Lecture 14 - Tissue remodelling (part 2)

Question 0

What are the stages of osteoblasts formation?

Answer 0

Mesenchymal stem cell -> osteogenesis progenitor cell -> transitory osteoblast -> mature osteoblast (then a bone osteocyte would form, that’s it’s terminal differentiation)

Question 1

What 2 types of components make up bone, and what are they exactly?

Answer 1

Mineral - calcium hydroxide

Organic - type 1 collagen, osteopontin, osteocalcin, osteonectin

Question 2

What are the primary functions of osteoblasts?

Answer 2

To line the bone surface and synthesise type 1 collagens and other bone matrix proteins
Initiate mineralisation

Question 3

What percentage of osteoblasts will become embedded osteocytes?

Answer 3

15%

Question 4

How are osteoblasts regulated (what type of signalling do they respond to)?

Answer 4

Regulated in an autocrine and paracrine manner by a range of growth factors (IGF, bFGF, TGFbeta, wnt, Bone Morphagenic Proteins <- bumps actually regulated themselves at local level).
Also respond to endocrine signals including:
Parathyroid hormone, vitamin d3, growth hormone, progesterone, oestrogen

Question 5

How are osteoblasts connected to one another?

Answer 5

Gap junctions
Integrins
Projections

Question 6

How do osteoblasts initiate mineralisation

Answer 6

1stly they’ve deposited the non-mineralised matrix, mainly made of collagen 1.
They then deposit matrix vesicles, where hydroxyapatite (HA) formation initiates
HA crystals cluster and coalesce

Question 7

How long is the delay from invitation to actual mineralisation?
What is new bone iniatlly formed as?

Answer 7

10 days

Un mineralised osteoid

Question 8

What are the possible fates of osteoblasts?

Answer 8

Most undergo apoptosis
Remaining ones become quiescent lining cells <-ready to be activated and lay down bone when needed
OR osteoblasts can differentiate into embedded osteocytes

Question 9

Describe the location and properties of the osteocyte habitat

Answer 9

Always remain enclosed in the lacuna, ie. they’re buried in the matrix forever
They reach out and connect to other osteocytes via extensions through the bone, but never actually see them
Remain viable for the life of the individual

Question 10

What is the purpose of osteocytes

Answer 10

To detect damage and changes to the environment
And subsequently when needed secrete sclerostin, which is involved in the wnt signalling pathway and is not really secreted by anything else

Question 11

What is the process of origin for an osteoclast

Answer 11

Haemapoetic stem cell -> CMP -> Oc- CFC (debatable) -> osteoclast

Question 12

General properties of osteoclasts (cell structure)

Answer 12

Large and multinucleated
Have a ruffled border (to increase SA)
Contains actin rings (which help with adhesion)

Question 13

What is the purpose of osteoclasts?

Answer 13

Overall: absorb and digest mineralised bone, have a clearance mechanism to absorb bone material and shunt it through apical membrane
How achieve this:
Secrete H+ and Cl- to form acidic environment
Also secrete enzymes (e.g. Tartare resistant acid phosphotases) to aid bone reabsoprtion
And secrete cathepisin k
And express calcitonin receptors

Question 14

Why do we ultimately have osteoblasts, osteocytes and osteoclasts?

Answer 14

BONE REMODELLING

Question 15

How often do you get a new skeleton?

Answer 15

Every 10 years

Question 16

Define bone remodelling and what it does

Answer 16

=Highly coordinated turnover of bone tissue involving sequential activity if blasts then clasts
It: repairs micro damaged bone
renews old bone tissue
Maintains ca2+ homeostasis (99% calcium store in bone, is released as needed)
Has a functional adaptation too-as increased muscular load bones get bigger as they’re used more

Question 17

What are the 2 types of bone remodelling

Answer 17

Cancellous and haversion

Question 18

Describe cancellous remodelling

Answer 18

Surface remodelling
Clasts attack and move along bone to the ‘furrow’
Then blasts arrive and begin remaking the bone
Osteoid mineralised to form bone
Technical stages of the process: quiescence -> resorption -> reversal-> formation -> mineralisation -> quiescence (CYCLE)

Question 19

Describe haver sham remodelling

Answer 19

Basically drilling through to the centre of the bone for repair there
Occurs in thickened bone
Head of the clasts tunnels through the bone (behind it is filled in)
-this gives rise to the concentric ring appearance
As the clasts tunnel down they are connected to other cells and blood vessels
-blood vessels provide nutrients and O2’ blasts are what lay down the new bone.

