Figure 8. LOH model for osteochondroma formation.

(A) EXT^−/− chondrocytes that originate or end up on the edge of the growth plate lose the ability to respond to a polarity signal. These chondrocytes adopt a round morphology and do not contribute to the neighbouring columns of chondrocytes. The LOH may occur during early skeletal development, before formation of the bony collar. (B) As EXT^−/− chondrocytes proliferate they form a clone of cells that begin to grow out from skeleton beneath the EXT^−/+ perichondrium. (C) EXT^−/+ chondrocytes join the clone of EXT^−/− cells, express collagen10 and become hypertrophic. EXT^−/+osteoblasts are recruited to form the bony collar. This results in the cartilage cap being predominantly composed of EXT^−/− chondrocytes, while the cells that make up the rest of the osteochondroma (hypertrophic chondrocytes, osteoblasts and perichondrial cells) are predominantly EXT^−/+.