Stem cells: a journey into a new frontier.

Stem cells are cells that are capable of self-renewal and are multipotent, meaning that they can differentiate into many specific cell types. For a long time, “stem cell” had been a concept in mammalian biology but not a reality that could be seen, touched, and expanded in vitro, until murine embryonic stem cell culture was established in 1981. Through recent progress made in adult stem cell research, we now can isolate various stem/progenitor cells derived from adult tissues, such as neural stem cells, vascular endothelial progenitor cells, and hepatic oval cells. Of the many adult stem cells described in the literature (hematopoietic, neural, hepatic, dermal, pancreatic, and intestinal), it once was thought that these cells could differentiate only into the cell type from which they originated. It now seems that the adult stem cell is more plastic, versatile, and capable of becoming many different types of cells, e.g., blood to brain/liver, brain to blood, liver to blood. These new findings give the once lonely adult stem cell a new lease on life and perhaps a better chance in enabling both investigators and clinicians to fight life-threatening human diseases. Recent stem cell–based cell therapies have been shown to be successful in animal models for various diseases, such as Parkinson’s disease and insulin-dependent diabetes mellitus. In the coming decade, the focus of research will be stem cell–based cell therapy (or a combination of cell and gene therapy techniques). In this article, we discuss the present status and the future of stem cell research via two major categories: embryonic stem (ES) cells and adult stem cells. Self-renewal is discussed, mostly from a point of view of gene regulation in the ES cell and from a point of view of the microenvironment in the adult stem cell. In addition, the pluripotency of stem cells is addressed. Finally, we refer to the controversies arising in stem cell research and the future applications of stem cells in medicine.