9–12

Indeed, many articles on adult stem cells have embedded somewhere in their introductions and/or discussions a distinct explanation why the adult stem cell system being studied circumvents the bioethics problem. However, with the exception of bone marrow transplants, adult stem cells, to date, still have their problems, which, similar to hESCs, have also kept them out of the clinic for use as stem cell therapies. Hence, human ingenuity has led to profound discovery that somatic cells could be induced to become pluripotent by simply adding four genes. Induced pluripotent stem cells (iPS cells) were first generated by two research teams led by Drs. Yamanaka and Thomson, respectively, who pioneered and generated stem cells see more from human skin through ectopic expression of four genes (Oct3/4, Sox2, c-Myc, CYC202 and Klf4, or Oct3/4, Sox2, Nanog, and Lin28).13–15 Since their discovery, improvements have been made in generating iPS cells including the ability to remove the inducing genes,16 the addition of only one or two genes in certain cell types,17, 18 and generation of iPS cells by chemical induction.19, 20 In each case, no matter the inductive route, human iPS cells have been shown to mimic hESCs in virtually all aspects of pluripotency and differentiation. These iPS cells are pluripotent because they can form all three germ layers. Moreover,

mouse iPS cells have been repeatedly shown to make chimeric mice, contribute to the germ line, and generate pups.21 However, to date, most of the in vitro investigations

into iPS cell differentiation have focused on mesodermal-derived cardiomyocyte and ectodermal-derived neuronal lineages—that is, until now. In this issue, two independent laboratories reveal, for the first time, complete derivation of iPS cells into endodermal-derived hepatocytes (Sullivan et al.22 and Si-Tayeb et al.23). While the elegance of each study enables them to stand alone, when taken together, they, in essence, delineate the true potential of iPS cells for the field of hepatology. The data clearly reveal that iPS cells can become fully functional liver cells. Both articles demonstrate that iPS cell–derived hepatocytes express distinct hepatocyte markers; however, and perhaps more importantly, both also show definitive medchemexpress function of their hepatocytes in vitro and in vivo. The magnitude of these investigations will probably be felt straight away because they represent a seminal advancement in current hepatocyte cell-culture technology. A constant problem experienced by many who try to culture hepatocytes is that current protocols generally revolve around the need for consistent derivation and culture of primary hepatocytes, which have the reputation for being difficult to cultivate, are generally scarce, and are usually rather heterogeneous once in culture.