The Cellular Structure of Bone

Question 1

What is bone?

Answer 1

INORGANIC - 65%
- calcium hydroxyapatite (Ca10(PO4)6(OH)2)
- is storehouse for 99% of Calcium in the body
- 85% of the Phosphorus, 65% Sodium, Magnesium
ORGANIC - 35%
- bone cells and protein matrix

Question 2

What are the different types of bone and classifications?

Answer 2

Bone types and classifications
Anatomical bones
- % types- Flat, long, short/cuboid, irregular, sesamoid
Macroscopic structure
- trabecular/cancellous/spongy- is the thin honeycomb like bone crisscrossing inside the bone shaft
- cortical/compact- the thick bone making up the shaft
Microscopic structure
- Woven bone (immature)- during initial bone development
- Lamellar bone (mature)

Question 3

What is the difference between cortical and trabecular bones?

Answer 3

CORTICAL:
	- Predominant type in long bones
	- 80% of skeleton
	- appendicular
	- 80-90% calcified
	- mainly structural, mechanical, and protective
TRABECULAR:
	- vertebrae & pelvis
	- 20% of skeleton
	- axial
	- 15-25% calcified
	- mainly metabolic
	- large surface area

Question 4

Describe the geography/areas of a typical bone.

Answer 4

The spacing closed by the cortical bone is called the medullary cavity
The main shaft of the bone is called the diaphysis
The ends of the bone are called the epiphysis
These are separated by called the growth plates (not visible in the diagram)
The region below the growth plate is called the metaphysis

Question 5

In what stages do bones develop?

Answer 5

Bones develop throughout growth and into adulthood
- Growth plate fusion and ossification completes development
- Clavicles growth plates fuse at ~20 years old
Intramembranous ossification
- Direct differentiation of osteoblasts from connective tissue
- Flat bones
Endochondral ossification
- Bones form from a cartilage model
- Long bones

Question 6

How does endochondral ossification happen in early foetal development?

Answer 6

In early foetal development, a cartilage scaffold will form within the limb buds
By birth this scaffold has begun to change a little
The scaffold has expanded in size but now lacks vasculature meaning the centre of the cartilage model becomes hypoxic
To correct this, blood vessels invade the scaffold and with them they bring the precursors for bone cells which then differentiate and convert the centre of the cartilage scaffold into bone
This is called the primary ossification centre first occurring in the centre of the diaphysis of the bone
Ossification then expands from the centre of the diaphysis moving gradually towards the epiphysis

Question 7

How does endochondral ossification happen in childhood?

Answer 7

In childhood, as the bones continues to increase in size, a secondary ossification centre will form in the epiphysis and that will remain separated from the primary ossification centre via a cartilage structure called the growth plates
This specialised structure is what allows us to have our rapid linear growth
Development continues as more and more of the cartilage scaffold becomes ossified into bone in the primary and secondary ossification centres
Growth finally finishes and bone development is complete when the cartilage growth plates finally becomes ossified and fuses the primary and secondary ossification centres together

Question 8

how do growth plates allow for rapid linear growth?

Answer 8

The growth plate is cartilage meaning it contains cells called chondrocytes
These are organised into a structure with three distinct zones and over their lifespans the chondrocytes will progress through these zones

Question 9

What zones of development do these chondrocytes ‘move’ through?

Answer 9

These zones are:
- The reserve zone- which is thought to contain the stem cell population for the growth plates
- They are close to their blood supply in the secondary ossification centre
- Proliferative zone- The chondrocytes become highly proliferative and form these distinct columnal structures
- As the chondrocytes get further away from the blood supply they undergo hypertrophic differentiation; they expand in size and start producing collagen10
- The further into the hypertrophic zone a cell is , the larger it will be and the further it is from the blood supply
- Eventually it becomes too much and they apoptose, leaving behind the cartilage mineral matrix that they’ve been producing which then calcifies and is finally ossified by bone cells
Note that the cells do not move around the growth plate, the growth plate moves around them as the bone elongates

Question 10

What are the specialised bone cells (3)?

Answer 10

Osteocytes - mechanosensory network embedded in mature bone
Osteoclasts - multinuclear cells that resorb/remove bone
Osteoblasts - produce osteoid to form new bone

Question 11

What do osteocytes do?

Answer 11

Osteocytes – mature bone cells
Embedded in lacunae in mature bone
Connected via processes through canalicular channels
Form a mechanosensory network throughout bone

Question 12

What do osteoclasts do?

