Chrondogenesis

Question 1

What 3 embryonic places to bone’s originate from and what bones do they form?

Answer 1

1. Cranial neural crest (craniofacial skeleton)
2. Somite’s (axial skeleton)
3. Lateral mesoderm (limb skeleton)

Question 2

What is the Axial skeleton and why do all the parts of it consist of different shapes and sizes?

Answer 2

bit in the middle so the ribs, vertebral column which forms from the paraxial mesoderm
- all come from the same cells but have diff sizes due to hox genes defining the A-P axis
- Hox also controls proliferation controlling size

Question 3

What are the three steps leading to axial skeleton formation?

Answer 3

1. Sclerotome induction
2. Cartilage formation = chrondogenesis
3. Ossification of axial skeleton = osteogenesis (formation of bone)

Question 4

What is the slightly more detailed sequence of events leading to axial skeleton formation?

Answer 4

Stem cell in somite
Scelrotomal cells
Chrondoblasts
Chrondocytes
Hypertorphic chrondocytes

Question 5

What precursors are found in the sclerotome?

Answer 5

sclerotome located on medial side of each somite adjacent to notochord

dorsal = pax3/7 for predetermined skeletal muscles
pax7= specifically for muscle repair
pax 1/9 involved in axial skeletal specification (similar structure to pax3/7)

Question 6

Where is pax 1/9 expressed?

Answer 6

• Both in ventral part of somite where sclerotome arises
• Pax1 expressed more medially, pax9 expressed more laterally

Question 7

What happens when you KO pax1, pax9 and then both?

Answer 7

Pax 1 KO:
- mice are viable with some defects but otherwise fairly normal

Pax9 KO:
- mouse dies after birth as they have craniofacial and limb impairments
- little defect in axial skeleton!

Both KO:
- mice completely lack vertebral bodies, vertebral discs and ribs
- functional redundancy

Question 8

What are the signals controlling sclerotome formation?

Answer 8

• Neural tube, notochord, ectoderm and lateral plate mesoderm contribute signals leading to specification of the sclerotome
• Shh from ventral and BMP4 from lateral plate prevent Pax1 from expanding into the lateral domain, this is how you restrict the domain

Question 9

How do we see if these sclerotome signals are the right ones for differentiation?

Answer 9

Ectopic signalling (GOF)
LOF removing SHH receptors etc.

Question 10

How do chrondoblasts form?

Answer 10

• Once you have specified cells in the sclerotome they will migrate laterally and will surround the notochord
• Downregulation of Pax1/9
• Condensation of cells
• Forms chrondoblast which proliferate induced by BMP family

Question 11

How does a chrondocyte form?

Answer 11

• Production of cartilage matrix through proliferation of chrondoblast requires Sox9
• Leads to final differentiated cartilage cell - a chrondocyte

Question 12

What are the two main modes of ossification?

Answer 12

1. Intramembranous ossification: mainly used for ossification of bones from skull
2. Endochondral ossification : used for ossification of most bones (limb)

Question 13

How does Endochronal ossification occur?

Answer 13

• Chronogenesis forms cartilage
• Chondrocytes stop dividing and become hypertrophic (enlargement)
• Triggers arrival of blood vessels to areas where chondrocytes have undergone apoptosis to form bone marrow
• Osteoblasts replace disappearing cartilage and form primary ossification centre (main shaft of bone)
• Secondary ossification centres are formed in epiphyses leaving a cartilage plate (growth plate) between epiphysis (rounded bit) and diaphysis

Question 14

How do you regulate the production of cartilage with the production of bones?

Answer 14

• Some bones form directly from the conversion of mesenchymal cells into osteoblasts and those subsequently differentiate into osteocytes
• In the case of the axial skeleton you first form a cartilage and THEN the bone comes when you have the full model of the cartilage

Question 15

What is Campomelic dysplasia? (CD)

Answer 15

• mutation in Sox 9 shows defect with short bones because there are issues with proliferation and formation of cartilage model
• Shows Sox9 is essential for subsequent proliferation of chondroblasts into chondrocytes

Question 16

What is Cleidocranial dysplasia? (CCD)

Answer 16

• mutation in Runx2 needed to drive mesenchymal cells into osteoblasts
• Runx2 controls expression of osterix, together responsible for osteogenesis

Question 17

How do you ensure cells forming cartilage are not differentiating into bones?

Answer 17

• Sox2 blocks expression of Runx2 so as long as its present, no osteoblasts
• No longer need Sox9 means Runx2 comes in place and starts inducing bone formation
• Wnt signalling in the tissues that need direct osteogenesis blocks the pathway of cartilage formation to refine and balance the decision of cells

Question 18

What three molecular molecules are essential for maintaining the growth plate and aligning the bones, what can defects in these cause?

Answer 18

FGFR3
Ihh (indian hedgehog)
PTHrP (Parathyroid hormone-related protein)

mutations = dwarfism

Question 19

How does the growth plate stimulate long bone formation?

Answer 19

• Growth plate resembles every stage from chondroblast to hypertrophic chondrocytes
• When it becomes hypertrophic chondrocyte it secretes Ihh which Chondoblasts respond to as they have the receptor
• Triggers signalling cascade where PTHrP is the end product
• PTHrP blocks the differentiation of this chondroblast and subsequently stimulates its proliferation
• Feedback mechanism to stop further progression