Lecture 11 - Specialised connective tissue and ECM: Cartilage

Question 1

What are the two components in cartilage?

Answer 1

Aggrecan and collagen

Question 2

What is cartilage and what are the main types?

Answer 2

A specialised connective tissue, which is the precursor for long bones in adults, it is very strong and allows tensile strength to be withstood
Main types: Hyaline (ribs, nose, larynx, trachea, articular joints), Fibro (joint capsules, ligaments), Elastic (ear, epiglottis, larynx)

Question 3

What kind of bones are tibia and fibia?

Answer 3

Long bones

Question 4

What are chondrocytes?

Answer 4

One of the key cells, they are the only cells in adult cartilage, they make the cartilage

Question 5

How is collagen synthesised?

Answer 5

3 polypeptides (alpha chains; rich in proline and glycine) form a coil, which can then aggregate into fibrils and then fibres
Pro collagen prevents aggregation in the cell

Question 6

What is Ehlers Danlos Syndrome?

Answer 6

Defective deposition of collagen - hyper flexible joints
If cartilage isn’t functioning, bones can move in anyway
This is due to a mutation in the collagen

Question 7

What are proteoglycans and what are the features of them?

Answer 7

GAGs (Glycosaminoglycan) attached to a core protein, they are highly anionic due to the presence of a negative sulphate group attached to the sugar in the GAG, therefore they attract a lot of water and form a hydrated gel

Question 8

What is function of the hydrated gel formed by proteoglycans?

Answer 8

Provides resistance to compression
Results in swelling pressure (turgor)
Provides strength and support

Question 9

Chondrocytes:

Answer 9

The only cell type in adult cartilage
Make up 5-10% of ECM volume
In groups of 2-8 cells
Groups are very spread
The cells are large and mature
High in RER and Golgi - shows they are making lots of proteins
Secrete high amounts of type II collagen and aggrecan

Question 10

What cells do chondrocytes form from?

Answer 10

During development some mesenchymal stem cells are becoming cartilage producing cells

Question 11

What key transcriptional factor do MSC that differentiate into chondrocytes express?

Answer 11

Sox-9 (HMG-box DNA binding transcription factor, leads to Col2A expression)
Col2A makes collagen type 2, which then drives these cells to become chondrocytes which are going to make cartilage

Question 12

What does chondrocytes proliferation and ECM synthesis require?

Answer 12

TGF-beta, FGF (fibroblast growth factor), IGF-1 (insulin-like growth factor) and PTHrP (Parathyroid hormone-related protein)

Question 13

What does PTHrP do?

Answer 13

Keeps cells proliferating and keeps them as chondrocytes

Question 14

Spatial patterning (location) of chondrocytes during bone formation:

Answer 14

This is coordinated by secreted signalling molecules termed morphogens, via inductive signalling pathway
Secreted factors can influence development of cells, cells closest to the signal will develop differently to cell furthest away
Morphogens as they are involved in the morphology of structures

Question 15

Hedgehog signalling:

Answer 15

In the absence of hedgehog, the genes that hedgehog would normally activate are turned off
They are being actively turned off
Hedgehog is a soluble protein, when it binds the receptor, it stops the repression of genes
Hedgehog turns on PTHrP genes, so it turns on protein expression, which keeps chondrocytes as chondrocytes
PTHrP binds a GPCR with then activate the G-protein pathway which leads to increased PTHrP production, so this is a positive feedback loop

Question 16

Where does formation of long bones from the cartilage model initiate?

Answer 16

From the primary ossification centres

Question 17

What happens at primary ossification centres?

Answer 17

Cartilage is catabolised and osteoid ECM becomes calcified, trapping osteoblasts, the cells in the centre turn to bone and get stuck the calcified environment

Question 18

What does osteoid ECM contain?

Answer 18

Hydroxyapatite and collagen

Question 19

What do osteoblasts do?

Answer 19

Breakdown newly formed bone in the centre to make a cavity into which bone marrow forms

Question 20

What do osteoblasts do?

Answer 20

Breakdown newly formed bone in the centre to make a cavity into which bone marrow forms

Question 21

What are the different members of the hedgehog family of proteins?

Answer 21

Sonic (SHH), Desert (DHH), and Indian (IHH)

Question 22

IHH/PTHrP feedback in long bone formation:

Answer 22

PTHrP is made by proliferating chondrocytes and keeps them proliferating
As these cells proliferate they become slightly differentiated and produce hypertrophic chondrocytes, these produce other factors too
As the distance between the proliferating chondrocytes and hypertrophic chondrocytes gets bigger, the amount of PTHrP that reaches the central cells is reduced, so IHH expression is reduced and so less PTHrP is being made by the cells, so eventually they stop proliferating
The central cells have now started to express other factors with further differentiates them into osteoblasts, which are now producing alternate ECM products which become mineralised and form the basis of bones and the ossification centres

Question 23

Spatial patterning during bone formation

Answer 23

The cells in the middle become bone like, as the matrix becomes less chondrocyte like and binds calcium, so cells also become osteoblasts which make the matrix and they get stuck in this matrix
Calcium binding the matrix makes it had and bone like

Question 24

What are the key features of articular cartilage?

Answer 24

Connective tissue ECM that direct bone development during embryogenesis
Essential for permitting mechanical load and movement of articulate bones
Contains a single cell type - chondrocytes
Avascular (hypoxic)
Alymphatic
Aneuronal
Articulated joints are at the two ends of long bone