So, to generate cultures of specific types of differentiated cells—^heart muscle cells,
blood cells, or nerve cells, for example—scientists try to control the differentiation of
embryonic stem cells. They change the chemical composition of the culture medium,
alter the surface of the culture dish, or modify the cells by inserting specific genes.
Through years of experimentation, scientists have established some basic protocols
or "recipes" for the directed differentiation of embryonic stem cells into some specific
cell types.
If scientists can reliably direct the differentiation of embryonic stem cells into
specific cell types, they may be able to use the resulting, differentiated cells to treat
certain diseases in the future. Diseases that might be treated by transplanting cells
generated from human embryonic stem cells include Parkinson's disease, diabetes,
traumatic spinal cord injury, Duchenne's muscular dystrophy, heart disease, and
vision and hearing loss.
• Induced pluripotent stem ceUs
Induced pluripotent stem cells (iPSCs) are adult ceUs that have been genetically
reprogrammed to an embryonic stem cell-like state by being forced to express genes
and factors important for maintaining the defining properties of embryonic stem
cells. Although these cells meet the defining criteria for pluripotent stem cells, it is
not known if iPSCs and embryonic stem cells differ in clinically significant ways.
Mouse iPSCs were first reported in 2006, and human iPSCs were first reported in
late 2007. Mouse iPSCs demonstrate important characteristics of pluripotent stem
cells, including expressing stem cell markers, forming tumors containing cells from
all three germ layers, and being able to contribute to many different tissues when
injected into mouse embryos at a very early stage in development. Human iPSCs
also express stem cell markers and are capable of generating cells characteristic of
all three germ layers.
Although additional research is needed, iPSCs are already useful tools for drug
development and modeHng of diseases, and scientists hope to use them in
transplantation medicine. Viruses are currently used to introduce the
reprogramming factors into adult cells, and this process must be carefully
controlled and tested before the technique can lead to useful treatments for humans.
In animal studies, the virus used to introduce the stem cell factors sometimes
causes cancers. Researchers are currently investigating non-viral delivery
strategies. In any case, this breakthrough discovery has created a powerful new way
to "de-differentiate" cells whose developmental fates had been previously assumed
to be determined. In addition, tissues derived from iPSCs wiU be a nearly identical
match to the ceU donor and thus probably avoid rejection by the immune system.
The iPSC strategy creates pluripotent stem cells that, together with studies of other
types of pluripotent stem cells, will help researchers learn how to reprogram cells to
repair damaged tissues in the human body.
1.5 Possibility of existence of adult pluripotent stem cells
Many important questions about adult stem cells remain to be answered. They
include:
How many kinds of adult stem cells
exist, and in whichtissues do they exist?
How do adult stem cells evolve during development and how are they maintained in
the adult? Are they "leftover" embrj'^onic stem cells, or do they arise in some other
way?
As described above, stem cells were discovered by studies of development. In
development, germ-layer differentiation is a critical point. All adult stem cells
discovered until today are known to be generated after germ-layer differentiation.
By this reason, adult stem cells cannot differentiate crossing germ layers.
Whereas, some scientists suggested that "left over" embryonic cells still reside in
adult body.
For example, MAPC, spore-like stem cells, MIAMI cells, VSESC and MUSE ceUs.
They suggest those theories because some phenomena can not be explained by
knowledge from development study. Adult stem cells have generally been felt to be
Hmited to multipotency and unable to cross germ layer lineages as they develop.
However, first, a part of mesenchymal stem ceUs were derived firom ectoderm not
mesoderm. Second, a part of mesenchymal stem cells can differentiate into cells
derived firom ectoderm. Thus, some of adult stem cells cross the germ layers. If the
origin of germ layer is critical for determining stem cells' fate, these phenomena can
not be explained. This is the reason why researchers suggest that "left over"
embryonic cells still reside in adult body.
Especially spore-like stem cell research suggested a very unique theory.
• Spore-like stem cells (Figure 2)
While the existence of adult stem cells has been reported for more than a decade,
our research team has advanced the theory that common adult stem cells reside in
all body tissues, they possess small figure and stress-tolerant property. These cells
were named spore-like stem cells.
1.6 Hypothesis of this research
• Sphere formation
Sphere formation is recognizes as one of isolation method of adult stem cells.
Because, stem cells should hold a strong proliferative potential and self-renewal
potency, sphere formation is recognized as a result of those potencies.
Interestingly, as presented by neurospheres, some sphere forming stem cells show
gene expression over lapping with ES cells.
• Immature adult stem ceUs
If stem cells which can cross the germ layer lineages
existing in native organisms. ES cells are artificial.
• Hypothesis
Considering all the various factors together.
1. Adult stem cells stem which can cross germ layers (very
immature adult stem cells) may exist.
2. They are very small, and stress-tolerant
3. They form spheres
In this study, we characterized cells isolated from three adult tissues (lung, muscle
and spinal cord) representative of the three different germ layers (endoderm,
mesoderm and ectoderm) and from bone marrow. The cells were triturated to break
mature cells and propagated as non-adherent clusters or spheres in a serum-free
culture medium. We found that cells from each source, initially expressed many of
the markers associated with ESCs and demonstrated differentiation potential into
all three germ layers at a time that neural Hneage markers had not yet been
expressed. Ultimately cells from each tissue, differentiated into cells representative
of all three germ layers in \itro. The isolation initially contained a significant
amount of floating debris, non-adherent cells, insoluble proteins or fibers, and other
extraneous materials, all of which appeared to participate in the formation of
non-adherent spherical clusters that contained the cells. The cellular make-up of
individual spheres was not identical; that is, spheres were composed of
heterogeneous populations of cells, even when the spheres were generated from
cells procured fi'om the same tissue at the same time. Similarities or differences
seen in the cell content of different spheres, were believed to be secondary to the
environment in which they were cultured.
