The role of extracellular mechanics in skin tissue engineering and ageing

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.