Resolving heterogeneity in liver development

项目摘要 定形内胚层（DE）在原肠胚形成期间作为上皮片层出现。这个单细胞片层， 产生整个肠管和相关器官，形成模式并分化为器官域， 对邻近中胚层衍生组织提供的诱导信号的反应。在诱导期间，肝脏 肝祖细胞，即成肝细胞，作为一组表达肝标志物的增厚细胞从DE中出现。 新生的成肝细胞形成肝芽，其将侵入邻近的中胚层组织。一旦入侵， 完整，明显可见单个中线喙叶和两个较小的双侧对称尾叶。 虽然成肝细胞和它们的中胚层都被认为是统一的群体，但我们认为， 发现了两者的显著异质性首先，肝母细胞起源于两个空间上不同的DE 每个种群都对肝芽有独特的贡献。腹侧中线祖细胞主要贡献 向位于前肝芽的成肝细胞，而两侧对称的侧向祖细胞 产生肝母细胞，形成后肝芽。令人惊讶的是，每个祖先都有独特的 这些细胞是该过程的必要条件，并且在诱导时各自与不同的支持性间充质相关。 例如，需要FGF信号来诱导静脉窦结合的前成肝细胞，而 横隔间充质结合的后肝母细胞需要诱导BMP信号传导 此外，我们小组最近的命运作图表明，前肝母细胞主要是 后肝母细胞主要贡献于喙叶。最后 未发表的组织学和分子数据表明，支持吻侧和尾侧的间充质 叶是不同的组织。这些观察结果表明，正常的发展遵循两条路线， 产生成肝细胞。我们推测在早期肝脏发育过程中产生的异质性 是正常肝功能所必需的，它有助于产生最近在 成年肝细胞和胆管细胞群。在概述的新发展框架的指导下， 以上，我们的建议的目的是利用我们在胚胎学方面的专业知识和小鼠遗传学的力量， 描述这种异质性，并确定它如何有助于正常发展。在目标1中，我们建议 使用条件性敲除策略来评估FGF和BMP信号如何促进成肝细胞诱导， vivo.在Aim2中，我们使用离体方法和新型小鼠体内报告基因来揭示发育过程中的基因表达。 的起源和成人的贡献，喙叶和尾叶间充质，以及确定分子 前叶和尾叶成肝细胞的特征。长期目标是了解 发育异质性有助于关键的成人反应，如稳态和再生， 利用这些发展的见解在体外产生功能性肝组织。

Project Summary The definitive endoderm (DE) emerges during gastrulation as an epithelial sheet. This single cell sheet, which produces the entire gut tube and associated organs, is patterned and differentiates into organ domains in response to inductive cues provided by adjacent mesoderm-derived tissues. During induction, the hepatic progenitor, the hepatoblast, emerges from the DE as a group of thickened cells that express hepatic markers. The nascent hepatoblasts form a liver bud that will invade adjacent mesodermal tissue. Once invasion is complete, a single midline rostral lobe and two smaller bilaterally symmetric caudal lobes are evident. Although hepatoblasts and their associated mesoderm have each been regarded as uniform populations, we have discovered remarkable heterogeneity in both. Firstly, hepatoblast arise from two spatially distinct DE populations that each contributes uniquely to the liver bud. The ventral midline progenitors contribute mainly to the hepatoblasts that populate the anterior liver bud while the bilaterally symmetric lateral progenitors produce hepatoblasts that contribute to the posterior liver bud. Surprisingly, each progenitor has unique requirements for this process and is each associated with different supportive mesenchyme upon induction. For example, FGF signals are required to induce the sinus venosus bounded anterior hepatoblasts, while the septum transversum mesenchyme bounded posterior hepatoblasts require BMP signaling to be induced Furthermore, recent fate mapping from our group demonstrates that the anterior hepatoblasts mainly contribute to the caudal lobes while posterior hepatoblasts mainly contribute to the rostral lobe. Finally unpublished histological and molecular data suggests that the mesenchyme supporting the rostral and caudal lobes are distinct tissues. Together these observations suggest that normal development follows two routes for generating hepatoblasts. We hypothesize that the heterogeneity generated during early hepatic development is essential for normal liver function and that it aids in generating the heterogeneity recently identified in adult hepatocyte and cholangiocyte populations. Guided by the new developmental framework outlined above, the aims of our proposal use our expertise in embryology and the power of mouse genetics to further delineate this heterogeneity and to identify how it contributes to normal development. In Aim1 we propose to use conditional knock-out strategies to assess how FGF and BMP signals contribute to hepatoblast induction in vivo. In Aim2, we use an ex vivo approach and novel mouse reporters in vivo to uncover the developmental origin and adult contribution of the rostral and caudal lobe mesenchyme as well as determine the molecular signature of the rostral and caudal lobe hepatoblasts. The long-term goals are to understand how developmental heterogeneity contributes to key adult responses such as homeostasis and regeneration and to use these developmental insights to produce functional hepatic tissues in vitro.