FUNCTION OF SOMITE AND SOMITOCOELE CELLS IN THE FORMATION OF THE VERTEBRAL MOTION SEGMENT IN AVIAN EMBRYOS

We have studied the distribution of thoracic somite and somitocoele-de rived cells using homotopical grafting between quail and chicken embry os and reincubation periods of 2-6 days. Serial sections were evaluate d with antibodies against quail cells, quail hemangiopoietic cells and desmin. With the exception of neural crest cells in the cranial scler otome half, all cells of the operated segment are quail cells derived from a single somite. These cells differentiate into sclerotome, myoto me and the anlage of the dermis of the back. After longer reincubation periods, the somite-derived quail cells form the neighboring halves o f 2 adjacent vertebral bodies and the intervening (disc-homologous) ti ssue. Resegmentation is furthermore visible in the lamina and the spin ous process. Somite cells also form the articular and transverse proce sses, and the intertransverse muscle including its insertion to the ne xt cranial transverse process. One thoracic somite forms the proximal part of 1 rib. In more distal parts, 1 somite forms the cranial half o f 1 rib and the caudal half of the next cranial rib, and the intercost al muscle and part of the connective tissue. Somite-derived quail cell s are found in muscle that bridges over 2 segments cranial and caudal from the operated segment. The craniocaudal distribution of endothelia l cells is approximately the same. Somitocoele cells that are located centrally in the epithelial somite express the sclerotome-markers Pax- 1 and Pax-9. After 2-3 days of reincubation, grafted thoracic somitoco ele cells are found mainly in the cranial part of the caudal sclerotom e half. They form an area representing the anlagen of the intervertebr al disc and the rib. After longer reincubation periods, the grafted qu ail somitocoele cells form the intervertebral disc-homologous tissue a nd the proximal part of the rib. In more distal parts of the rib they are located in the cranial half of 1 rib and the caudal half of the ne xt cranial rib. The somitocoele cells also form the surface of the int ervertebral joint, and give rise to a small number of endothelial cell s that are found up to 1 segment cranial and caudal to the operation s ite. Our studies show that resegmentation is found in most parts of th e vertebra and in the distal ribs. One somite forms the origin and ins ertion of the segmental muscle. Therefore, the somite can be regarded as the ancestor of the vertebral motion segment. Somitocoele cells are located centrally both in the epithelial somite and in the vertebral motion segment.