Developmental and Cell Biology, University of California, Irvine, Irvine, CA 92697-2300, USA.
Integr Comp Biol. 2008 Nov;48(5):658-67. doi: 10.1093/icb/icn081. Epub 2008 Aug 5.
Segmentation of the vertebrate head emerges out of earlier processes that establish the anterior-posterior (A-P) axis. Recent genetic studies and comparisons across species have led to a better understanding of the links between A-P patterning and segmentation. These point to similar signals acting on both head and trunk, such as retinoic acid and fibroblast growth factors. These form interacting networks of diffusible morphogen gradients that pattern both hindbrain rhombomeres and mesodermal somites. New computational models, particularly for retinoic acid, have revealed how morphogen gradients are established and made robust to changes in signaling levels. However, the orientations of these gradients, as well as how they interact to generate segments, differ remarkably between germ layers and body regions. Thus, the vertebrate head is, in part, built through modifications of the same processes that link A-P patterning and segmentation in the trunk, but fundamental differences in how these processes are deployed lend further doubt to the notion that head and trunk segments are homologous.
脊椎动物头部的分割是由更早的过程产生的，这些过程建立了前后（A-P）轴。最近的遗传研究和跨物种比较，使我们更好地理解了 A-P 模式和分割之间的联系。这些研究表明，类似的信号作用于头部和躯干，如视黄酸和纤维母细胞生长因子。这些信号形成了相互作用的扩散形态发生梯度网络，模式化后脑菱形和中胚层体节。新的计算模型，特别是对于视黄酸，揭示了形态发生梯度是如何建立的，以及如何使其对信号水平的变化具有鲁棒性。然而，这些梯度的方向，以及它们如何相互作用产生片段，在胚层和身体区域之间有显著的差异。因此，脊椎动物的头部部分是通过对连接躯干 A-P 模式和分割的相同过程进行修改而构建的，但是这些过程的部署方式存在根本差异，这进一步怀疑了头部和躯干片段是同源的观点。
