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(Odorico et al. 2001 ). Later, clonally derived human ES cells were produced, as
reported by Amit et al. ( 2000 ), that demonstrate pluripotency, maintained over an
extended period of culture, at the level of a single cell that can self-renew for long
periods of time (Amit et al. 2000 ).
Schuldiner et al. ( 2000 ) showed that both undifferentiated ES cells and EBs
express receptors for different growth factors which effect the differentiation into
cells with different epithelial or mesenchymal morphologies (Schuldiner et al.
2000 ). The overall effects of these factors were described in three categories: (1)
growth factors mainly responsible for inducing mesodermal cells, (2) factors that
activate ectodermal and mesodermal markers and (3) factors that direct differentia-
tion into all three embryonic germ layers, including endoderm. However, none of
these growth factors were found to direct differentiation solely to one cell type. For
the success of ES cell-based therapy in treating human diseases, we must be able to
direct human ES cells differentiation towards a particular cell type of interest and be
able to obtain this lineage from the mixed population (Amit et al. 2000 ). This
remains a challenge as hardly ever have specifi c growth factors or culture conditions
resulted in cultures containing a single-cell type, and signifi cant culture-to-culture
variability remains even when identical growth factor and conditions are maintained
(Amit et al. 2000 ). Various human cell types may be developed in vitro by using
Fig. 9.4 Timeline of the development of human ES cell culture over the years
D.M. Kalaskar and S.M. Shahid