3 Patterning and Differentiation of the Vertebrate Spine

Abstract

One of the most striking features of the human spine is its periodic organization. This so-called “segmental” arrangement of the vertebrae along the anteroposterior body axis is established during embryonic development. Structures called somites, which contain the precursors of the vertebrae, form in a rhythmic fashion at the posterior end of the embryo during the process of somitogenesis. Somites are sequentially added to the growing axis, thus establishing the characteristic periodic pattern of the future vertebral column. The primary segmentation of the vertebrate embryo displayed by somitic organization also underlies much of the segmental organization of the body, including muscles, nerves, and blood vessels. In amniotes, somites are the major component of the paraxial mesoderm that form bilaterally along the nerve cord as a result of primitive streak and tail bud regression during body axis formation. Somites bud off from the anterior presomitic mesoderm (PSM) as epithelial spheres surrounding a core of mesenchymal cells called the somitocoele. The dorsal portion of the somite remains epithelial and forms the dermomyotome, which differentiates into muscle and dermis while its ventral moiety undergoes an epithelio-mesenchymal transition, leading to the formation of the sclerotome. The sclerotome gives rise to the skeletal elements of the vertebral column: the vertebrae, ribs, intervertebral disks, and tendons. Most of our understanding of amniote somitogenesis at the morphogenetic and molecular levels results from studies involving the chicken (Gallus gallus) and the mouse (Mus musculus). In this chapter, we essentially focus on the patterning and development of the spine in...