Biomedical Engineering Reference
In-Depth Information
to the drug(s) being tested and so offer safer, and potentially cheaper, models for
drug screening. The reasons for using human ES cells to screen potential toxins
closely resemble those for using human ES-derived cells to test drugs. Toxins often
have different effects on different animal species, which makes it critical to have the
best possible in vitro models for evaluating their effects on human cells. Finally,
human ES cells could be used to develop new methods for genetic engineering.
Currently, the genetic complement of mouse ES cells in vitro can be modified easily
by techniques such as homologous recombination. This is a method for replacing or
adding genes, which requires a DNA molecule artificially introduced into the
genome and then expressed. Using this method, genes to direct differentiation to a
specific cell type or genes that express a desired protein product might be intro-
duced into the ES cell line. Ultimately, if such techniques could be developed using
human ES cells, it may be possible to devise better methods for gene therapy [
38
] .
What Are Critical Questions Regarding Human
Embryonic Stem Cells?
There is the difference between ES cells and human EG cells which are apparently
not equivalent in their potential to proliferate or differentiate. Both kinds of cells
spontaneously generate neural precursor-type cells (widely regarded as a default
pathway for differentiation), both generating cells that resemble cardiac myocytes
[
17,
35
]. However, ES cells, derived from the inner cell mass of the preimplantation
blastocyst, approximately 5 days post-fertilization, should be distinguished from
human EG cells, derived from fetal primordial germ cells, 5-10 weeks post-
fertilization. ES cells can proliferate for up to 300 population doublings, while cells
derived from embryoid bodies that are generated from embryonic germ cells (fetal
tissue) double a maximum of 70-80 times in vitro. ES cells appear to have a broader
ability to differentiate. Both human ES and EG cells in vitro will spontaneously
generate embryoid bodies that consist of cell types from all three primary germ lay-
ers (Pera M, personal communication) [
1,
18,
29
]. Many key questions regarding
ES remain unanswered or only partly answered. For example, in order to refine and
improve ES cell culture systems, we have to identify the mechanisms that allow
human ES cells in vitro to proliferate without differentiating [
26
] . Once the mecha-
nisms that regulate human ES proliferation are completely known, it will be highly
likely to apply this knowledge to the long-standing challenge of improving the
in vitro self-renewal capabilities of adult stem cells.
It is of critical significance to determine whether the genetic imprinting status of
human ES cells plays any significant role in maintaining the cells, directing their
differentiation, or determining their suitability for transplant. One of the effects of
growing mouse blastocysts in culture is a change in the methylation of specific
genes that control embryonic growth and development [
21
]. Do similar changes in
gene imprinting patterns occur in human ES cells (or blastocysts)? If so, what is
their effect on in vitro development and on any differentiated cell types that may be
derived from cultured ES cells?
