normally, nor whether hES cells have the capacity to participate in normal development in the context of the three-dimensional embryo in the reproductive tract. Such conclusive evidence requires testing in blastocyst chimeras as is routinely done with mES cells.

Understanding why ES cells are able to proliferate essentially indefinitely and retain the ability to be induced to differentiate and stop proliferating will provide important information about the regulation of normal embryonic development and the uncontrolled cell division that can lead to cancer. It is known that external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and molecules in the microenvironment. Identifying such factors would allow scientists to find methods for controlling stem cell differentiation in the laboratory and thereby allow growth of cells or tissues that can be used for specific purposes, such as cell-based therapies.

Several methods have been shown to be effective for delivering exogenous genes into hES cells, including transfection by chemical reagents, electroporation, and viral infection (Eiges et al., 2001; Gropp et al., 2003; Ma et al., 2003; Pfeifer et al., 2002; Zwaka and Thomson, 2003). Those are all critical methodological objectives that must be met if hES cells are to be used as the basis of therapeutic transplantation.


Most work on hES cells has taken place with a relatively small number of cell lines obtained from excess blastocysts donated from in vitro fertilization (IVF) programs. The genetic makeup of the cells is not controlled in any way, and genetic variation among lines needs to be considered when results from different lines are compared. Experience from research with mES cells shows that ES cell lines can differ markedly in their differentiation efficiencies. Being able to control the genotype of ES cells would be valuable for various reasons, most notably the desire to generate ES cells with genotypes known to predispose to particular diseases. In the case of single-gene defects, one could achieve that goal by deriving hES cells from discarded morulae or blastocysts that were identified with preimplantation genetic diagnosis (PGD) procedures (Verlinsky et al, 2005) as carrying mutations or by generating the appropriate mutation by gene targeting of established hES cell lines. However, such approaches cannot be used if the genetic predisposition has an unknown basis or arises from multiple gene effects. Availability of hES cell lines from patients with Alzheimer’s disease, type I diabetes, or many other complex diseases would provide a source of cells that could be differentiated into appropriate cell types; and the progression of the disease could then be modeled and potentially modified in culture. Given the complex interplay between genotype and environment that typifies complex chronic diseases, the availability of cell-line models would provide major new tools for diagnosis and therapy. In this context, hES cells are research tools for the study of disease, not therapeutic agents themselves.

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