Introduction
Embryonic stem (ES) cells are continuous proliferating stem cell lines of embryonic origin first
isolated from the inner cell mass (ICM). Two distinguishing features of ES cells are their ability to be
maintained indefinitely in an undifferentiated state and their potential to develop into any cell within
the body. Based on previous methods developed for mouse ES cells, human ES cell lines were first
established by Dr. James Thomson and colleagues. Like mouse ES cells, human ES cells express high
levels of membrane alkaline phosphatase (AP) and Oct-4, a transcriptional factor critical to ICM and
germline formation. However, unlike mouse ES cells, hES cells do not express stage-specific
embryonic antigen (SSEA-1). In addition, prolonged propagation of hES cells is typically achieved by
coculture with primary mouse embryonic fibroblasts (MEFs) serving as feeder cells. Human ES cell
lines are not able to maintain their undifferentiated state in the absence of supporting feeder layer cells,
even when exogenous cytokines such as leukemia inhibitory factor (LIF) and gelatin-coated plates are
used.