Introduction

One of the most remarkable processes in nature is the process of replacing or regenerating damaged tissue. Some salamander species possess the capacity to regenerate a variety of tissues and organs as adult organisms. Other higher vertebrate species also possess regenerative abilities, but these are limited to early embryonic stages and⁄or tissues that can undergo renewal (Tsonis, 2000, 2001). Lens regeneration in the adult urodele amphibian represents one of these unique processes in which major cellular events such as dedifferentiation and transdifferentiation regulate tissue replacement. Dedifferentiation involves terminally differentiated cells reentering the cell cycle and losing the typical characteristics of their origin, whereas transdifferentiation allows a cell to change its identity and become a completely different cell type. During lens regeneration, the cells that undergo this transformation are the pigment epithelial cells (PECs) of the dorsal iris. This cell-type conversion is not usually observed in terminally differentiated cells that have followed a developmental path and had been determined in phenotype and function. Cancer cells share similarities with the PECs that undergo the regenerative process. In the former, during oncogenesis, the original phenotype is destabilized and the cells divide, resulting in uncontrolled growth, eventual invasion to other organs/tissues, and the production of tumors. During lens regeneration, there must be a mechanism or program that destabilizes the cell phenotype but at the same time carefully directs these cells to divide, reorganize, and redifferentiate to new cell types that will be responsible for replacing the lost parts.