Lecture 13 Chrondogenesis

Question 1

The genes that regulate organogenesis/cartilage/bone derive from?

Answer 1

craniofacial skeleton, axial skeleton and limb skeleton

Question 2

Each bony element from the axial skeleton derives from the same…

Answer 2

Somitic domain

Question 3

How do the bony elements differ?

Answer 3

Not identical as they differ depending on position along AP axis

Question 4

What do Hox genes control

Answer 4

Hox genes expressed control antero-posterior patterning of somites i.e. control segment identity

Question 5

Hox genes expressed control …. of somites i.e. control …

Answer 5

Hox genes expressed control antero-posterior patterning of somites i.e. control segment identity

Question 6

Why did mammals evolve to have the same number of vertebrae, but different sized vertebrae?

Answer 6

Mammals evolved to have the same number, but different sized vertebrae because Hox genes also control cell proliferation. Evidence from studies linking childhood cancer with defect in Hox gene regulation and homeotic transformation (i.e. formation of ribs on cervical vertebrae).

Question 7

What are sclerotome?

Answer 7

Proliferating progenitor cells

Question 8

What are the three main steps which lead to the formation of axial skeleton

Answer 8

1. Sclerotome induction
   - Sclerotome are proliferating progenitor cells
2. Cartilage formation = chrondogenesis
3. Ossification of axial skeleton = osteogenesis

Question 9

Describe the pathways of how a stem cell forms a myofiber or a hypertrophic chondrocyte

Answer 9

See notes

Question 10

Where are Pax3/7 expressed?

Answer 10

Dorsally

Question 11

Where are Pax1/9 expressed?

Answer 11

Ventrally

Question 12

Describe where the Pax genes are expressed during sclerotome formation

Answer 12

• Pax1/9 are expressed in the ventral somite as it undergoes EMT
• Pax1 is expressed more medial in sclerotome cells lying right next to neural cord and notochord
• Pax9 is expressed more distal sclerotome cells near the outside of the forming embryo and closer to the lateral mesoderm.

Question 13

TYPICAL EXAM Q: What is the experimental evidence that a particular gene is involved in a particular process?

Answer 13

Must discuss GOF assays (gene/mRNA/protein placed into ectopic place to see if it can instruct cell fate) and LOF assay (KO gene or protein/knock down mRNA encoded by gene).

Question 14

What does the phenotype of Pax1-/-;Pax9-/- demonstrate?

Answer 14

That these genes have an essential role in axial skeleton formation

Question 15

Compare WT to Pax1-/-;Pax9-/-

Answer 15

The photo shows that the neural arches have lost their middle section and are smaller. The vertebral body has been lost. There is a loss of intervertebral disc. So it can be concluded Pax1/9 play an essential role in formation of axial skeleton

Question 16

TYPICAL EXAM Q: What is experimental evidence that Shh regulates sclerotome induction?

Answer 16

We haven’t been told this but need to combine different components of the lecture. E.g. LOF on pax1 or pax9, or genetic knock downs. Include how mice lacking shh do not get pax1 or pax9 induction.

Question 17

Besides Pax1/9, what else play a role in sclerotome formation?

Answer 17

Scleraxis

Question 18

Shh and BMP4 and involved in the … and … of Pax1/9 expression

Answer 18

Induction

Patterning

Question 19

Describe the induction and patterning of Pax1/9 expression

Answer 19

INDUCTION:
- i.e. transcriptional upregulation. Shh induces a gene, which means that the signal is received by the responding sclerotome cell to initiate Shh signalling cascade. Gli activators go into nucleus and bind to Pax1/9 promotors to turn on transcription of pax1/9 genes
PATTERNING:
- i.e. how in a field of cells, you begin to break symmetry - here symmetry is broken along the medial lateral axis

Question 20

What is the cartilage matric made up of?

Answer 20

Proteins including collagen II/IX/XI

Question 21

Disorders affecting differentiation of cartilage and bone cells have features of both…

Answer 21

Dysostosis (defective formation of bone) and osteochondrodysplasia (defective bone/cartilage development).

Question 22

Describe the Wnt signalling in chrondogenesis

Answer 22

A bistable switch is regulated by Wnt signalling. 2 cells types form depending on whether mesenchymal condensations are switched on or not:
• If wnt is inhibitory, chondrocytes are formed
• If wnt is not inhibitory, osteoblasts are formed