An in vitro model of region-specific rib formation in chick axial skeleton: Intercellular interaction between somite and lateral plate cells.

The axial skeleton is divided into different regions based on its morphological features. In particular, in birds and mammals, ribs are present only in the thoracic region. The axial skeleton is derived from a series of somites. In the thoracic region of the axial skeleton, descendants of somites coherently penetrate into the somatic mesoderm to form ribs. In regions other than the thoracic, descendants of somites do not penetrate the somatic lateral plate mesoderm. We performed live-cell time-lapse imaging to investigate the difference in the migration of a somite cell after contact with the somatic lateral plate mesoderm obtained from different regions of anterior-posterior axis in vitro on cytophilic narrow paths. We found that a thoracic somite cell continues to migrate after contact with the thoracic somatic lateral plate mesoderm, whereas it ceases migration after contact with the lumbar somatic lateral plate mesoderm. This suggests that cell-cell interaction works as an important guidance cue that regulates migration of somite cells. We surmise that the thoracic somatic lateral plate mesoderm exhibits region-specific competence to allow penetration of somite cells, whereas the lumbosacral somatic lateral plate mesoderm repels somite cells by contact inhibition of locomotion. The differences in the behavior of the somatic lateral plate mesoderm toward somite cells may confirm the distinction between different regions of the axial skeleton.