Lecture 16

Question 1

What happens when chondrocytes mature and stop dividing

Answer 1

They become hypertrophic and increase their cell volume

Question 2

As a result of a change in area at the periphery of the future bone, the region becomes known as the perichondrium. What is the significance of this region

Answer 2

The perichondrium is the precursor of osteoblasts

Question 3

What is meant by the collinearity of hox gene expression

Answer 3

Hox genes are expressed along the anterior-posterior axis in the same sequence that they occur in the genome. 5’ genes are expressed most posteriorly and 3’ are expressed anteriorly

Question 4

What is the first stage of endochondral ossification

Answer 4

Chondrogenesis the formation of a cartilaginous model of the bone

Question 5

What is the result of Ihh signalling?

Answer 5

Ihh binding to its receptors on the osteoblasts of the perichondrium causes the release of PTHrP. PTHrP acts on receptors on the chondroblasts and pre-hypertrophic chondrocytes to inhibit their differentiation and promote their proliferation. This negative feedback mechanism acts to maintain a progenitor pool in the bone.

Question 6

What is meant by the term epiphyses

Answer 6

The ends of the bones

Question 7

Hox gene expression provides the positional information responsible for the adoption of different cell fates along the anterior-posterior axis. What is the effect of this on formation of bones

Answer 7

The morphological structure of the axial skeleton throughout the anterior-posterior axis (i.e. ribs and vertebrae)

Question 8

What are the role of SOX9 in skeletal development implicated by conditional mice knockouts

Answer 8

Early inactivation of SOX9 reveals its role as a transcription factor that controls the expression of cartilage extracellular matrix proteins such as collagen II and XI. Later SOX9 knockouts indicated a second role for SOX9 in balancing the proliferative and hypertrophic chondrocyte levels.

Question 9

How has the study of the disease campomelic dysplasia implicated the role of a gene in regulation cartilage and bone production

Answer 9

Campomelic dysplasia (CD) is a disease that causes deformations of the long bones and axial skeleton. It is a dominant disease also characterised by defective airway cartilage and a smaller rib cage with fewer ribs. These defects are like those caused by mutations in the cartilage collagens II and XI. It was identified that CD is causes by a loss of function mutation in the SOX9 gene which is important for the formation of cartilage.

Question 10

Which cells does Ihh act on?

Answer 10

Ihh acts on cells in the perichondrium that are the precursors of osteoblasts

Question 11

What do the boundaries between the expression of different hox genes/hox profiles indicate in the axial skeleton

Answer 11

Boundaries between different hox expression correlates with a transition in the type of vertebrae

Question 12

Explain how studying cleidocranial dysplasia revealed another key gene involved in skeletal development

Answer 12

Cleidocranial dysplasia is a dominant disease caused by a mutation in Runx2. More than one mutation in the Runx2 gene is embryo lethal however patients with one mutation will show defective bone formation.

Question 13

Explain how Ihh or PTHrP knockout results in dwarfism

Answer 13

This leads to a loss of promotion of proliferation chondroblasts and pre-hypertrophic chondrocytes and the progenitor cells are exhausted quickly hence resulting in smaller bone growth

Question 14

Below is a list of the embryonic origins and aspects of the skeleton that they give rise to. Match up the embryonic origin with the bone structure. A - cranial neural crest cells, B - lateral mesoderm, C - somites. i - limb skeleton, ii - axial skeleton, iii - craniofacial skeleton

Answer 14

a – iii, b – ii, c – I

Question 15

What is the growth plate of long bones

Answer 15

The growth plate is a region of long bones that never ossifies and is located between the primary and secondary ossification centres. It is a stratified structure containing cells in all stages of bone development; pluripotent cells, chondroblasts, chondrocytes, pre-hypertrophic chondrocytes and hypertrophic chondrocytes

Question 16

What can be seen in conditional SOX9 mouse knockouts

Answer 16

Early inactivation of SOX9 lead to a failure of the embryo to form chondrocytes. Later SOX9 inactivation lead to a defected ability of the already formed chondrocytes to go hypertrophic

Question 17

What embryonic precursor structure gives rise to the appendicular skeleton

Answer 17

Lateral mesoderm

Question 18

Which cells release Ihh and when?

Answer 18

The chrondrocytes release Ihh as they differentiate and go hypertrophic.

Question 19

What structure gives rise to the somites that go on to give rise to the vertebral column and ribs

Answer 19

Paraxial mesoderm

Question 20

Recall the signalling pathway that regulates bone formation

Answer 20

FGFR3 –| Ihh –> PTHrP

Question 21

What is the significance of the growth plate

Answer 21

Disruption of this area leads to dwarfism

Question 22

What is the role of Runx2 in skeletal development

Answer 22

Runx2 is a transcription factor that is required to drive expression of the osterix gene that is required for bone development and ossification.

Question 23

Which paired box gene is expressed laterally in the sclerotome and which is expressed medially?

Answer 23

Pax1 is expressed medially and Pax9 is expressed laterally.

Question 24

Following the maturation of chondrocytes, they then undergo apoptosis. What fills the space left behind by this cell death

Answer 24

Blood vessels and osteoblasts from the perichondrium enter the space left by chondrocyte death and begin to secrete the bone matrix. This region will become the bone marrow.

