Bone and Cartilage Formation

Question 1

What are the 4 origins of cartilage and bone?

Answer 1

1. Somites (sclerotome) - form vertebrae, ribs, and occipital bone
2. Somatic lateral plate Mesoderm - limb and girdle bones, sternum
3. Cranial Neural Crest - craniofacial bones
4. Head mesoderm (prechordal plate and paraxial) - forms part of the skull (parietal bones)

Question 2

What are the two ways to make bones?

Answer 2

1. Intramembranous Ossification - mesenchymal cells make osteoblasts, osteoblasts make bone
   - makes the most of the skull, clavicle, sesamoid bones (like the patella)
2. Endochondral Ossification - mesenchymal cells make chondrocytes, chondrocytes make cartilage, cartilage makes bone

Question 3

What is the purpose of hypertrophic chondrocytes?

Answer 3

They secrete VEGF which simulates blood vessel invasion

Question 4

How do chondrocytes help bone development?

Answer 4

Chondrocytes express Sox9, when BMPs and Wnt is expressed it becomes a hypertrophic chondrocyte. The hypertrophic chondrocyte allows for expression of Runx2 which activates osterix, osterix changes a prechondrial cell into an osteoblast.

Question 5

Is Runx2 needed for bone development?

Answer 5

yes otherwise the boned made from endochondral ossification will only be cartilage (this is lethal because the ribs wouldn’t help the lungs with breathing)

Question 6

Where do limb buds and limb bones come from?

Answer 6

The somatic lateral plate mesoderm (the buds also have and overlaying ectoderm)