Although mammalian hearts show virtually no ability to regenerate, there is a growing initiative to determine whether existing cardiomyocytes or progenitor cells can be coaxed into eliciting a regenerative response. In contrast to mammals, a number of non-mammalian vertebrate species are able to regenerate their hearts1–3, including the zebrafish4,5, which can fully regenerate its heart following amputation of up to 20% of the ventricle. To directly address the source of newly formed cardiomyocytes during zebrafish heart regeneration, we first established a genetic strategy to lineage-trace cardiomyocytes in the adult fish, based on the Cre/lox system widely used in the mouse6. Using this system, we show here that regenerated heart muscle cells are derived from the proliferation of differentiated cardiomyocytes. Furthermore, we show that proliferating cardiomyocytes undergo limited dedifferentiation characterized by the disassembly of their sarcomeric structure, detachment from one another and expression of regulators of cell cycle progression. Specifically, we show that polo-like kinase1 (plk1) is an essential component of cardiomyocyte proliferation during heart regeneration. Our data provides the first direct evidence for the source of proliferating cardiomyocytes during zebrafish heart regeneration and indicates that stem/progenitor cells are not significantly involved in this process.

The utility of induced pluripotent stem (iPS) cells for investigating the molecular logic of pluripotency and for eventual clinical application is limited by the low efficiency of current methods for reprogramming. Here we show that reprogramming of juvenile human primary keratinocytes by retroviral transduction with OCT4, SOX2, KLF4 and c-MYC is at least 100-fold more efficient and twofold faster compared with reprogramming of human fibroblasts. Keratinocyte-derived iPS (KiPS) cells appear indistinguishable from human embryonic stem cells in colony morphology, growth properties, expression of pluripotency-associated transcription factors and surface markers, global gene expression profiles and differentiation potential in vitro and in vivo. To underscore the efficiency and practicability of this technology, we generated KiPS cells from single adult human hairs. Our findings provide an experimental model for investigating the bases of cellular reprogramming and highlight potential advantages of using keratinocytes to generate patient-specific iPS cells.