taking place
• Occupy pits called resorption bays or lacunae of Howship
• Are very large cells (20 to 100 μm or more in diameter) with numerous nuclei and mitochondria
• Have a ruffled membrane at sites of bone resorption
• Removal of bone by osteoclasts involves demineralization and removal of matrix
• Bone removal can be stimulated by factors secreted by osteoblasts, macrophages, lymphocytes, and parathyroid hormone

Bone Matrix

• Composed of:
  • Inorganic salts (about 65% of dry weight)
  • Organic ground substance and collagen fibers (about 35%)
• Organic matrix consists of:
  • Glycosaminoglycans
  • Proteoglycans
  • Water
  • Special glycoproteins osteonectin and osteocalcin
• Collagen fibers (Type I) are synthesized by osteoblasts and embedded in the ground substance

Periosteum and Endosteum

• Periosteum:
  • An outer fibrous layer of dense connective tissue
  • Contains bundled type I collagen, fibroblasts, and blood vessels
  • Penetrates the bone matrix and binds to the bone
• Endosteum:
  • A thin layer covering small trabeculae of bony matrix
  • Contains osteoprogenitor cells, osteoblasts, and bone lining cells
  • Within a sparse, delicate matrix of collagen fibers

Types of Bone

• Woven bone:
  • Nonlamellar, with random disposition of type I collagen fibers
  • First bone tissue to appear in embryonic development and fracture repair
  • Usually temporary, replaced by lamellar bone in adults, except in a few places
• Lamellar bone:
  • Characterized by multiple layers or lamellae of calcified matrix
  • Typical of compact or cancellous bone in adults
  • Organized as parallel sheets or concentrically around a central canal
  • Contains osteocytes in lacunae, interconnected by canaliculi with dendritic processes

Compact Bone

• Compact bone:
  • Also includes parallel lamellae organized as multiple external circumferential lamellae
  • Inner circumferential lamellae around the marrow cavity
  • Lamellae of outer and innermost areas of compact bone enclose and strengthen the middle region containing vascularized osteons

This quiz covers the structure, composition, and functions of bones, including types of bone, bone cells, and their roles in the body.