New principles of cell plasticity.

Recent discoveries demonstrating surprising cell plasticity in animals and humans call into question many long held assumptions regarding differentiative potential of adult cells. These assumptions reflect a classical paradigm of cell lineage development projected onto both prenatal development and post-natal maintenance and repair of tissues. The classical paradigm describes unidirectional, hierarchical lineages proceedings step-wise from totipotent or pluripotent stem cells through intermediate, ever more restricted progenitor cells, leading finally to 'terminally differentiated' cells. However, in light of both the recent discoveries and older clinical or experimental findings, we have suggested principles comprising a new paradigm of cell plasticity, summarized here.