Chondrogenesis (L13) Flashcards
What is chondrogenesis?
The process by which cartilage and bones form
Where do different parts of the skeleton come from? What are they regulated by?
The cells that make up the axial skeleton are regulated by patterning genes such as Pax genes. they come from the somites. The craniofacial skeleton comes from the cranial neural crest cells and the limb skeleton comes from the lateral mesoderm. the 3 skeletons are controlled by genes that regulate organogenesis (transcription factors, extracellular matrix proteins). The axial skeleton (vertebral column and ribs) forms from the paraxial mesoderm. Although each bony element from the axial mesoderm derives from the same somatic domain, they aren’t identical and differ depending on their position along the AP axis.
What evidence led to the idea that there is a positional mechanism that tells sclerotome cells to indicate their fate?
The fact that cervical and thoracic vertebrae are so different suggests that there must be a mechanism that provides positional information to sclerotomal cells to indicatetheir axial level and therefore their future identity and morphology.
What controls the patterning of somites?
HOX genes yay!! Mammals have a fixed number of cervical vertebrates (7) including giraffes and camels, it’s just the size and their structure changes e.g. the joint in giraffes is more flexible. It is thought that HOX genes control cell proliferation and therefore control the size of the vertebrae in mammals. Slightly different Hox gene codes cause the variation in vertebrae between birds and mammals. E.g. Chicks have many more cervical vertebrae than mice.
What is the transition between cervical and thoracic vertebrae dictated by?
By the boundaries between somites, which is determined by the AP axis.
What are the steps leading to axial skeleton formation?
- sclerotome induction - takes place in the somite - located in the ventral part.
- cartilage formation (chondrogenesis)
- Ossification of the axial skeleton (osteogenesis)
It works in quite a similar way to myogenesis.
Stem cells -> sclerotomal cells -> chondroblasts -> chondrocytes -> hypertrophic condrocytes
Similar genes are also involved. (Pax genes)
Explain the expression of Pax genes in the sclerotome formation
You can see that Pax1 and Pax9 are expressed during sclerotome formation (the ventral part of the somite- which has undergone the mesenchymal transition to form the sclerotome) Pax1 is more strongly expressed in the medial domain and Pax 9 in the lateral domain.
Explain the results of Pax KOs in skeletal development
In Pax1 KOs, mice are viable but they have slight abnormalities in the vertebral column, the sternum and the scapula. But overall, they’re okay. In Pax9 KOs, the mice have fairly normal bone development except limb and craniofacial. However, because of this craniofacial problem, they die shortly after birth. To test if the genes have functional redundancy you can do a double KO. In this case, a lack of Pax 1 and 9 causes the mice to have a complete lack of medial sclerotome, vertebral bodies, intervertebral discs and proximal ribs. Showing that Oax1 and Pax9 are required for medial sclerotome development. However, neural arches and distal ribs are retained, suggesting they are controlled by something else. These experiments have allowed scientists to fate map where each part of the vertebrae and rib comes from
What different parts of the sclerotome do the different parts of the axial skeleton come from?
The vertebral bodies come from the medial sclerotome, the proximal ribs come from the lateral sclerotome. The neural arches come from the dorsal sclerotome and the distal and sternal part of the ribs comes from the dermomyotome rather than the sclerotome.
Explain the different signals used in sclerotome formation
Sclerotome formation is not controlled by intrinsic factors - i.e. the cells do not know themselves that they need to activate these genes but are controlled by surrounding tissues through the release of signalling molecules. For sclerotome formation, we know that you need Shh released from the notochord and diffuses in the ventral part of the somite and induces the expression of Pax and Pax9. We know this because if you KO Shh, you get similar phenotypes to a Pax1/Pax9 KO and you also don’t an expression of the TFs in the sclerotome. The expression of these genes in localised to the sclerotome by BMP4 being released from the lateral plate. BMP4 inhibits Shh expression. This not only keeps the Pax localised but it also causes the boundary between the medial and lateral sclerotome by mainly restricting Pax1 diffusion into the lateral sclerotome
How do cells differentiate to form cartilage?
The cells start migrating around the notochord. They then down-regulate Pax1 and Pax9, and upregulate the number of extracellular matrix proteins. This leads to the condensation of cells. This forms the chondroblasts (the non-differentiated cartilage cells). The chondroblasts then need to differentiate into chondrocytes. they do this by increasing proliferation, which is induced by BMP2,4 and 5, Then a cartilage matrix is produced by upregulating proteins like collagen II, IX and XI. However, this requires Sox9 (an HMG-box TF) You can see that after condensation, you start to get a structure that already is starting to look like a vertebral body.
What is osteogenesis? what are the 2 models?
Also, know as ossification. When cartilage turns into bone. After the cartilage has formed, it serves as the base for the model of the skeleton.
- intramembranous ossification: Mainly used for ossification of bone from the skull - involved mesenchymal cells to nodules to osteoblasts to osteocytes to bone (no chondroblasts or cytes)
- Endochondral ossification: Used for ossification of most bones (e.g. limbs) Bone develops by replacement of a cartilage model.
Explain how endochondral ossification works.
Chondrocytes in the model stop diving and become hypertrophic. They die by apoptosis - blood vessels and osteoblasts enter this space which will become bone marrow. Osteoblasts replace the disappearing cartilage and form primary ossification centres. Blood vessels enter the epiphyses (ends of the structure). Secondary ossification centres are formed at the epiphyses, leaving a cartilage plate (growth plate) between the epiphysis and diaphysis.
Why is a balance between cartilage-producing and bone-producing cells important?
If it is imbalanced, you get bone disorders like campomelic dysplasia (a mutation sox9) (airway cartilage is effected so babies die) and cleidocranial dysplasia (a mutation in RunX2) - causes problems with the intramembranous ossification in the skull
What does the growth of long bones depend on?
Growth of long bones all depends on the growth plate. Negative feedback loop between IHh (indian hedgehog) and PTHrP (parathyroid hormone-related protein) controls chondrocyte differentiation and maturation. FGFr3 inhibits Ihh which activates PTHrP
