Cell sheet engineering for integrating functional tissue in vivo : Successes and challenges
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mensional versus three-dimensional cell culture for tissue engineering The direct integration of fully functional, natural cell-based materials as a therapeutic for damaged endogenous tissue dates back to 2500 BC when Susruta excised skin from one person and sewed it onto another.1 Tissue engineering aims to develop functional tissue constructs from cultured cells that can be directly implanted into damaged tissue, either for local repair or for full replacement. Modern regeneration techniques lie between the two extremes as a means of integration into living organisms—“top-down” assembly of individual cells and “bottom-up” assembly of pre-assembled cells. The top-down approach involves seeding cells into three-dimensional (3D) scaffolds that guide attachment and proliferation. The bottom-up approach involves aggregating cells from simple two-dimensional (2D) culture to achieve functional tissue without the need for an intermediate scaffold. These extremes exist because functional tissue cannot be regenerated simply by mixing the requisite cells and allowing them to assemble into structures that possess the higher-order organization of native tissue with respect to both the spatial distribution of the cells and the extracellular matrix (ECM).
The ECM is a collection of noncellular components that is formed during tissue development, providing both biochemical and structural support for the cells. Consequently, some sort of templating must exist to at least drive initial organization. The simplest approach to organizing cells is 2D cell culture. This was first achieved in 1910, when a fragment of tadpole nerve cord was placed into a droplet of frog lymph and new nerve fibers were observed.2 However, 2D culture does not provide the necessary guidance to engender a specific tissue architecture with the level of complexity that replicates developing embryos, wound healing, or the regeneration activities of living animals.3 This is not entirely surprising, as 2D environments (at least those imposed by the ubiquitous petri or agar dishes) are not representative of the cellular environment found in physiological tissues. As a result, there has been significant research into 3D cell culture. The first known example was demonstrated in 1972, when fetal lung fibroblasts were cultured on hydrated collagen lattices.4 Novel cellular activity was immediately observed. Cells attached and extended within the lattices unlike their behavior on soft agar. It is now commonly accepted that 3D cell culture has several advantages over 2D culture with respect to
Nicholas Baksh, Department of Mechanical Engineering, University of South Florida, USA; [email protected] Nathan D. Gallant, Department of Mechanical Engineering, University of South Florida, USA; [email protected] Ryan G. Toomey, Department of Chemical and Biomedical Engineering, University of South Florida, USA; [email protected] doi:10.1557/mrs.2017.91
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