Definitive endoderm: a key step in coaxing human embryonic stem cells into transplantable beta-cells

Using the Edmonton protocol, a number of patients with Type 1 diabetes mellitus have remained insulin-independent for prolonged periods of time. In spite of this success, transplantation of islets from cadaver donors will remain a therapy for very few patients owing to a lack of donors. Thus, if cell therapy should be widely available, it will require an unlimited source of cells to serve as a 'biological' insulin pump. At this time, the development of beta-cells from hESCs (human embryonic stem cells) has emerged as the most attractive alternative. It is envisioned that ultimate success of an in vitro approach to programme hESCs into beta-cells will depend on the ability, at least to a certain degree, to sequentially reproduce the individual steps that characterizes normal beta-cell ontogenesis during fetal pancreatic development, including definitive endoderm from which all gastrointestinal organs, including the pancreas, originate. In the present article, differentiation of hESCs into putative definitive endodermal cell types is reviewed.