Background: Somites, direct products of the segmentation of the paraxial mesoderm give rise to the vertebrse. Vertebral organogenesis is submitted to various tissue interactions.

Subjects: Avian (chick and quail) embryos, used individually or in quail-chick-chimera constructions for a cell marking technique (Le Douarin, 1969).

Interventions:

  • Quail-chick chimeras. In 2-day (E2) chick embryos in ovo, one pair of the last segmented somites is microsurgically removed and replaced by the equivalent pair excised from a quail embryo of the same stage. Histological studies, at various stages, using DNA staining or a quail specific antibody (QCPN) allow to distinguish quail and chick cells and to know their derivatives.

  • Unilateral excision. The caudal half and the rostral half, respectively, of two consecutive somites are ablsted at the cervical level of E2 chick embryos. Skeleton preparations are performed at EIO.

Results:

  • Quail cells of one grafted pair of somites participate to the formation of two consecutive vertebras.

  • Unilateral excision of consecutive caudal and rostral somites produces hemivertebrae of different sorts according to the stage of development of the excised somites: free hemivertebrae with intervertebral discs on both sides(rostral and caudal) of the vertebral body after late excision, and fusion with the rostral and caudal consecutive vertebral bodies after early excision.

Conclusion: Our work demonstrates the resegmentation theory for development of vertebrae and suggests that accidental events at precise stages of development can explain different from of vertebral congenital anomalies. Developmental expression of rostro-caudal molecular markers in normal and excised embryos, will inform us about the formation of the formation of the intervertebral discs.