Iimura Tadahiro, Denans Nicolas, Pourquié Olivier
Tokyo Medical and Dental University, Tokyo, Japan.
Curr Top Dev Biol. 2009;88:201-34. doi: 10.1016/S0070-2153(09)88007-1.
The vertebrate spine exhibits two striking characteristics. The first one is the periodic arrangement of its elements-the vertebrae-along the anteroposterior axis. This segmented organization is the result of somitogenesis, which takes place during organogenesis. The segmentation machinery involves a molecular oscillator-the segmentation clock-which delivers a periodic signal controlling somite production. During embryonic axis elongation, this signal is displaced posteriorly by a system of traveling signaling gradients-the wavefront-which depends on the Wnt, FGF, and retinoic acid pathways. The other characteristic feature of the spine is the subdivision of groups of vertebrae into anatomical domains, such as the cervical, thoracic, lumbar, sacral, and caudal regions. This axial regionalization is controlled by a set of transcription factors called Hox genes. Hox genes exhibit nested expression domains in the somites which reflect their linear arrangement along the chromosomes-a property termed colinearity. The colinear disposition of Hox genes expression domains provides a blueprint for the regionalization of the future vertebral territories of the spine. In amniotes, Hox genes are activated in the somite precursors of the epiblast in a temporal colinear sequence and they were proposed to control their progressive ingression into the nascent paraxial mesoderm. Consequently, the positioning of the expression domains of Hox genes along the anteroposterior axis is largely controlled by the timing of Hox activation during gastrulation. Positioning of the somitic Hox domains is subsequently refined through a crosstalk with the segmentation machinery in the presomitic mesoderm. In this review, we focus on our current understanding of the embryonic mechanisms that establish vertebral identities during vertebrate development.
脊椎动物的脊柱呈现出两个显著特征。第一个特征是其组成部分——椎骨——沿前后轴的周期性排列。这种分段组织是体节发生的结果，体节发生在器官发生过程中。分段机制涉及一个分子振荡器——分段时钟，它传递一个控制体节产生的周期性信号。在胚胎轴伸长过程中，这个信号通过一个移动信号梯度系统——波前——向后移动，波前依赖于Wnt、FGF和视黄酸信号通路。脊柱的另一个特征是椎骨组细分为解剖学区域，如颈、胸、腰、骶和尾区。这种轴向区域化由一组称为Hox基因的转录因子控制。Hox基因在体节中呈现嵌套的表达域，这反映了它们沿染色体的线性排列——这一特性称为共线性。Hox基因表达域的共线性排列为脊柱未来椎骨区域的区域化提供了蓝图。在羊膜动物中，Hox基因在胚泡的体节前体细胞中按时间共线性序列被激活，有人提出它们控制着这些细胞逐渐进入新生的近轴中胚层。因此，Hox基因表达域沿前后轴的定位在很大程度上由原肠胚形成过程中Hox激活的时间控制。体节Hox域的定位随后通过与前体节中胚层的分段机制的相互作用而得到细化。在这篇综述中，我们重点关注目前对脊椎动物发育过程中建立椎骨特征的胚胎机制的理解。
