Bone Tissue

Bone Tissue has special functions.

Outline the functions of the Bone Tissue

1. Bone provides a mechanical framework on which muscles can act to provide movement.
   
   
   

2. Bone provides a protective casing. Eg: Ribcage around thoracic organs. Skull for the brain
   
   
   

3. Haematological/ Immunological : Bone contains bone marrow from which bone blood cells and immunological cells are synthesised.
   
   
   

4. Pharmacological: Bone is a site of drug and fluid infusions
   
   
   

5. Physiological: Bone is part of fluids and electrolytes.
   
   
   

Outline the structural components of the Bone Tissue

Bone is made up of:

• 70% Salt deposits
  
  
  

• 30% of organic matrix
  
  
  

The salt deposits are then further made up of:

• Calcium (Ca2+)
  
  
  

• Phosphate
  
  
  

• Carbonate
  
  
  

• Magnesium (Mg2+)
  
  
  

• And other trace elements (Na+ and K+)

Calcium (Ca2+)

What are the functions of Calcium ?

• Calcium has Structural, Cofactor and Signaling Functions.
  
  
  

1. Structural Functions
   

• Intercellular Adhesions

  
  

• Provides structural integrity of bone and teeth

  
  
  
  

2. Cofactor Functions include
   

• Calcium acts as a cofactor in reactions such as the Clotting / Complement cascading process
  
  
  

• Calcium also acts as a cofactor in protein synthesis and apoptosis
  
  
  

3. Signaling
   

• Calcium acts as a second messenger and can induce gene expression
  
  
  

• Calcium is able to induce excitation-contraction and secretion
  
  
  

• Calcium can also modulate nerve/ membrane excitability (formation of action potentials)

  
  
  
  • Hypercalcemia's membrane excitability is low as Calcium repels the Sodium Ions, vice versa

Calcium Metabolism

Outline the Calcium distribution in the body

• 99% of Calcium is placed in the bone
  
  
  

• 1% is in the cells
  
  • Where o.1% is in the plasma
    
    
    

• We can ONLY measure the calcium in the plasma
  
  
  

Outline the distribution of Calcium in the Plasma

• 50% of Calcium in the plasms is bound to proteins. eg: Albumin
  
  
  

• 9% of the Calcium in the plasma is bound to Anions. Eg: Bicarbonate or citrate
  
  
  

• 41% of Calcium in the plasma is known as the free portion.

  
  
  
  • If an individual as low albumin levels in the plasma
  • There is less albumin to bind proteins
  • Therefore, more proteins in the free portions
    
    
    

• Ionized/ Free portion of calcium executes physiological reactions - Calcium is bioactive portion
  
  
  

• Portion is tightly regulated by Parathyroid Hormone (PTH)
  

Describe the sections of the body which can affect the Plasma Calcium

1. Intestine:
   

• Calcium can come from diet through the Splanchnic Blood Flow from the intestine.

  
  

• It can then be lost through fecal loss.

  
  
  
  

2. Cells:
   

• Cellular components can also provide or take away calcium from the Plasma
  
  
  

3. Kidney
   

• Plasma calcium can also communicate with the renal components

  
  

• This is because kidneys have the ability to store calcium and loss it through urine loss

  
  

• Secondary regulatory site of plasma calcium

  
  
  
  

4. Bone:
   

• Calcium is taken from the plasma to make bone tissue

  
  

• Bone tissue are dissolved and calcium is retuned back to plasms

How is Calcium Handled in the GIT ?

• Bowel Input: Calcium in the intestinal system comes from diet, bowel juices or tissue breakdown
  
  
  

• Absorption: The calcium is then absorbed into the duodenum
  
  
  

• Within the duodenum there are specialised epithelial cells, that can allow calcium to pass from one end to another.
  
  
  

• Calcium does this in 2 ways:
  

1. Passive absorption
   

• Depends on the concentration of calcium on the lumen side.
  
  
  

• Allows ONLY a small portion to be absorbed
  
  
  

2. Active Transport
   

• Active transport mechanism is Vitamin D dependent
  
  
  

• Vitamin D activation is enhanced by the PTH (Parathyroid Hormone)
  
  
  

• PTH stimulated the synthesis of a Calcium (Ca2+) binding protein known as Calbindin
  
  
  

• Calbindin will bind calcium so that calcium ends up accumulating in the epithelial cell.
  