Question 20

Through what receptor and ligand to blasts and clasts communicate?

Answer 20

RANK and RANK ligand

Question 21

Describe the process of how new osteoclasts are signalled to form

Answer 21

Macrophage stimulating colonies released from blast
These attaché to mCSF receptors in monocytes
- this encourages the monocytes to become clast precursors (aka NFKB) (which have rank on the surface)
The blast has rank ligand in it’s surface, and this fuses with rank on clast precursors
-stimulates precursors to become proteoclasts, which then become quiescent osteoclasts then functional osteoclasts

Question 22

Can quiescent osteoclasts be stimulated directly to form functional osteoclasts?

Answer 22

Yes. They have rank so rank ligand can attach and make them become functional

Question 23

How is osteoclast formation regulated?

Answer 23

Indirectly. (Because blasts being signalled not clasts)
PTH and 1,25(OH)2D5 are signalling molecules, which are bone resorting agents
To increase clast formation:
They act directly on blasts and increase rank ligand expression
MCSF expression also increases so more monocytes are created
All together this stimulates osteoclasts genesis
Simulataneously (still for increasing clasts), calcitonin acts DIRECTLY in osteoclasts to inhibit resorption

Question 24

What can cause bone loss, also give details of how much and when

Answer 24

Being in space-1%lost per month in 0 gravity cos just not needed
Aging-bone mass increases till you’re 30
Then falls: steadily for men, sharp drop at menopause for females because sudden loss of protective oestrogen

Question 25

What is a dodgy consequence of bone loss

Answer 25

Elevated circulatory ca2+ levels

Question 26

Is oestoporosis bad and big problem

Answer 26

Yes. There is no way we need to know the monetary statistics etc. just do that there’s no screening, it’s expensive, and deadly

Question 27

What are the features of osteoporosis

Answer 27

There is less thickening and less connectivity in the bone
Osteoclasts act in ‘nibbled’ sites (how they appear to look at)
Clasts perforate the bone, resulting in more nibbling
BINER REABSORPTION IS GREATER THAN BONE FORMATION (called uncoupled modelling)

Question 28

What are current osteoporosis treatments?

Answer 28

Calcium supplementation
Hormone replacement therapy
Biphosphonates - potent inhibitors of bone REABSORPTION it will now only write that in capitals
Calcetonin
Selective oestrogen receptor modulators
Raloxifone
Drugs to target wnt signalling

Question 29

Does wnt signalling promote bone formation?

Answer 29

Yar

Question 30

Describe the Process of wnt signalling

Answer 30

Wnt binds to frz and lrp5/6 (which come together to form it’s receptor)
This activates dishevelled
A signalling cascade occurs
This turns on TCF gene transcription

Question 31

Describe how sclerostin acts on wnt

Answer 31

It’s a wnt INHIBITOR, it prevents it binding to it’s receptor so no signalling cascade and no ctf gene script ion

Remember sclerostin is released by osteocytes

Question 32

What are the effects of having a mutation in LRP5/6?

Answer 32

If the mutation is activating: higher bone mass because more wnt signalling
Inactivating mutation: lower bone mass

Question 33

Describe general high bone mass disorders and what they’re caused by

Answer 33

Bone thickening throughout skeleton, especially skul
Cranial nerves can be trapped causing deafness and facial palsy
Caused by: mutation in sost gene (encodes sclerostin)

Question 34

What is the evidence that sclerostin regulates wnt signalling?

Answer 34

Mutation in sclerostin gene leads to unregulated bone formation therefore higher bone mass

Question 35

Talk through the process of bone remodelling in the context if wnt and sclerostin

Answer 35

Normal bone-chutes secrete sclerostin, wnt signalling inhibited
Micro fracture-osteocyte apoptosis. Decrease in sclerostin so more wnt signalling. Osteoblast differentiation and rank ligand expression promoted (<-what wnt cascade actually leads to). Therefore more clasts now present at injury site because blast have differentiated. These resorb damaged bone.
Mature osteoblast replace damaged bone, including osteocytes and sclerostin levels return to normal
The end.