Arthrotome: a specific joint forming compartment in the avian somite.

Somitocoele cells previously have been shown to form the proximal part of the ribs, the intervertebral discs, and the intervertebral joints (synovial joints). To determine whether the somitocoele cells are necessary for the development of axial skeleton joints, we microsurgically ablated the somitocoele cells in epithelial somites of 2-day-old chick embryos. The operated embryos were analyzed after whole-mount skeletal preparations and in sections. Removal of the somitocoele cells led to two major outcomes: (1) Intervertebral joints failed to develop and resulted in the fusion of the superior articular process and the inferior articular process; (2) Adjacent vertebral bodies fused and lacked the intervertebral disc. These results demonstrate that somitocoele cells specifically give rise to intervertebral joints and discs. Furthermore, these results suggest that neighboring sclerotome cells cannot adapt to form vertebral joints in the absence of the somitocoele compartment. Thus, we provide for the first time experimental evidence for the existence of a joint forming compartment in the somites, which we term the "arthrotome."