  
  

• When Calcium builds up, it forms a gradient between the amount of calcium in the cell compared to the amount of calcium outside the cell.
  
  
  

• Therefore, by Facilitated Protein through a transport protein it can take advantage of this gradient to move calcium out of the cell.
  
  
  
  

• Bowel Output: Calcium can be lost through fecal loss.

Renal Ca2+ Handling

• The nephron is the smallest functional unit of the kidney
  
  

List the components of the nephron

• Glomerulus
• Proximal Tubules
• Loop of Henle
• Distal Tubule
• Collecting Tubule
  
  

How is Calcium handled in the Kidney ?

1. Glomerulus
   

• Ionized (Free) calcium is filtered in the glomerulus.
  
  
  

• Afterwards it ends up the tubular system
  
  
  

2. Proximal Tubule
   

• Bulk of (65%) calcium in the tubular system is reabsorbed in the proximal tubule.
  
  
  

• Through Paracellular Transport or through Ca Channels.
  
  
  

3. Loop of Henle
   

• 25% of Calcium is reabsorbed in the Loop of Henle
  
  
  

4. Distal Tubule
   

• 10% is reabsorbed in the Distal Tubule
  
  
  

• Through Epithelial Calcium Channels (ECaC)
  
  
  

• Protein involved is Transient Receptor Potential V5 (TRPV5)
  
  
  

• Reabsorption is enhanced by Vitamin D and PTH to prevent Ca excretion
  
  
  

• Vitamin D is also activated by the kidneys
  
  
  

5. Collecting Tubule
   

• If things are not reabsorbed then they are lost with urine.

Bone Components: Phosphate

• Phosphate is predominantly an intracellular ion
  
  

Describe the distribution of Phosphate in the body

• 85% of phosphate is in bone

  
  

• 15% of phosphate is in the cells

  
  

• Less than 1% is in the plasma

  
  
  
  

List the functions of Phosphate

• Phosphate has Metabolic, Homeostasis and Structural Functions
  
  
  

1. Metabolic
   
   

Phosphate is responsible for a variety of metabolic reactions:

• Enzyme regulation

  
  

• Oxygen Transport through 2.3 DPG

  
  

• Energy storage via the ATP molecule

  
  
  
  

2. Homeostasis
   

• Acid-base balance: Phosphate can bind Hydrogen which is an acid.

  
  

• Phosphate acts as a buffer system

  
  
  
  

3. Structural
   

• Phosphate is used to make bone. (using Bisphosphonates)

  
  

• Phosphate is used to make the membrane structure: Phospholipid Bilayer

  
  

Phosphate Metabolism

How is Phosphate Metabolized in the Plasma and GIT ?

1. Plasma
   

• The plasma has 0.8 - 1.4 mmol/L
  
  
  

• Phosphate is not as tightly regulated in the plasma as Calcium (Ca2+)
  
  
  

• Phosphate is able to bind to Hydrogen to form phosphate buffer
  
  
  

• Phosphate in the plasma acts as a storage site for Hydrogen ions.
  
  
  

2. GIT
   

• Almost all dietary Phosphate is absorbed
  
  
  

• Vitamin D promotes the trans-epithelial fluxes

Renal Phosphate Handling

How is the Phosphate handled in the Kidney ?

1. Glomerulus
   

• Phosphate is filtered in the glomerulus
  
  
  

2. Proximal Tubule
   

• 80% of Phosphate is reabsorbed in the proximal tubule
  
  
  

• This occurs through co-transporters: co-transport transport phosphate back into blood from the proximal tubule.
  
  
  

3. Distal Tubule
   

• 10% is reabsorbed in the distal tubule
  
  
  

• Phosphate is regulated in the Distal Tubule.
  
  
  

• Parathyroid Hormone will increase the loss of Phosphate
  
  
  

• Vitamin D will stimulate the conservation of Phosphate
  
  
  

• When we measure in blood that a person has a high PTH, their Blood Phosphate level is low. As the phosphate has been lost in the kidney.

  
  
  
  • Therefore, a high Ca2+ and a low Phosphate level will indicate that there is a high PTH level and activity.
    
