Topic 70 - The role of the osteoblast, osteoclast and osteocyte

Question 1

Words to include

Answer 1

Osteoblast

• Mesenchyma tissue → oseteoprogenitor cell → osteoblast → octeocyte
• Role:
  
  • ​Osteogenesis
  • Ca/P deposition & mobilization in adults
• Non ionized osteoid (OS)
  
  • Active osteoblasts secrete a thick matrix layer
  • Organ substance: collagen
• Mineralized layer
  
  • Calcium-salt
    
    • Deposited in matrix
• Resorptive osteoblast
  
  • Osteoclasts inactivated
• ​Functions:
  
  • ​Calcium concentrations
    
    • EC: mmol/l
    • IC: μmol/l
  • Calcium pathways
    
    • Transcellular: regulated
    • Paracellular: gradient dependent pathway
  • Calcium mobilization
    
    • Under direct regulation by parathormone
  • D-hormone effect
    
    • D-hormone activity of ATP-dependent calcium pump ↑ on the plasma side of resorptive osteoblasts ↑
  • Bone formation
    
    • PTH directly stimulates bone formation
      
      • PTH ↓
      • Calcium permability ↓
      • Osteoblast facilitation of PTH suspended

Osteocyte

• Mesenchyma tissue → oseteoprogenitor cell → osteoblast → octeocyte
• Role: Maintains normal metabolism in bone
• Characteristics:
  
  • ​Lacunae cells
  • Alkaline phosphataes activity (ø)
  • Involvement in resorption
  • Trophic activity
  • Metabolite elimination

Osteoclast

• Macrophages
  
  • Role: Structural development in young age
• Regulation:
  
  • ​Signal peptides
    
    • Secreted by osteoblast layer stimulated by PTH
      
      • PTH induces production of a group of peptides
      • Reach osteoclast in a paracrine way
      • Facilitate bone-resorbing activity
• Processes in activated osteoblast:
  
  • Collagenase enzyme
    
    • Production & release ↑
  • Facilitation of proton pump and glycolytic activity
    
    • Local pH ↓
    • Hydroxyapatite crystals dissolved
• Processes converge to resorption of organic and inorganic components of bone:
  
  • Osseus derbis from resorption
    
    • Phagocytosis
    • Diffusion
  • Hydroxyproline
    
    • Detected in urine
      
      • Intensive collagenolysis

Osteoclast figure:

1. Parathormone
2. Glycolysis (glycolytic activity in cell)
3. Lactic acid, citric acid dissolving of Ca-salts (lactic acid & citric acid)
4. Primary lysosome
5. Collagenase protease
6. Re-uptake or phagocytosis of osseus derbis and Ca, P (phagocytosis)
7. Secondary sysosome
8. Blood & plasma
9. Kidney: increase of hydroxyproline clerance

Question 2

What are the main constituents of bone tissue?

1. Organic components
2. Mineral components

Answer 2

1. Organic components:
   
   1. _​_95% collagen
   2. 5% chondritinsulphate, glucoproteins
2. Mineral components
   
   1. _​_85% hydroxyapatite
   2. 10% CaCO₃ (calcium carbonate)
   3. 5% Mg, Na, K⁺ salts

Question 3

Give the origin of osteoblast, osteoclast and osteocyte

Answer 3

• Specialized cells of bone resorption are macrophages having hemopoetic origin
• Osteoblast** → osteocyte**
  • ​Due to agening and change of location

Hemopoietic progenitor cell → monocyte → preosteoclast → osteo_clast_

Mesenchyma cells → osteoprogentior cells → osteoblast** → osteocyte**

Question 4

Osteoblast

Answer 4

• Derived from mesenchyma tissue
• ​Role:
  
  • Prematur phase: Major cells of osteogenesis
  • Adults: Calcium/phosphorus deposition and mobilization
• Osteoblast** → osteocyte**
  
  • ​Due to agening and change of location

Histology:

• Active osteoblast layer
  
  • ​Organic substance production
  • Ca & P transport
• Non ionized osteoid
  
  • ​Organic substance: collagen
• Mineralized layer
  
  • ​Calcium-salts is deposited in the matrix
• Resorptive osteoblast
  
  • ​On the other side of this bone osteoclasts are inactivated

Functions of osteoblast:

1. ​Calcium concentrations
   
   • ​EC: mmol/l
   • IC: μmol/l
2. Calcium pathways
   
   • ​Transcellular: regulated
   • Paracellular: gradient dependent pathway
3. Calcium mobilization
   
   • ​PTH ↑
   • Osteoblasts are under direct regulation by parathormone
4. D-hormone effect
   
   • ​By the action of D-hormone activity of ATP-dependent calcium pump is increased on the plasma side of the resorptive osteoblasts
   • Calcium, which is resorbed can pass into plasma against concentration gradient
   • The effect is due to increased synthesis of de novo calcium pump
5. Bone formation
   
   • ​PTH ↓
   • Termination of PTH effect in bone forming osteoblast

Question 5

Osteocyte

Answer 5

• Osteoblast** → osteocyte**
• Role: Maintains normal metabolism in bone
• Characteristics:
  
  • Cells are in lacunae
  • No alkaline phosphatase activity
  • Involvement in resorption
  • Trophic activity
  • Metabolite elimination

Question 6

Osteoclast

Answer 6

• Macrophages
• Osteoclasts are crucial during osteolysis and remodeling
• Regulation: osteoclasts are regulated by unidentified signal peptides which are secreted by the osteoblast** layer** stimulated by PTH
• Intensive bone resorption conducted by osteoclasts results in the removal of matrix and mineral content of the bone

Figure:

1. The macrophage/osteoclast penetrate the finished osseus tissue
2. This cell is under indirect hormonal control of PTH
   
   • ​This hormone induces production of a group of peptides, which will reach the osteoclasts in a paracrine way and facilitate their bone-resorbing activity
3. In activated osteoblast there are two parallel processes:
   
   1. Production and release of collagenase enzyme ↑
   2. By facilitation of proton pump and glycolytic acitvity in the cell:
      
      1. Local pH ↓
      2. Hydroxyapatite crystals are dissolved

Question 7

Answer 7

1. Parathormone
2. Glycolysis (glycolytic activity in cell)
3. Lactic acid, citric acid dissolving of Ca-salts (lactic acid & citric acid)
4. Primary lysosome
5. Collagenase protease
6. Re-uptake or phagocytosis of osseus derbis and Ca, P (phagocytosis)
7. Secondary sysosome
8. Blood & plasma
9. Kidney: increase of hydroxyproline clerance