Answer 12

Osteoclasts – the bone breaking cells
Giant multinuclear cells formed from the fusion of macrophages
They seal off a portion of bone beneath them using what is called an actin ring
They secrete acids to break down the inorganic portion of the bone and enzymes for the organic to ‘resorb’ the sealed off bone

Question 13

What do osteoblasts do?

Answer 13

Osteoblasts – form new bone
Secretes osteoid; the organic component of new bone
Osteoid is mineralised over time to become mature bone
Some osteoblasts are embedded in the new bone and differentiate into osteocytes- this is how the osteocyte network is maintained in newly formed bone

Question 14

Overall what is the reason for these specialised bone cells?

Answer 14

The skeleton is not a fixed or stagnant organ, it is dynamically regulated to maintain health
Small portions of bone are constantly being removed and replaced
- Your whole skeleton has been replaced after 7 years
This is done by the specialised bone cells via the bone remodelling cycle

Question 15

Describe the bone remodelling cycle.

Answer 15

As you move around your bone is exposed to stresses and strains that can cause microfractures, typically in older bones
These are detected by the osteocyte mechanosensory network which signals for osteoclasts to come in and differentiate at the site of the damage and resorb away that old damaged bone
When they’re finished they undergo fission, dedifferentiating back into individual mononuclear cells
The osteoblasts then arrive and secrete the osteoid to produce an amount of new bone equal to that removed by the osteoclast
The bone becomes mineralised and the bone has been repaired
Bone disease occurs when that balance is lost

Question 16

How can bone loss and bone gain occur from an error in the bone remodelling cycle?

Answer 16

The Bone Remodelling Cycle – bone loss
If you have excess osteoclast bone resorption or insufficient bone formation at the end of the cycle you have a net loss of bone mass

The Bone Remodelling Cycle – bone gain
Conversely, if you have insufficient bone resorption or excess osteoblastic bone formation, you will end up with a net gain in bone mass

The Bone Remodelling Cycle – bone disease
Loss of bone seen in osteoporosis
Osteosclerosis is the gain of bone mass

Question 17

How can bone remodelling be controlled through endocrine and paracrine means?

Answer 17

Endocrine
- Oestrogen, Thyroid hormone, PTH
Paracrine
- RANKL, Wnt signalling

Question 18

What is RANKL and how does it regulate osteoclast differentiation?

Answer 18

When it comes to osteoclasts, RANK ligand is the master regulator
In the absence of it, you will not have differentiation of osteoclasts and all
RANK receptor – Activation required for osteoclast differentiation and survival.
RANK Ligand – produced by osteocytes and osteoblasts
Cells of the osteoblast lineage also produce a substance called OPG (osteoprotegerin) – decoy receptor for RANKL also produced by osteocytes and osteoblasts.
OPG can inhibit the activation of osteoclast differentiation and survival

Question 19

What are Wnts? How does Wnt signalling lead to bone formation?

Answer 19

The Wnts are a family of secreted glycoprotein ligands
Highly complex pathway involved in many different organ systems
Stimulates osteoblast differentiation
The Wnt ligand binds onto a cell membrane receptor, the frizzled receptor, a 7 transmembrane receptor with similarities to a G-coupled protein receptor
Before it can be activated by Wnt it must be in complex with a co receptor
In bone the relative co receptor is made up of proteins called LRP5 and LRP6
When Wnt binds to this complex of LRP5/6 and frizzled it canonically sets of a signalling cascade that results in the translocation of beta catenin into the cell nucleus, causing a change in gene expression and leading to osteoblast differentiation
Inhibited by Sclerostin and Dkk-1 which bind to LRP5/6 and prevent it from interacting with the frizzled receptor

Question 20

Why are osteocytes key regulators of remodelling?

Answer 20

Osteocytes are the key regulators of the other bone cells types via this paracrine signalling
You can see that they regulate osteoclast differentiation and activity by producing RNAK ligands as well as inhibiting it by producing OPG
They also produce sclerostin and DKK1 so they also can regulate the differentiation of osteoblasts

Question 21

What is osteopetrosis?

Answer 21

Stone bones
LRP5 activating mutations- resulting in increased osteoblast differentiation throughout their lifetime
The family this was discovered in was able to survive a high impact car crash unscathed but have difficulty swimming
van Buchem’s and SOST- in this disorder patients have a loss of function in the SOST gene that encodes for the production of sclerostin meaning there is no inhibition of Wnt pathway

Question 22

What is osteoporosis?

Answer 22

Defined as having a bone mass greater than 2.5 SD below average peak bone mass
Estimated that it will effect 1:2 women and 1:5 men.
- Causes 500,000 low impact bone fractures every year in the UK
Can be primary (menopause, aging), or secondary (drugs, disease, lifestyle)