Background
Embryonic stem (ES) cells have been derived from the inner cell masses (ICM) of blastocysts in many
species. They are capable of unlimited, undifferentiated proliferation on feeder cell layers and remain
karyotypically normal and phenotypically stable. In addition, ES cells have the ability to differentiate into
a wide variety of cell types in vitro and in vivo. In mES cell culture, the feeder layer can be replaced by
the addition of LIF in the growth medium. However, LIF does not have the same effect on hES cell
culture as mES. Therefore, both the derivation and maintenance of hES cells require the use of feeder
cells.