Cebra-Thomas Judith, Tan Fraser, Sistla Seeta, Estes Eileen, Bender Gunes, Kim Christine, Riccio Paul, Gilbert Scott F
Department of Biology, Swarthmore College, Swarthmore, Pennsylvania 19081, USA.
J Exp Zool B Mol Dev Evol. 2005 Nov 15;304(6):558-69. doi: 10.1002/jez.b.21059.
We propose a two-step model for the evolutionary origin of the turtle shell. We show here that the carapacial ridge (CR) is critical for the entry of the ribs into the dorsal dermis. Moreover, we demonstrate that the maintenance of the CR and its ability to attract the migrating rib precursor cells depend upon fibroblast growth factor (FGF) signaling. Inhibitors of FGF allow the CR to degenerate, with the consequent migration of ribs along the ventral body wall. Beads containing FGF10 can rearrange rib migration in the chick, suggesting that the CR FGF10 plays an important role in attracting the rib rudiments. The co-ordinated growth of the carapacial plate and the ribs may be a positive feedback loop (similar to that of the limbs) caused by the induction of Fgf8 in the distal tips of the ribs by the FGF10-secreting mesenchyme of the CR. Once in the dermis, the ribs undergo endochrondral ossification. We provide evidence that the ribs act as signaling centers for the dermal ossification and that this ossification is due to bone morphogenetic proteins secreted by the rib. Thus, once the ribs are within the dermis, the ossification of the dermis is not difficult to achieve. This relatively rapid means of carapace formation would allow for the appearance of turtles in the fossil record without obvious intermediates.
我们提出了一个关于龟壳进化起源的两步模型。我们在此表明，背甲嵴（CR）对于肋骨进入背部真皮至关重要。此外，我们证明CR的维持及其吸引迁移的肋骨前体细胞的能力取决于成纤维细胞生长因子（FGF）信号传导。FGF抑制剂会使CR退化，导致肋骨沿腹侧体壁迁移。含有FGF10的珠子可以重新排列鸡胚中的肋骨迁移，这表明CR中的FGF10在吸引肋骨原基方面起着重要作用。背甲板和肋骨的协调生长可能是一个正反馈回路（类似于肢体的情况），由CR分泌FGF10的间充质在肋骨远端诱导Fgf8所引起。一旦进入真皮，肋骨就会经历软骨内成骨。我们提供的证据表明，肋骨作为真皮骨化的信号中心，并且这种骨化是由肋骨分泌的骨形态发生蛋白引起的。因此，一旦肋骨进入真皮，真皮的骨化就不难实现。这种相对快速的龟壳形成方式将使得龟类在化石记录中出现时没有明显的中间过渡形态。
