Spheroids as microtissues are a powerful alternative to standard 2D cell culture for in vitro studies. 3D scaffold-free spheroids are formed within a few days from a cell suspension using hanging drop technology. The advantages of spheroids exclusively composed of fibroblasts rely on the physiological production of the extracellular matrix thanks to the aggregative capability of fibroblasts to self-assemble in a round tridimensional structure. This microdermis presents a complex tissue organization that closely mimics the architecture and composition of the human dermis in vivo. The aim of this study was to characterize structural and biomechanical properties of spheroids composed of normal human fibroblasts. The density and the structural organization of the elastic fiber network within spheroids were firstly characterized using second harmonic generation microscopy, a powerful technique for autofluorescent observation of dense and ordered fibrillary macrostructures. Biomechanical properties of spheroids were then evaluated using atomic force microscopy, a nanoindentation tool that can determine the Young’s modulus and therefore elastic properties of the microdermis. The combination of these two analysis techniques on a same spheroid allowed us to establish a perfect correlation between the fluorescent signal indicating elastic fiber presence and the mechanical properties of the spheroid. Indeed, we demonstrated that the increase of elastin amount within a spheroid resulted in the decrease of the elastic modulus value. These results highlight the elastic properties of the microdermis which is mechanically softer thanks to the presence of elastic fibers. Therefore, a spheroid is a biomimetic model in which mechanical and structural elastic properties are related to each other. This correlative study outlines spheroids as a reliable microdermis model to test active ingredient efficiency on elastin synthesis and biomechanical properties of the microtissue.

Structural and biomechanical properties of a novel 3D microdermis model: the spheroid