Brown D, Wagner D, Li X, Richardson J A, Olson E N
Department of Molecular Biology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75235-9148, USA.
Development. 1999 Oct;126(19):4317-29. doi: 10.1242/dev.126.19.4317.
Scleraxis is a basic helix-loop-helix (bHLH) transcription factor shown previously to be expressed in developing chondrogenic cell lineages during embryogenesis. To investigate its function in embryonic development, we produced scleraxis-null mice by gene targeting. Homozygous mutant embryos developed normally until the early egg cylinder stage (embryonic day 6.0), when they became growth-arrested and failed to gastrulate. Consistent with this early embryonic phenotype, scleraxis was found to be expressed throughout the embryo at the time of gastrulation before becoming restricted to chondrogenic precursor cells at embryonic day 9.5. At the time of developmental arrest, scleraxis-null embryos consisted of ectodermal and primitive endodermal cell layers, but lacked a primitive streak or recognizable mesoderm. Analysis of molecular markers of the three embryonic germ layers confirmed that scleraxis mutant embryos were unable to form mesoderm. By generating chimeric embryos, using lacZ-marked scleraxis-null and wild-type embryonic stem cells, we examined the ability of mutant cells to contribute to regions of the embryo beyond the time of lethality of homozygous mutants. Scleraxis-null cells were specifically excluded from the sclerotomal compartment of somites, which gives rise to the axial skeleton, and from developing ribs, but were able to contribute to most other regions of the embryo, including mesoderm-derived tissues. These results reveal an essential early role for scleraxis in mesoderm formation, as well as a later role in formation of somite-derived chondrogenic lineages, and suggest that scleraxis target genes mediate these processes.
硬骨素是一种基本的螺旋-环-螺旋（bHLH）转录因子，先前已证明其在胚胎发育过程中在软骨生成细胞谱系中表达。为了研究其在胚胎发育中的功能，我们通过基因靶向产生了硬骨素缺失小鼠。纯合突变胚胎在早期卵柱期（胚胎第6.0天）之前发育正常，之后它们停止生长并无法进行原肠胚形成。与这种早期胚胎表型一致，在原肠胚形成时发现硬骨素在整个胚胎中表达，之后在胚胎第9.5天局限于软骨生成前体细胞。在发育停滞时，硬骨素缺失胚胎由外胚层和原始内胚层细胞层组成，但缺乏原条或可识别的中胚层。对三个胚胎胚层的分子标记物分析证实，硬骨素突变胚胎无法形成中胚层。通过使用lacZ标记的硬骨素缺失和野生型胚胎干细胞生成嵌合胚胎，我们检查了突变细胞在纯合突变体致死时间之后对胚胎区域的贡献能力。硬骨素缺失细胞被特异性地排除在形成轴向骨骼的体节的生骨节隔室和发育中的肋骨之外，但能够对胚胎的大多数其他区域做出贡献，包括中胚层衍生组织。这些结果揭示了硬骨素在中胚层形成中的早期关键作用，以及在体节衍生的软骨生成谱系形成中的后期作用，并表明硬骨素靶基因介导了这些过程。