    
    

4. Collecting Tubule
   

• Phosphate Buffer system acts in the Collecting Tubule.
  

Magnesium (Mg2+)

How is Magnesium distributed in the body ?

• 53% of Magnesium is in the Bone
  
  
  

• 27% is in the muscle
  
  
  

• 19% is in the soft tissue
  
  
  

• 0.3% is in the Plasma
  
  
  

How is Magnesium distributed in the Plasma ?

• 33% of Mg2+ in the plasma is Protein-Bound
  
  
  

• 5% of Mg2+ in the plasma is anions
  
  
  

• 62% of Mg2+ in the plasma is ionized (free)
  
  
  

Outline the functions of Magnesium

Magnesium has Metabolic, Structural and Other Functions.

1. Metabolic
   

• Magnesium acts as an enzyme cofactor, and because of its ability to bind to ATP, it is a cofactor which makes ATP useful for cells.
  
  
  

• Magnesium is an energy substrate as it can bind to ATP to form MgATP
  
  
  

2. Structural
   
   

-Magnesium is also used in the making of bone structure

• Magnesium is used to make structures of Ribosomes, nucleic acids and proteins
  
  
  

3. Other
   

• Magnesium interact with Calcium: Magnesium has a greater charge than Ca2+. Therefore it can repel Calcium.

  
  
  

  • If someone has Hypermagnesemia, the individual will have muscle paralysis as the Magnesium will repel calcium by electrostatic repulsion from calcium channels.

    
    

  • Calcium channels will therefore not be able to enter a muscle to trigger an excitation. Person will become Hypotonic with Hypermagnesemia

    
    
    
    

• Magnesium regulates protein functions.

GIT Mg2+ Handling

Explain how Magnesium handled in the GIT ?

1. Absorption
   

• Magnesium is absorbed in the GIT at the Jejunum and Ileum
  
  
  

• Absorption is promoted by the Vitamin D.
  
  
  

• The entry of Magnesium from the lumen and into the Blood occurs via 2 transport system:

  
  
  
  • Paracellular Transport
  • Transcellular Transport
    
    
    

1. Paracellular Transport
   

• Paracellular transport is when Magnesium can pass through cells to reach the blood.
  
  
  

• This transport system is dependent on the concentration gradient
  
  
  

• As long as there is greater Magnesium concentration the lumen of the gut compared to the blood.
  
  
  

• The Magnesium will find it way between the cells to the blood.
  
  
  
  

2. Transcellular Transport
   

• Transcellular transport is when Magnesium has to find its way through the epithelial cells
  
  
  

• Magnesium will enter the Apical Side of the epithelial cell via the TRPM6 (Transient Receptor Potential Melastatin Type 6) Channels
  
  
  

• Channels will allow Magnesium into the epithelial cell via the Electrochemical Gradient as the inside of the cell is negatively charged
  
  
  

• This transport system is dependent on the
  
  
  

• Once inside the epithelial cell, the Magnesium is then transported via transport proteins to the blood through the process of Facilitated Diffusion.
  
  
  

• Facilitated Diffusion takes advantage of an existing electrochemical gradient to move objects down the concentration gradient

Bone Structure

Describe the structure of a typical long bone

• A long bone has an Epiphysis on either end of the bone.
  
  
  

• A diaphysis (Shaft) between the 2 Epiphysis.
  
  
  

• The Epiphyses contain articular cartilages to form joints with the other bones.
  
  
  

• The bone has and Epiphyseal Plate that allows for bone growth and lengthening of the bone.
  
  
  

• Diaphysis contains a Periosteum which can be modified to allow bone growth and widening of the bone.
  
  
  
  

Describe the cross-sectional structures seen in the bone

• Outer Periosteum known as the Compact Bone.
  
  
  

• Inner Part is known as Spongy Bone and is close to the bone marrow.
  
  
  

• Osteocytes that form a pattern in the bone
  
  
  

• Lamella which is made up of concentric layers of compact tissue that surround the Haversian canals
  
  
  

• Blood vessels moves within Haversian Canals
  
  
  

• The combination of pattern where Blood vessels are surrounded by Lamella is known as the Osteons/ Haversian system
  
  

NOTE: Bone is a living structure along with Osteocytes.

