Human embryonic stem cells: isolation, maintenance, and differentiation.

The isolation of pluripotent human embryonic stem (hES) cells having the capacity to differentiate in vitro to numerous cell types generated much excitement and promise in the field of regenerative medicine. However, along with great enthusiasm came hot controversy for stem cell research and researchers alike because available hES cell lines were isolated from "excess" embryos from in vitro fertilization clinics. Despite ethical and political debates, the methods and protocols to study diverse lineages are developing. Furthermore, strategies using specific growth factor combinations, cell-cell and cell-extracellular matrix induction systems are being explored for directed differentiation along a desired lineage. However, there is a great need to characterize the mechanisms that control self-renewal and differentiation and a necessity to improve methodologies and develop new purification protocols for the potential future clinical application of hES cells. After the scientific and political obstacles are overcome, it is anticipated that the hES cell field will make a tremendous difference in conditions, such as burn traumas and diabetic foot ulcers, as well a number of degenerative diseases such as Parkinson's disease, type 1 diabetes, rheumatoid arthritis, and myocardial infarction. In this introductory chapter, we will summarize and review recent progress in the field of hES cell differentiation protocols and discuss some of the current issues surrounding hES cell research.