Nature has created numerous elegant living systems, including the human, based on the hierarchical functional units—molecule, cell, tissue, and organ. A living system develops through a long evolutionary process, during which the system undergoes genotypic and phenotypic changes in response to environmental simuli. Whereas the environmental and genetic factors play critical roles in evolutionary development, they may induce disorders and injuries of the cell, tissue, or organ, resulting in impairment or destruction of the functional units and preventing the living system from functioning and survival. Since these disorders and injuries are inevitable events during the evolutionary process, Nature has designed various mechanisms for the repair or replacement of.
