The role of extracellular mechanics in skin tissue engineering and ageing

皮肤包含生物力学/化学和拓扑学上不同的壁龛，适合其常驻细胞。表皮细胞位于坚硬的(MPA)IV/VI型胶原和富含层粘连蛋白的细胞外基质(ECM)上，称为基底膜(BM)，而真皮成纤维细胞则位于相当柔软(0.1-10kPa)的III/I型胶原和基于纤维蛋白/弹性蛋白的基质上。时序老化对皮肤生物力学有深远的影响。由于细胞外基质合成减少，I型胶原/弹性蛋白断裂和重塑，皮肤逐渐失去其自然弹性，变得皱纹和脆弱。随着年龄的增长，真皮中的III型胶原比例增加，而BM则表现出明显的III型胶原丢失和脆性。这些变化是如何影响机械感觉和老化皮肤细胞生理学的，目前还知之甚少。然而，众所周知，衰老会导致细胞僵硬，这会影响细胞的收缩和流动性。然而，在组织工程中，地形和时间上的皮肤组织力学变异性在很大程度上被忽略了。标准的细胞培养程序包括在非生理硬质(GPA)表面(即聚苯乙烯)上繁殖。然而，主要的细胞功能，如身份、增殖、信号、行为和结构，是由细胞外环境的刚性决定的。因此，目前体外设计的皮肤替代品并不能准确地代表天然皮肤也就不足为奇了。具体地说，塑料培养皿上的生长异常地激活了真皮成纤维细胞(例如纤维化)，诱导了压力信号通路，改变了细胞结构，同时大大限制了细胞的增殖。因此，我们的假设是，利用仿生2D表面和研究支持角质形成细胞和成纤维细胞机械传感的分子机制将有助于开发合适的全厚度3D青年/老年皮肤模型。开发这样的模型将有利于个人保健和化妆品行业(靶标验证、技术识别)，也是研究皮肤体外老化机制的关键工具。

Skin contains bio-mechanically/-chemically and topologically distinct niches for its resident cells. Epidermal cells rest on a stiff (MPa) collagen IV/VII and laminin-rich extracellular matrix (ECM) termed the basement membrane (BM), while dermal fibroblasts reside on considerably softer (0.1-10kPa) collagen III/I and fibrillin/elastin-based matrixes. Chronological ageing has profound effects on skin biomechanics. Skin progressively loses its natural elasticity and becomes wrinkled and fragile due to reduced ECM synthesis, collagen I/elastin fragmentation and remodelling. The collagen III/I ratio increases in the aged dermis, while the BM exhibits marked collagen VII loss and fragility. How these changes affect mechano-sensing and aged skin cell physiology is poorly understood. However, it is well established that ageing yields cell stiffening, which affects contractility and mobility. The topographic and chronological skin tissue mechanical variability is however, largely ignored in tissue engineering. Standard cell culture procedures involve propagation on non-physiologically hard (GPa) surfaces (i.e. poly-styrene). Yet, major cellular functions such as identity, proliferation, signalling, behaviour and architecture are determined by the stiffness of the extracellular environment. It is thus not surprising that current in vitro-engineered skin-substitutes do not accurate represent native skin. Specifically, growth on plastic dishes abnormally activates dermal fibroblasts (e.g. fibrosis), induces stress signalling pathways, and alters cell architecture, while it substantially limits cell proliferation. Therefore our hypothesis is that the use of biomimetic 2D surfaces and the study of the molecular mechanisms that underpin keratinocyte and fibroblast mechano-sensing will facilitate the development of proper full thickness 3D young/aged skin models. Developing such models will be beneficial for the personal healthcare and cosmetics industry (target validation, technology identification) and a pivotal tool for the study of the mechanisms that underpin skin ageing in vitro.