Bone & joints

• Bone is a type of mesenchymal connective tissue derived from common primitive mesenchymal precursors
• Structure primarily consists of an extracellular matrix produced and modified by bone cells
• It is a composite biphasic mix of organic and inorganic components

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• Bone cells belong to the osteoprogenitor series and osteoclast series
• Osteoprogenitor series (primitive committed mesenchymal cells) is derivative of tissue bound mesenchymal stem cells
• Mesenchymal stem cells are multipotential stromal precursor cells with potential to form adipocytes, chondrocytes, myocytes and fibroblasts
• Present in the perianlage tissue of the fetus, periosteum, Haversian systems, Volkmann and medullary canals
• Osteoprogenitor cells have the ability to fabricate osteoblasts only; they have basic spindle morphology
• Osteoclast series is derived from monocyte granulocyte colony precursors in the bone marrow

Osteoblasts

• Essential to the formation of bone tissue
• Located on actively growing bone surfaces
• Lifespan may range from months to many years
• Metabolic state is intimately associated with their morphology
  • Active osteoblasts resemble plasma cells and range in size from 10 - 80 μm (average: 20 - 30 μm)
    • Large cuboidal to pear shaped, polyhedral mononuclear cells when rapidly forming bone
    • Contain copious amphophilic to basophilic cytoplasm that is in intimate contact with bone
    • Nuclear membranes are distinct with eccentric (polarized away from the matrix) vesicular nuclei that possess 1 or more prominent nucleoli
    • Visible perinuclear halo representing Golgi apparatus
  • Inactive osteoblasts / lining cells are spindle shaped to flattened
    • Present on the surface of bony trabeculae
    • Have flattened, nonreactive nuclei with scant cytoplasm
    • ~60 - 80% of osteoblasts go through apoptosis
    • Remaining osteoblasts either develop into osteocytes enveloped by matrix or flatten and stretch forming cellular coating on bone surfaces (J Bone Miner Res 2021;36:1432)

Osteocytes

• Most abundant cell type of bone (> 90% of bone cells in adult bone)
• Terminally differentiated osteoblasts that incorporate into bone matrix
• Half life approaches 25 years
• Appear as small blue dots within lacunar spaces in the bone matrix
• Cell body, nucleus and surrounding scant cytoplasm dwell inside a lacunar space
• Nuclei are small and not evident in every plane of section; therefore, in most slides of bone tissue, lacunae are empty
• Part of osteocyte cytoplasm pinches off into long and delicate cytoplasmic processes resembling axons
• These processes lie in canaliculi (channels traversing through the bone matrix)
• The ends of osteocyte cell processes connect to those of adjoining osteocytes and to surface osteoblasts through gap junctions (Annu Rev Physiol 2020:82:485)
• This creates a tremendous surface area of contact between the osteocyte and the matrix and extracellular fluid surrounding each cell
• In this way, osteocytes communicate with one another forming intricate and integrated systems all over the bone tissue
• Number, size, shape and position of osteocytes depend upon the type of bone they reside in
  • Woven bone
    • Abundant, big and plump
    • Appears disordered and random microscopically as their long axes are parallel to the direction of adjacent collagen fibers
  • Lamellar bone
    • Less in number, smaller, spindly
    • Seem to be organized as the cells are arranged in the same direction as the surrounding lamellae
    • Osteocyte apoptosis increases with aging, causing reduced osteocyte density

Osteoclasts

• Osteoclasts are terminally differentiated cells
• Lifespan of days to weeks
• Present in scalloped, cup shaped depressions on bone surface called Howship lacunae
• 40 - 100 μm in diameter
• Polarized with one part of cell membrane closely adhered to bone (ruffled border) while the other pole is exposed to extracellular fluid
• Have granular eosinophilic cytoplasm, may have 4 - 20 nuclei
• Metabolic activation of osteoclasts is initiated by anchorage
• Activated nuclei coordinate the complex and transitory cytoplasmic and cell membrane alterations necessary for bone digestion
• After osteoclastic activity, macrophages mobilize to the base of the resorption pit and phagocytize the organic remnants (Connect Tissue Res 2018;59:99)
• Osteoclast-like giant cells are multinucleated cells that were once osteoclasts but could not be defined at the moment because of absence of residual bone

Contributed by Nasir Ud Din, M.B.B.S.

• Osteoblasts
  • Cytoplasm of active osteoblasts contains widespread granular endoplasmic reticulum, a large, prominent Golgi apparatus, abundant mitochondria and lysosomes
  • In contrast, the cytoplasm of inactive osteoblasts resembles that of quiescent fibroblasts
• Osteoclasts
  • In the cytoplasm, a network of interrelated actin filaments extends from the site where the osteoclast cell membrane attaches to the bone (clear zone) directly to the nuclei
• Reference: StatPearls: Histology, Osteoblasts [Accessed 7 November 2023]

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Which of the following about this bone is true?

1. Collagen bundles are organized in parallel or concentric layers
2. Hypocellular with smaller lacunae than lamellar bone
3. Low mineral content
4. Synthesized slowly
5. Weaker, less rigid and more flexible than lamellar bone

E. Weaker, less rigid and more flexible than lamellar bone. The image depicts woven bone, which is weaker, less rigid and more flexible than lamellar bone. Answer C is incorrect because woven bone has a higher mineral content than lamellar bone. Answer B is incorrect because woven bone is more hypercellular than lamellar bone. Answer D is incorrect because woven bone is fabricated during rapid bone formation. Answer A is incorrect because collagen bundles are arranged haphazardly in woven bone.

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Reference: Histology-bone

Which of the following corresponds to this structure?

1. Does not go through apoptosis
2. Has abundant cytoplasm and is multinucleated
3. Most abundant cell type of bone tissue
4. Resembles plasma cells, is polyhedral, with copious amphophilic cytoplasm and eccentrically placed nuclei with 1 or more nucleoli
5. Spindle shaped

D. Resembles plasma cells, is polyhedral, with copious amphophilic cytoplasm and eccentrically placed nuclei with 1 or more nucleoli. This is a description of activated osteoblasts, which resemble plasma cells in that they have an eccentrically placed nucleus with copious amphophilic cytoplasm. Answer E is incorrect because inactive osteoblasts are spindle shaped to flattened, not active osteoblasts as shown here. Answer B is incorrect because osteoclasts have abundant cytoplasm and multiple nuclei, not active osteoblasts. Answer A is incorrect because ~80% of active osteoblasts undergo apoptosis. Answer C is incorrect because osteocytes are the most abundant cell type in bone tissue, not active osteoblasts.

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Reference: Histology-bone