Stem cell research is a fairly current field. The first major advance was in 1956 when it was first shown that the injection of bone marrow into a subject could regenerate a healthy blood-forming system (Domen et al., 2006). Then in the 1960s it was discovered that there were two different types of stem cells in bone marrow (NIH & DHHS, Stem cell basics, 2006). The first time that embryonic stem cells were derived was from mice in 1981. Scientists then moved on to taking these embryonic stem cells from primates, specifically the rhesus monkey and the common marmoset, in the 1990s. (Yu & Thomson, 2006) The final step of taking human embryonic stem cells and growing them in a lab was first achieved in 1998 (NIH & DHHS, Stem cell basics, 2006). A new method of obtaining embryonic stem cells, somatic cell nuclear transfer (SCNT) is where the nucleus of an adult stem cell is injected into an enucleated egg cell and an embryo is formed with the DNA of the adult donor. Embryonic stem cells were first obtained in this manner in 2004.
The first step in creating a group of stem cells in research is to isolate them
from an embryo or from adult tissue. Embryonic stem cells are found in the
blastocyst and adult stem cells exist in many different organs of adults. (NIH &
DHHS, Stem cell basics, 2006) Adult stem cells must be purified, since they are
mixed in with specialized cells. One way scientists do this is by using certain
cell
markers that react differently depending on whether or not there is stem
cell activity.
There are also some dyes that stain stem cells slightly
differently than other cells.
(Princeton University, 2005) Figure 1 shows a stem cell that has been
magnetically
identified using iron markings, which are the pieces surrounding the cell (Dusik &
Morley, 2004).
However this purification process is not perfect
so a completely pure stem cell
sample cannot usually be obtained (Princeton University, 2005) .
The embryonic stem cells once obtained are cultured to create a larger sample. The inner cell mass of the blastocyst is placed in a culture dish that has a nutrient-rich substance in it with a layer of cells on top. These cells are usually embryonic mouse skin cells that have been altered so they won’t divide. The purpose of these cells is to provide a sticky surface for the embryonic stem cells to attach to, as well as to secrete nutrients into the dish. These cells are a problem because viruses or other unwanted molecules contained in them may transfer to the stem cells, contaminating the culture. (NIH & DHHS, Stem cell basics, 2006) There have been some stem cell cultures that have used a layer of human cells instead of mouse cells. This solves some of the problems of cross-species contamination, but even the human cells could contaminate the culture. (Yu & Thomson, 2006) Scientists are working on culturing stem cells without this layer of cells to support them.
Embryonic stem cells have been cultured for over a year in a lab without differentiating, but adult stem cells have not been able to self-replicate for anywhere near this amount of time. The embryonic cells remain undifferentiated as long as they are not allowed to clump up into embryoid bodies. If this clumping is allowed the cells begin to spontaneously differentiate. There are various test that determine the fitness of the stem cells. Embryonic stem cells must have grown in culture for six or more months without differentiating, be pluripotent, and be genetically normal. Adult stem cells should be able to produce cells that are identical genetically to themselves and they should be able to differentiate into any type of cell of the tissue from which they were taken. (NIH & DHHS, Stem cell basics, 2006)
Scientists then must try to differentiate the stem cells into the specific types
of cells needed for research or medical applications. At this time scientists
are largely unaware of the conditions needed to make stem cells differentiate
into the various specific cell types. For some types of cells they have set
methods, and a couple of these methods are shown in Figure 2. However, there still needs to be research done on stem cell differentiation.
(NIH & DHHS, Stem cell basics, 2006)
