Roles of BMPs and WNTs in endochondral bone formation. (a) Physiological endochondral bone formation is stimulated by bone morphogenetic proteins (BMPs). Wingless-type like (WNT) signaling plays a supportive role in relation to BMPs. However, some WNTs have a negative effect on early chondrocyte differentiation. (b) In the presence of inflammation, tumour necrosis factor (TNF) may stimulate BMP signalling but also the expression of DKK1, which acts a WNT antagonist. The balance between TNF, BMP and WNT signalling may determine the onset and progression of ankylosis. DKK, dickkopf.