bone

• Bone

  A specialized connective tissue composed of calcified extracellular material and the bone matrix, with three major cell types
• Three major bone cell types

  • Osteocytes
  • Osteoblasts
  • Osteoclasts
• Osteocytes


  • Found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes extending into small canaliculi (canals) between lamellae
• Osteoblasts


  • Synthesize the organic components of the matrix
• Osteoclasts


  • Multinucleated, giant cells involved in the resorption and remodeling of bone tissue
• Mesenchyme


  A loosely organized, mainly mesodermal embryonic tissue which develops into connective and skeletal tissues, including blood and lymph
• Canaliculi


  Microscopic canals between the lacunae of ossified bone
• Haversian canals / Osteons (Haversian systems)

  The morphological and functional unit of the bone, surrounding blood vessels and nerve fibers throughout the bone and communicating with osteocytes
• Periosteum

  • A layer of dense connective tissue on the outer surface of bone, bound to bone matrix by bundles of type I collagen called perforating (or Sharpey) fibers. Regions adjacent to bone are rich in osteoprogenitor cells and osteoblasts that mediate increases in bone thickness by appositional growth.
• Endosteum


  • A thin layer of active and inactive osteoblasts, which lines all the internal surfaces within bone; osteoblasts here are also required for bone growth.
• Bone formation

  1. Intramembranous ossification: Osteoblasts differentiate directly from mesenchyme and begin secreting osteoid

  2. Endochondral ossification: A pre-existing matrix of hyaline cartilage is eroded and invaded by osteoblasts, which then begin osteoid production
• Osteoblast differentiation and bone matrix secretion

  1. Osteoprogenitor cells in the periosteum and endosteum differentiate into osteoblasts

  2. Osteoblasts secrete osteoid containing collagen type I, proteoglycans and other molecules, and matrix vesicles

  3. As osteoid undergoes calcification, hardens, and traps the osteoblasts, these cells differentiate further as osteocytes occupying lacunae surrounded by bony matrix

  4. Osteoclasts, produced by the fusion of blood monocytes, reside on bony surfaces and erode the matrix during bone remodeling
• Compact lamellar bone

  • Transverse perforating (Volkmann) canals connecting adjacent osteons
  • Remnants of eroded osteons seen as irregular interstitial lamellae