RUI: Paracrine Factors and Bone Development in the Turtle Shell

[00:79 . 34]吉尔伯特龟壳是一种进化上的新奇事物，代表着与脊椎动物骨骼通常发育方式的独特不同。龟壳是一种几乎完全由骨头制成的外壳，它由两个主要部分组成，背甲壳和腹板。在它们之间的两侧，有一座桥。甲壳包含50块骨头，由肋骨支撑，这些肋骨是背外侧而不是腹侧生长的。海龟发育和进化的关键事件被认为是肋骨被甲壳脊“诱捕”，甲壳脊是外胚层和中胚层的凸起，类似于肢芽。壳的骨头被认为是由真皮层通过膜内骨化形成的，并有这些肋骨的贡献。吉尔伯特博士希望对龟壳发育的三个假设进行检验：(1)甲壳脊的形成是由形成肢芽的发育程序共同决定的；(2)肋诱导真皮膜内骨化；(3)肋软骨前体的背侧生长取决于它们与腕嵴的接触。为了验证第一个假设，他将在甲壳嵴中检查已知启动肢体形态发生的基因（特别是编码成纤维细胞生长因子的基因）的时空表达谱。为了测试肋骨是否会诱发膜内骨化，他将观察那些编码旁分泌因子（尤其是骨形态发生蛋白）的基因在肋骨和其他区域的表达，这些基因可以诱导真皮层骨化。他还将插入含有已知骨诱导旁分泌因子抑制剂的小珠，以观察这些蛋白质的抑制是否会抑制局部骨发育。从龟体移植到鸡胚胎的龟肋的行为将告诉我们龟肋是否需要甲壳脊来背侧生长，或者这是否是肋骨软骨前体的固有特性。这项研究的结果应该能让我们深入了解发育指令的共同选择可以用来创造进化上新颖的结构的机制。它还有助于重建导致三叠纪海龟“突然”出现的系统发育路径。

0079341GilbertThe turtle shell is an evolutionary novelty representing a unique departure from the usual way that vertebrate skeletons develop. The turtle shell is a casing made almost exclusively of bone, and it is composed of two main parts, the dorsal carapace and the ventral plastron. Between them, on the lateral sides, is a bridge. The carapace contains fifty bones and is supported by the ribs, which grow dorsolaterally rather than ventrally. The critical event in turtle development and evolution is thought to be the "ensnarement" of the ribs by the carapacial ridge, a bulge of ectoderm and mesoderm that resembles the limb bud. The bones of the shell are thought to be formed from the dermis by intramembranous ossification, with contributions from these ribs. Dr. Gilbert wish to test three hypotheses for the turtle shell development: (1) that the carapace ridge forms by the co-option of the developmental program that forms the limb buds; (2) that the ribs induce intramembranous ossification in the dermis; and (3) that the dorsal growth of the rib cartilage precursors depends on their encountering the carpacial ridge. To test the first hypothesis, He will examine in the carapacial ridge the spatiotemporal expression profile of genes (especially those encoding fibroblast growth factors) known to initiate limb morphogenesis. To test whether the ribs induce intramembranous ossification, He will look at the expression, in the ribs and in other regions, of those genes encoding paracrine factors (especially the bone morphogenetic proteins) that could induce the ossification of the dermis. He will also insert beads containing inhibitors of known bone-inducing paracrine factors to see if the inhibition of these proteins inhibits regional bone development. The behavior of turtle ribs from turtle somites transplanted into chick embryos will tell us if the turtle ribs need the carapacial ridge to grow dorsolaterally or if this is an intrinsic property of the rib cartilage precursors. The results of this research should give insights into the mechanisms by which the co-option of developmental instructions can be used to create evolutionarily novel structures. It should also aid in the reconstruction of the phylogenetic path leading to the "sudden" emergence of turtles in the Triassic.