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Since 1998, when the first human embryonic stem (hES) cell lines were reported (Thomson et al., 1998), a large number of hES cell lines have been derived from the inner cell mass of preimplantation embryos donated at in vitro fertilization (IVF) clinics (Guhr et al., 2006). These hES cell lines possess remarkable ability of both unlimited self-renewal and pluripotency to generate any cell type differentiated from three germ-layers ectoderm, mesoderm and endoderm. Thus, these hES cell lines have great potentials for cell therapies in regenerative medicine and experimental models for drug discovery and toxicity testing in addition to basic studies on stem cell biology and molecular embryogenesis (Wobus & Boheler, 2005). The proliferation of undifferentiated hES cells can be maintained on either mitotically inactivated mouse embryonic fibroblasts (MEF) as feeder or Matrigel-coated plastic surfaces in MEF-conditioned medium (Xu et al., 2001). The continuous culture of undifferentiated hES cells either on MEF feeder or in the MEF-conditioned medium bears the risk of transmitting animal pathogens, and limits future medical applications of hES cells. A few human cell systems, including hES-derived fibroblast-like cells as feeder (Stojkovic et al., 2005), with capacity to support the growth of undifferentiated hES cells have been developed to replace the use of MEF. Activin A was previously reported to be necessary and sufficient for the maintenance of self-renewal and pluripotency of hES cells in long-term feederand serum-free culture (Xiao et al., 2006). It would be of interest to compare the gene expression profiles of these undifferentiated hES cells grown under these different conditions in order to better understand their common molecular mechanisms of unlimited self-renewal and pluripotency. The genome-wide gene expression analyses using high-throughput microarray have been used to identify key “stemness” genes responsible for the unlimited self-renewal and pluripotency of hES cells (Ivanova et al., 2002; Ramalho-Santos et al., 2002; Sperger et al., 2003). A meta-analysis of 20 previously reported transcriptomes had identified 48 genes overexpressed in hES cells compared to differentiated cell types (Assou et al., 2007), and these 48 genes, including transcription factors such as OCT4 (also known as POU5F1), SOX2 and NANOG, may be responsible for the unlimited self-renewal and pluripotency of hES cells. However, molecular mechanisms involved in unlimited self-renewal and pluripotency of hES cells remain to be fully understood.

Target Identification of MicroRNAs Expressed Highly and Regulated by Activin A in Human Embryonic Stem Cells