Epithelial Sheet Dynamics during Primitive Streak Formation as Active Matter

研究包括人类在内的高等动物发育的一个重要目标是了解原肠胚形成。原肠胚形成是早期胚胎发育的关键阶段，在此阶段胚胎的主体计划被确定，主体轴出现。它涉及大范围的、长距离的细胞运动，在此过程中，三个组织层，外胚层、中胚层和内胚层的细胞在胚胎中占据正确的位置。内胚层位于胚胎和成体的最里面，包裹着消化道和相关腺体。它被产生肌肉和骨骼的中胚层所包围，而中胚层又被最外层外胚层所覆盖，外胚层将形成表皮和神经系统。原肠形成过程中细胞运动的缺陷导致严重的死亡，在不太严重的情况下，形成许多出生缺陷的基础。高等脊椎动物（如人类）原肠胚形成的细胞过程和化学信号在所谓的模型系统中进行了实验研究，特别是鸡和老鼠的胚胎。鸡胚的优点是发育发生在母体外，因此很容易在实验中获得。它也是平坦和半透明的，这有助于观察细胞在原肠形成过程中的运动。鸡胚胎的原肠胚形成与人类的原肠胚形成非常相似，这意味着研究结果可以外推到人类的发育过程中。在非常早期的发育阶段，鸡胚由位于蛋黄顶部的两个同心圆盘组织组成；内部的一层细胞厚的环将形成胚胎。外胚层圆盘一侧镰状区域内的细胞分化为中胚层和内胚层。在原肠胚形成过程中，中胚层细胞的镰状区域变形为从胚胎的一端延伸到中央中线的条状组织；这种结构被称为原始条纹。原始条纹的中心细胞随后向内和向外移动，形成胚胎的内中胚层和内胚层。在这个项目中，我们将使用两种互补的方法来研究鸡胚的原肠胚形成。首先，我们使用实验在细胞水平上详细跟踪发育胚胎中机械和化学细胞间信号传导。为了做到这一点，我们开发并制造了一种新型的显微镜，一种光片荧光显微镜，它使我们能够看到一种特殊的鸡品种胚胎中的几乎所有细胞（50,000-200,000），其中所有细胞的细胞膜都标记着绿色荧光蛋白。我们研究不同的细胞行为，如分裂、形状变化和运动是如何协调产生这些组织的，以及哪些化学和机械细胞-细胞信号机制控制它们。其次，我们利用集体运动物理学的概念建立了一个基于活跃的、相互作用的细胞的计算模型，并用它来理解局部和整个胚胎尺度上的细胞流动。我们对细胞-细胞信号传导、细胞分化、增殖和迁移之间相互作用的研究不仅对研究胚胎发生的研究人员很重要，而且对研究伤口愈合、组织修复和再生等过程的科学家也很重要。此外，为了推进所提出的研究，我们将开发几种新的数学和计算技术，这些技术有望对其他生物和生物医学/工程问题的进一步数学研究具有重要价值。

An important goal of the study of development of higher animals including humans is understanding gastrulation. Gastrulation is a critical stage in early embryonic development where the main body plan of the embryo is laid down and the main body axes emerge. It involves large-scale, long-range cell movements during which cells of the three tissue layers, the ectoderm, the mesoderm and endoderm take up their correct positions in the embryo. The endoderm is located innermost in the embryo and adult, lining the digestive tract and associated glands. It is surrounded by the mesoderm that will give rise to the muscles and the skeleton, which is in turn covered by the outmost layer, the ectoderm, which will form the epidermis and the nervous system. Defects in cell movements during gastrulation result in severe cases in death and in less severe cases form the basis of many birth defects.The cellular processes and chemical signalling underlying gastrulation in higher vertebrates (such as humans) are experimentally studied in so-called model systems, especially chick and mouse embryos. The chick embryo has the advantage that development takes place outside the mother and is therefore easily experimentally accessible. It is also flat and translucent which helps observation of cell movement during gastrulation. Gastrulation in chick embryos greatly resembles gastrulation in humans, which means that findings can be extrapolated to human development. During very early stages development the chick embryo consists of two concentric disks of tissue sitting on top of the yolk; the inner, one cell layer thick, ring will form the embryo proper. Cells in a sickle shaped domain on one side of this epiblast disc will differentiate into the mesoderm and endoderm. During gastrulation this sickle shaped domain of mesendoderm cells deforms into a stripe of tissue extending from one edge of the embryo through the central midline; the structure is known as the primitive streak. The central cells of the primitive streak then move inwards and away from this site of ingression to form the inner mesodermal and endodermal layers of the embryo.In this project we will study gastrulation in the chick embryo using two complementary approaches. First, we use experiments to follow the mechanical and chemical cell-to-cell signalling in the developing embryo at a cell-level detail. In order to do so we have developed and built a novel type of microscope, a light-sheet fluorescence microscope, that allows us to see almost all the cells in the embryo (50,000-200,000) in a special chick strain in which the cell membranes of all cells are marked with a green fluorescent protein. We study how different cell behaviours such as division, shape changes and motion are coordinated to generate these tissues and which chemical and mechanical cell-cell signalling mechanisms control them. Second, we build a computational model based on active, interacting cells using concepts from the physics of collective motion and use it to understand cell flow both at the local and the full embryo scale. Our study of the interplay between cell-cell signalling, cell differentiation, proliferation and migration is not only important to the community of researchers whose interest is focused on embryogenesis but will also be of great importance to scientists whose research is centred on processes such as wound healing, tissue repair and regeneration. Furthermore, in order to progress with the proposed research we will develop several new mathematical and computational techniques which are expected to be of great value for further mathematical investigation of other biological and biomedical/engineering problems.