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Embryonic stem cell researchers comment on the challenges of using stem cells from embryos:
Rarely have specific growth factors or culture conditions led to establishment of cultures containing a single cell type….[T]he possibility arises that transplantation of differentiated human ES cell derivatives into human recipients may result in the formation of ES cell-derived tumors.
J.S. Odorico et al., “Multilineage differentiation from human embryonic stem cell lines,” 19 Stem Cells 193-204 (2001).
Transplanted ES cells spontaneously differentiate into any of a variety of ectodermal, endodermal and mesodermal cell types – sometimes into a disorganized mass of neurons, cartilage and muscle; sometimes into terratomas containing an eye, hair or even teeth.
R. Lanza et al., “Human therapeutic cloning,” 5 Nature Medicine 975-977 (1999).
There are still many hurdles to clear before embryonic stem cells can be used therapeutically. For example, because undifferentiated embryonic stem cells can form tumors after transplantation in histocompatible animals, it is important to determine an appropriate state of differentiation before transplantation. Differentiation protocols for many cell types have yet to be established. Targeting the differentiated cells to the appropriate organ and the appropriate part of the organ is also a challenge.
E. Phimister and J. Drazen, “Two Fillips for Human Embryonic Stem Cells,” 350 New England Journal of Medicine 1351 ( March 25, 2004 ).
Normally, if you take an embryonic stem cell, it will make all kinds of things, sort of willy-nilly.
[Harvard ESC researcher Doug] Melton. J. Mitchell, “Stem Cells 101,” PBS Scientific American Frontiers, May 28th, 2002 , www.pbs.org/saf/1209/features/stemcell.htm.