Atypical molecular profile for joint development in the avian costal joint.

Development of synovial joints involves generation of cartilaginous anlagen, formation of interzones between cartilage anlagen, and cavitation of interzones to produce fluid filled cavities. Interzone development is not fully understood, but interzones are thought to develop from skeletogenic cells that are inhibited from further chondrogenic development by a cascade of gene expression including Wnt and Bmp family members. We examined the development of the rarely studied avian costal joint to better understand mechanisms of joint development. The costal joint is found within ribs, is morphologically similar to the metatarsophalangeal joint, and undergoes cavitation in a similar manner. In contrast to other interzones, Wnt14/9a, Gdf5, Chordin, Barx1, and Bapx1 are absent from the costal joint interzone, consistent with the absence of active β-catenin and phosphorylated Smad 1/5/8. However Autotaxin and Noggin are expressed. The molecular profile of the costal joint suggests there are alternative mechanisms of interzone development.