Differential tissue growth and cell adhesion alone drive early tooth morphogenesis: An ex vivo and in silico study

From gastrulation to late organogenesis animal development involves many genetic and bio-mechanical interactions between epithelial and mesenchymal tissues. Ectodermal organs, such as hairs, feathers and teeth are well studied examples of organs whose development is based on epithelial-mesenchymal interactions. These develop from a similar primordium through an epithelial folding and its interaction with the mesenchyme. Despite extensive knowledge on the molecular pathways involved, little is known about the role of bio-mechanical processes in the morphogenesis of these organs. We propose a simple computational model for the biomechanics of one such organ, the tooth, and contrast its predictions against cell-tracking experiments, mechanical relaxation experiments and the observed tooth shape changes over developmental time. We found that two biomechanical processes, differential tissue growth and differential cell adhesion, were enough, in the model, for the development of the 3D morphology of the early tooth germ. This was largely determined by the length and direction of growth of the cervical loops, lateral folds of the enamel epithelium. The formation of these cervical loops was found to require accelerated epithelial growth relative to other tissues and their direction of growth depended on specific differential adhesion between the three tooth tissues. These two processes and geometrical constraints in early tooth bud also explained the shape asymmetry between the lateral cervical loops and those forming in the anterior and posterior of the tooth. By performing mechanical perturbations ex vivo and in silico we inferred the distribution and direction of tensile stresses in the mesenchyme that restricted cervical loop lateral growth and forced them to grow downwards. Overall our study suggests detailed quantitative explanations for how bio-mechanical processes lead to specific morphological 3D changes over developmental time.从原肠作用（gastrulation）到器官发生后期，动物发育过程涉及上皮组织与间充质组织之间大量的遗传与生物力学（bio-mechanical）相互作用。外胚层器官（ectodermal organs），如毛发、羽毛与牙齿，是一类发育依赖上皮-间充质相互作用（epithelial-mesenchymal interactions）的经典研究模型，其发育均始于相似的原基（primordium），通过上皮褶皱形成并与间充质组织发生相互作用完成形态建成。尽管学界对其涉及的分子通路已有较为深入的认知，但对于生物力学过程在这类器官形态发生中的作用仍知之甚少。我们针对其中一类器官——牙齿的生物力学机制，构建了一个简易的计算模型，并将模型预测结果与细胞追踪实验、力学松弛实验以及发育过程中观测到的牙齿形态变化进行比对验证。研究发现，在该模型中，仅需两种生物力学过程——差异性组织生长与差异性细胞黏附，即可复现早期牙蕾（tooth germ）的三维形态发育过程。该三维形态的形成主要由牙釉质上皮（enamel epithelium）的侧向褶皱——颈环（cervical loops）的生长长度与方向决定。研究表明，颈环的形成需要相对于其他组织更快的上皮生长速率，而其生长方向则取决于三种牙齿组织间特异性的差异性细胞黏附作用。这两种生物力学过程以及早期牙蕾的几何约束条件，同样解释了侧向颈环与牙齿前后侧形成的颈环之间的形态不对称性。通过开展离体（ex vivo）与计算机模拟（in silico）条件下的力学扰动实验，我们推断出间充质组织内张应力的分布与方向——该应力可限制颈环的侧向生长并迫使其向下延伸。综上，本研究为生物力学过程如何在发育过程中驱动特定的三维形态变化提供了精细化的定量解释。