Grainger R M, Mannion J E, Cook T L, Zygar C A
Department of Biology, University of Virginia, Charlottesville 22903, USA.
Dev Genet. 1997;20(3):246-57. doi: 10.1002/(SICI)1520-6408(1997)20:3<246::AID-DVG7>3.0.CO;2-7.
Cell determination in vertebrates requires integration of many events, although the mechanisms controlling the different stages in this process are poorly understood. While studies of lens determination have helped define some of these stages, we know very little about the intermediate steps involved in the commitment of ectoderm to lens formation. Lens determination begins during gastrulation when ectoderm is briefly competent to respond to lens-inducing signals and progresses to a point, at the neural tube stage, when the presumptive lens ectoderm is specified. Between these two stages important regulatory genes are activated in the presumptive lens ectoderm, including the transcription factor Pax-6, and transplantation experiments presented here suggest that the presumptive lens ectoderm is becoming "biased" toward lens formation. We show that competent ectoderm from Xenopus laevis embryos forms well-differentiated lenses in most cases when transplanted to the presumptive lens area of neural plate stage hosts, where the lens-inductive environment is strong. When placed into later, neural tube stage hosts, optimally competent ectoderm does form small lenses in about half the cases, but the overall response is much weaker. Even in this weakly inducing environment, however, lens ectoderm that is part way through the inductive process (at the neural plate stage) is shown to have a lens-forming bias, since it forms well differentiated lenses in nearly all cases. While we find that ectoderm surrounding the lens-forming area at neural plate stages does not have a lens-forming bias, non-lens head ectoderm at the neural tube stage does, suggesting that a large region of head ectoderm is biased during neurulation. Using Rana palustris embryos, a species used in the earliest lens induction studies, we were also able to show that the optic vesicle can induce lenses in non-lens head ectoderm at neural tube stages. These results lead us to refine the definition of lens cell determination and provide a context that should allow clarification of determination mechanisms.
脊椎动物中的细胞决定需要整合许多事件，尽管控制这一过程中不同阶段的机制还知之甚少。虽然对晶状体决定的研究有助于定义其中一些阶段，但我们对外胚层向晶状体形成定向分化所涉及的中间步骤了解甚少。晶状体决定始于原肠胚形成期，此时外胚层短暂地能够对晶状体诱导信号作出反应，并发展到神经管阶段，此时预定晶状体外胚层被确定。在这两个阶段之间，预定晶状体外胚层中重要的调控基因被激活，包括转录因子Pax-6，此处进行的移植实验表明预定晶状体外胚层正朝着晶状体形成“偏向”。我们发现，非洲爪蟾胚胎的感受态外胚层在大多数情况下移植到神经管阶段宿主的预定晶状体区域时，能形成分化良好的晶状体，该区域的晶状体诱导环境很强。当置于稍后的神经管阶段宿主中时，最佳感受态外胚层在大约一半的情况下确实能形成小晶状体，但总体反应要弱得多。然而，即使在这种弱诱导环境中，处于诱导过程中途（神经管阶段）的晶状体外胚层也显示出有形成晶状体的偏向，因为它在几乎所有情况下都能形成分化良好的晶状体。虽然我们发现神经管阶段围绕晶状体形成区域的外胚层没有形成晶状体的偏向，但神经管阶段的非晶状体头部外胚层有，这表明头部外胚层的一大区域在神经胚形成过程中是有偏向的。使用最早用于晶状体诱导研究的物种北美锄足蟾胚胎，我们还能够证明视泡在神经管阶段可诱导非晶状体头部外胚层形成晶状体。这些结果使我们完善了晶状体细胞决定的定义，并提供了一个有助于阐明决定机制的背景。
