Fig. 1. Lack of nuclei pulposi in Sox5^–/–/Sox6^–/– fetuses. (A) Histological analysis of mid-sagittal sections of the thoracic vertebral column of E15.5 control and Sox5^–/–/Sox6^–/– fetuses. In the control (top), intervertebral discs (arrows) feature a nucleus pulposus (NP), a cartilaginous inner annulus (IA) and a mesenchymal outer annulus (OA). Vertebral bodies (VB) exhibit a core of hypertrophic chondrocytes, surrounded with chondroblasts juxtaposed to inner annulus chondroblasts. In the mutant (bottom), the segmentation of the vertebral column is only visible in outer annuli. Nuclei pulposi are lacking. The cartilage matrix of inner annuli and vertebral bodies is deficient (pale Alcian Blue staining), and chondroblasts are all small and round. Groups of condensed cells (arrowheads), remnants of the notochord, are seen in vertebral body regions, but not in intervertebral regions. (B) Histology of cross sections of E15.5 control and Sox5^–/–/Sox6^–/– fetuses in the upper lumbar vertebral column. In control intervertebral regions (top), the nucleus pulposus is surrounded with inner and outer annulus. Notochord cells are no longer seen in vertebral bodies or between intervertebral and vertebral regions (arrowheads). The mutant intervertebral and vertebral regions (bottom) show no nucleus pulposus or notochord cells. Residual notochord cells form trains between the center of cartilages and the outer mesenchyme between vertebral and intervertebral regions (arrow).