Tissue restoration: approaches and prospects.

This article summarizes the results of three basic research approaches directed toward achieving the restoration of injured or diseased human tissues. The three approaches are (1) to understand the differences in the molecular characteristics of cells and their environments in tissues which exhibit regenerative capacity at one stage of the life cycle but not at another; (2) to understand the molecular mechanisms whereby tissues regenerate by means of reserve progenitor cells or progenitor cells formed by dedifferentiation; and (3) to design artificial tissues for implantation into the body. These strategies should allow us to locate the key switchpoints which determine regeneration versus repair and how to reconfigure those switchpoints into a regenerative circuit where necessary or to build tissues in vitro that can serve as in vivo substitutes. For these strategies to be successful, we must understand the role of the immune system in repair and regeneration and the developmental roles of regulatory molecules, such as growth factors, trophic factors, extracellular matrix components, and the protein products of patterning genes, as well as the intracellular signaling systems activated by these molecules. The examples used to illustrate these themes are repair versus regeneration in wounded mammalian skin, the regeneration of injured mammalian bone and muscle by reserve progenitor cells, and the regeneration of the urodele limb, neural retina, and lens by progenitor cells produced by dedifferentiation. In addition, several approaches used in the construction of bioartificial tissues are discussed.