The development of the avian vertebral column

Segmentation of the paraxial mesoderm leads to somite formation. The underl ying molecular mechanisms involve the oscillation of "clock-genes" like c-h airy-1 and lunatic fringe indicative of an implication of the Notch signali ng pathway. The cranio-caudal polarity of each segment is already establish ed in the cranial part of the segmental plate and accompanied by the expres sion of genes like Delta1, Mesp1, Mesp2, Uncx-1, and EphA4 which are restri cted to one half of the prospective somite. Dorsoventral compartmentalizati on of somites leads to the development of the dermomyotome and the scleroto me, the latter forming as a consequence of an epithelio-to-mesenchymal tran sition of the ventral part of the somite. The sclerotome cells express Pax- 1 and Pax-9, which are induced by notochordal signals mediated by sonic hed gehog (Shh) and noggin. The craniocaudal somite compartmentalization that b ecomes visible in the sclerotomes is the prerequisite for the segmental pat tern of the peripheral nervous system and the formation of the vertebrae an d ribs, whose boundaries are shifted half a segment compared to the sclerot ome boundaries. Sclerotome development is characterized by the formation of three subcompartments giving rise to different parts of the axial skeleton and ribs. The lateral sclerotome gives rise to the laminae and pedicles of the neural arches and to the ribs. Its development depends on signals from the notochord and the myotome. The ventral sclerotome giving rise to the v ertebral bodies and intervertebral discs is made up of Pax-1 expressing cel ls that have invaded the perinotochordal space. The dorsal sclerotome is fo rmed by cells that migrate from the dorso-medial angle of the sclerotome in to the space between the roof plate of the neural tube and the dermis. Thes e cells express the genes Msx1 and Msx2, which are induced by BMP-4 secrete d from the roof plate, and they later form the dorsal part of the neural ar ch and the spinous process. The formation of the ventral and dorsal sclerot ome requires directed migration of sclerotome cells. The regionalization of the paraxial mesoderm occurs by a combination of functionally Hox genes, t he Hox code, and determines the segment identity. The development of the ve rtebral column is a consequence of a segment-specific balance between proli feration, apoptosis and differentiation of cells.