![]() Lineage-tracing approaches have localized the PSM precursor cells (P-PSM) in the early chick embryo: bilateral to the midline in the epiblast (3HH), in the PS anterior region at stage 4HH, and later in the tail-bud (Figure 1). From 16 to 20 somite stages, new mesodermal cells contributing to more caudal fates arise from the tail-bud, a mass of highly packed undifferentiated cells which corresponds to a functional remnant of the primitive streak. ![]() In the chick embryo, PS regression is completed around the 16-somite stage. As a consequence, avian and mammalian embryos display a clear A-P gradient of developmental maturity: as cell ingression occurs, the HN regresses to a more posterior position, laying down the axial and anterior structures while gastrulation is still taking place at the embryo tail. Cells migrating through the PS undergo an epithelial-to-mesenchymal transition and become organized in a head-to-tail fashion: the earliest cells to ingress will be positioned more anteriorly than cells migrating later in development. In the chick embryo, the fully extended PS corresponds to the developmental stage 4 of Hamburger and Hamilton (HH), where Hensen’s node (HN), which constitutes the embryonic organizer, can be detected as a cellular thickening at the PS tip (Figure 1). As epiblast cells ingress and adopt distinct fates, the PS elongates towards the future anterior region and the body axes are defined. Distinct models have been proposed to explain the specific cellular mechanism underlying PS formation (reviewed in ). Gastrulation begins with the formation of the primitive streak (PS), first identified as a posterior thickening of the epiblast. PSM is formed during gastrulation, in which extensive cellular rearrangements take place to form the three embryonic germinative layers: ectoderm, mesoderm, and endoderm. Somites are blocks of cells formed from the anterior end of the mesenchymal presomitic mesoderm (PSM) and have a key role in the subsequent patterning of the body giving rise to all segmented structures in the adult body, such as vertebrae, intervertebral disks and ribs, the dermis of the back, and body skeletal muscles, except those of the head. ![]() Early Events in Vertebrate Developmentīody segmentation can be detected early in development through the formation of repeated segments, the somites, along the anterior-posterior (A-P) body axis. Thus, a better comprehension of the molecular mechanisms regulating somite formation is required in order to fully understand the origin of human skeletal malformations. Human congenital vertebral malformations have been associated with perturbations in these oscillatory mechanisms. ![]() Herein, we provide an overview of the molecular clock operating during somite formation and its underlying molecular regulatory mechanisms. The sequential formation of the segmented precursors of the vertebral column during embryonic development, the somites, is governed by an oscillating genetic network, the somitogenesis molecular clock. All vertebrate species present a segmented body, easily observed in the vertebrate column and its associated components, which provides a high degree of motility to the adult body and efficient protection of the internal organs. ![]()
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