Question 25

Explain the role of sonic hedgehog signalling in the control of sclerotome formation

Answer 25

Sonic hedgehog secreted from the ventral floor plate and notochord activates Pax1/9. Shh (-/-) fail to activate Pax1/9 and don’t development axial skeletons

Question 26

What is meant by chondrogenesis and osteogenesis

Answer 26

Chondrogenesis is the generation of cartilage and osteogenesis is the formation of bone

Question 27

Endochondral ossification is the other main type of ossification, when is this kind used and how does it work

Answer 27

Used for ossification of most bones other than those of the skull. It involves the development of bones by the replacement of a cartilage model

Question 28

Below is a list of cells in the chondrogenesis pathway and the corresponding processes they undergo. Put the cells in chronological order in which they appear and determine which process each undergoes to produce the next cell. A - Chondroblasts, B - Stem cells, C - Specification, D - Hypertrophic chondrocytes, E - Differentiation, F - Sclerotomal cells, H - Maturation, I - Chondrocytes, J - Determination

Answer 28

b(c), f(i), a(e), h(g), d

Question 29

What is seen in mouse models of Cleidocranial dysplasia

Answer 29

Runx2 knockout mice show cartilage formation but this cartilage fails to form bone and never ossifies

Question 30

Explain how BMP4 signalling acts to control Pax gene expression in the ventral somite

Answer 30

BMP4 from the lateral mesoderm prevents Pax1 expression from expanding into the Pax9 expression domain.

Question 31

What are the three stages of axial skeleton formation

Answer 31

Sclerotome induction, cartilage formation and ossification

Question 32

What is seen in mice knockouts for the Pax genes involved in chondrogenesis

Answer 32

Pax1 knockout produces viable mice that do show abnormalilites in vertebral column, sternum and scapula. Pax9 knockout produces mice that die shortly after birth due to abnormal craniofacial, visceral and limb skeletogenesis

Question 33

What four processes to sclerotomal cells undergo as they give rise to chondroblasts

Answer 33

Migration of cells around the notochord, downregulation of Pax1 and Pax9, the condensation of the cells and expression of extracellular matrix proteins

Question 34

Which two paired-box genes with homology to the Pax3/7 genes involved in myogenesis play a key role in chondrogenesis

Answer 34

Pax1/9

Question 35

Explain how primary and secondary ossification centres form

Answer 35

Osteoblasts begin to replace the disappearing cartilage and form a primary ossification centre where the centre of the bone is devoid of hypertrophic chondrocytes and full of a bone matrix. Blood vessels enter at the epiphyses which have remained cartilaginous during bone development. These epiphyses form the secondary ossification centres and leaves a cartilage growth plate between the epiphysis and diaphysis

Question 36

Each different embryonic precursor gives rise to a different bone structure using completely different mechanisms, T or F

Answer 36

F – the process of forming the bones is very similar in all regions, with the extracellular matrix playing a big role

Question 37

Explain the negative feedback loop between Indian hedgehog and parathyroid-related hormone that controls chondrocyte differentiation and maturation

Answer 37

As bone grows the chondrocytes become hypertrophic and release Ihh which signals in the same way as Shh. Cells in the perichondrium will respond to Ihh binding to their receptors and release PTHrP. The PTHrP receptor is only expressed in the chondroblasts and the pre-hypertrophic chondrocytes and hence these cells are the only ones that can respond to PTHrP. PTHrP signalling then results in the promotion of proliferation and inhibition of progression of the cells towards differentiation. As the cells differentiate they secrete a signal that will promote the proliferation of progenitors. This negative feedback loop ensures that as differentiation continues a pool of proliferative progenitors are maintained to cater for future growth

Question 38

What is significant about the need to ensure that chondrocytes of the secondary ossification centre don’t differentiate into osteoblasts too early

Answer 38

This is vital to ensure normal growth of the bone which would be prevented if the chondrocytes differentiated prematurely

Question 39

What is the result of knocking out both pax genes involved in skeletogenesis in mice and what does this show

Answer 39

The mice completely lack derivatives of the medial sclerotome including the vertebral bodies, interverbal discs and proximal rib regions. The distal and sternal rib portions are maintained however. This shows that Pax1 or Pax9 are required for medial sclerotome development

Question 40

A balance between which two genes allows the bones of the axial skeleton to grow but not maturate and differentiation too early

Answer 40

SOX9 and Runx2

Question 41

Which regions of the somite are the specific pax genes involved in axial skeleton development localised in

Answer 41

Ventral somite – Pax1 tends to be medial whereas Pax9 is lateral

Question 42

As chondroblasts differentiate into chondrocytes, proliferation is induced by expression of BMP2, BMP4 and BMP5 and a cartilaginous matrix is produced. What gene expression is required for this process

Answer 42

SOX9 HMG-box transcription factor

Question 43

What method of ossification is seen in the bones of the skull

Answer 43

Intramembranous ossification – this doesn’t involve chondrocytes or chondroblasts and instead includes nodules (mesenchymal cells --> nodules --> bone)