Does transdifferentiation occur in regeneration?

Abstract The problem of the flexibility in cell differentiation originally stemmed from two main sources, one from medical observations of metaplasia, and the other from zoological studies of regeneration in various animals. In early regeneration studies, it was taken for granted that the major cellular basis for the dramatic changes during regeneration should be metaplasia, or the conversion of pre-existing cell types. The introduction of labelling techniques to mark precursors of regenerating cells has shown that a switch of differentiation pathway, or transdifferentiation, of the precursors is not an universal occurrence. Nevertheless, unequivocal demonstrations of transdifferentiation have also been gained from observations of regeneration in some systems. The aim of this chapter is to critically review the occurrence of transdifferentiation in regeneration.