Osteon

Describe the structure of an Osteon

1. Osteon has a blood vessel at the center of the Haversian Canal
   
   
   

• Osteoblast follow after the blood vessel
  
  
  

2. Osteoblast are a stem cell derived cells
   
   
   

• Osteoblasts are responsible for building bone tissue
  
  
  

• Osteoblasts then connect to Osteocytes
  
  
  

3. Osteocytes are quiescent cells which form the particular pattern of the bone
   
   
   

• Connections of the Osteocytes allow for spaces to form around these cells.
  
  
  

• Spaces are known as Canaliculi, which contain communicating substances
  
  
  

• Between the Osteoblasts and the Osteocytes there are Gap Junctions which allow for intercellular communication
  
  
  

• Osteocytes "builders" are trapped within their concrete. They can be recruited through communication systems and can become Osteoblasts
  
  
  

4. Osteoclasts
   

• Osteoclasts ae connected to Osteocytes
  
  
  

• Osteoclasts are a form of Macrophages
  
  
  

• As a result: Osteoclasts dissolve or destroy bone tissue to prepare for bone remodeling
  
  
  

Bone Formation: Deposition and Calcification

How do Osteoblasts build bone tissue ?

• Osteoblasts are responsible for building bone tissue.
  
  
  

• Osteoblasts secret collagen, ground substance, alkaline and phosphatase.
  
  
  

• These structures and fibres will then polymerize to form soft tissue also known as Osteoid Tissue
  
  
  

• Sometimes, osteoblasts become trapped in the osteoid tissue and become Osteocytes (form of bone), (quiescent forms of osteoblasts)
  
  
  

• How does this Osteoid Tissue harden ?
  
  
  

• With the deposition of Hydroxyapatite crystals from the Calcium/ Phosphate precipitation the osteoid bone is able to harden
  
  
  

• This process of hardening is regulated by precipitation Inhibitors called Pyrophosphate that prevent Ectopic Calcification.
  
  
  

• When precipitation (hardening of bone) occurs after a few days bone will form Amorphous Bone which is fairly soft.
  
  
  

• After a few weeks the bone will form Crystalline Hydroxyapatite (Bone)

How do Osteoclasts breakdown bone tissue ?

• Osteoclasts are regulated by the Parathyroid Hormone (PTH)
  
  
  

• PTH aims to breakdown bone to release more Calcium into the plasma
  
  
  

• Because osteoclasts come from a lineage of macrophages they produce villus-like extensions that can phagocytose bone tissue
  
  
  

• Osteoclasts release:
  

1. Proteolytic enzymes that digest bone matrix
2. Acids such as lactic acid, citric acid dissolve the salts in the bone tissue
   
   
   

• Osteoclasts will phagocytose other debris.

Regulation: Cellular Signals

Outline how do cellular signals regulate bone changes ?

1. Osteoblasts are regulated by the PTH
   
   
   

• Osteoblasts can produce molecules that can bind to macrophages or pro-osteoclasts to stimulate them to become osteoclasts.
  
  
  

• This is done through the production of molecules that can bind to receptors on the macrophages.
  
  
  

• These receptors are known as RANK (Receptor Activator of Necrosis Factor)
  
  
  

• Osteoblasts produce the RANK ligand
  
  
  

• When the RANK Ligand binds to RANK receptor, the macrophages proliferate and become typical Osteoclasts.
  
  
  

• This promotes bone destruction.
  
  
  

• In turn the Osteoclasts have receptors known as Osteoprotegerin
  
  
  

• Osteoblasts will then produce an Osteoprotegerin Ligand that binds to the Osteoprotegerin of the Osteoclasts.
  
  
  

• the binding of the Osteoprotegerin Ligand to the Osteoprotegerin receptor will inhibit the Osteoclast activity.
  
  
  

• This promotes bone formation.
  
  
  

How is Osteoclast activity regulated

• RANK binding stimulates osteoclast activity

  
  

• Osteoprotegerin binding inhibits osteoclast activity

  
  
  
  

How do we measure osteoclast activity ?

To monitor osteoclast activity we:

• Measure the RANK Ligand and compare it to the amount of Osteoprotegerin Ligand
  
  
  

• This can tell us whether a bone is being formed or a the bone is undergoing destruction, by measuring bone density