Biomedical Engineering Reference
In-Depth Information
The research is occurring in several laboratories, but is limited because so few
laboratories have access to human ES cells. Thus, at this stage, many therapies
based on the use of human ES cells are still hypothetical and highly experimental
[
20,
26,
28
] .
One of the
current advantages
of using ES cells as compared to adult stem cells
is that ES cells have an unlimited ability to proliferate in vitro and are more likely
to be able to generate a broad range of cell types through directed differentiation.
Ultimately, it will also be necessary to both identify the optimal stage(s) of differen-
tiation for transplant and demonstrate that the transplanted ES-derived cells can
survive, integrate, and function in the recipient.
The potential
disadvantages
of the use of human ES cells for transplant therapy
include the propensity of undifferentiated ES cells to induce the formation of tumors
(teratomas), which are typically benign. Because it is the undifferentiated cells—
rather than their differentiated progeny—that have been shown to induce teratomas,
tumor formation might be avoided by devising methods for removing any undif-
ferentiated ES cells prior to transplant. Also, it should be possible to devise a fail-
safe mechanism—i.e., to insert into transplanted ES-derived cells suicide genes that
can trigger the death of the cells should they become tumorigenic. The potential
immunological rejection of human ES-derived cells might be avoided by geneti-
cally engineering the ES cells to express the MHC antigens of the transplant recipi-
ent, or by using nuclear transfer technology to generate ES cells that are genetically
identical to the person who receives the transplant, as already mentioned [
26
] .
What Are Other Potential Uses of Human
Embryonic Stem Cells?
Many potential uses of human ES cells have been proposed that do not involve
transplantation. For example, human ES cells could be used to study early events in
human development and so make it possible to identify the genetic, molecular, and
cellular events that lead to birth defect problems and identify methods for their pre-
vention [
20,
35,
38
]. They could also be used to explore the effects of chromosomal
abnormalities in early development, including ability to monitor the development of
early childhood tumors, many of which are embryonic in origin [
39
] .
Testing candidate therapeutic drugs and even toxins by using ES cells involves
human cell lines. Although animal model testing is a mainstay of pharmaceutical
research, it cannot always predict the effects that a candidate drug may have on
human cells. For this reason, cultures of human cells are often employed in preclini-
cal tests. These human cell lines have usually been maintained in vitro for long
periods and as such often have different characteristics than do in vivo cells. These
differences can make it difficult to predict the action of a drug in vivo based on the
response of human cell lines in vitro. Therefore, if human ES cells can be directed
to differentiate into specific cell types that are important for drug screening, the
ES-derived cells may be more likely to mimic the in vivo response of the cells/tissues
