Intervertebral discs are derived from embryonic structures called the sclerotome and notochord (Paavola et al. 1980; Theiler 1988; Rufai et al. 1995). The nucleus pulposus, the cushioning core of the mature intervertebral disc, is derived from the notochord, while the annulus fibrosus, which provides the structural properties of the disc, is derived from sclerotome (Christ et al. 2004, 2007; Christ and Scaal 2008). The sclerotome is derived from the somites, transient structures that determine the segmented nature of the embryo. In response to signals from the notochord and floor plate of the neural tube, the maturing somites undergo dorsal-ventral compartmentalization establishing the dermomyotome and sclerotome, the latter forming most of the connective tissues of the future axial skeleton. The development of the sclerotome is characterized by proliferation and expansion of cells as well as the formation of three subcompartments: ventral, lateral, and dorsal. The ventral sclerotome gives rise to the vertebral bodies and annulus fibrosus and is made up of Pax-1-expressing cells that have invaded the perinotochordal space (Monsoro-Burq et al. 1994; Peters et al. 1999).