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For any tissue engineering or regenerative medicine strategy its success is dependent largely on controlling the biology of the cells at the site of repair or regeneration, since it is the cells that constitute and co-ordinate the basic structure and function of tissues. The ideal situation would be to augment intrinsic self-repair mechanisms by stimulating the mobilisation, recruitment and activity of cells within the body. At present, however, this approach is quite limited, but as our knowledge of cell biology, cell environments, cell signalling and cell trafficking increases such activation of self-repair mechanism might become possible. The alternative is to supply cells exogenously and this raises a number of questions and challenges, such as what are the most appropriate sources and types of cell, how to control the growth and differentiation of the cells and how to deliver the cells to site of repair. This chapter provides a brief summary of some of the various cell types, including differentiated somatic adult cells, somatic stem cells, foetal cells and embryonic stem cells that are being and which might be used in promoting and understanding tissue repair and regeneration.

Aggregation of cells into clusters is a tissue engineering approach used for rapid and controlled formation of structures that have certain architectural and functional properties of native tissue. Some mammalian cells aggregate spontaneously in suspension, for others, methods have been devised to force them to aggregate. This chapter focuses on approaches based on biomaterials and bioreactors to induce or accelerate cell aggregation. It also describes a recently introduced cell surface engineering method used to induce aggregation of homotypic as well as heterotypic cell types.