Embryonic Stem Cell Primer
A review of the scientific literature for laymen
The first mention I found of the term "stem cell" was from 1982,
these derived from mouse embryos. They were culturing and maintaining these cells way back then.
Most of the subsequent data is from the 1990's, and mostly continued in rodent, rabbit, pig and cow cells. First human cell data around 1998. Stem cells are
the totipotent (able to differentiate into any cell type) cells that form the earliest embryo (blastocyst).
To be called stem cells they must have 3 properties: 1). derivation from preimplantation embryo; 2).prolonged undifferentiated proliferation; 3). ability to
differentiate into all 3 embryonic germ layers (endoderm, mesoderm and ectoderm). From these layers all tissues are derived. This ability to form different
tissues and to divide seemingly indefinitely is what researchers get so excited about. Differentiated cells that are not transformed, either artificially or by
mutations (usually tantamount to malignant cell transformation, or indeed derivation from tumors), are extremely difficult to culture and grow and do not last
long. That is the nature of the mature cell, to do its duty then to die. It is an ingrained biological quality because mature cells acquire mutations that lead to
malignant transformation.
While we don't understand cell senescence very well (the inevitable degeneration cells leading to aging and death), at least one important factor is the
telomere. These are the genetic sequences at the ends of chromosomes. They start out long and get shorter and shorter as we grow older. One theory says
that once they get too short, the cell is signaled to die. On an organism wide level, this leads to organ failure and death. Exogenous factors (diet,
environmental, infectious, etc.) may contribute to the acceleration of degeneration of particular tissues. ESCs have high levels of expression of an enzyme
called telomerase, which is responsible for adding telomeres to the ends of chromosomes. Is that the key? One lab found that telomere lengths were maintained
through cell passages in ESCs. Another lab found that if the telomerase was deleted, the cells died much more quickly.
Much of the literature on stem cells is reporting how to maintain viable growth in culture media. Their maintenance is described as very labor intensive.
Apparently they are very picky about their diet (called media). They have a tendancy to spontaneously differentiate (thus become useless). This appears to be
especially true for human ESCs. Those currently in existance cannot be used in human trials because no one has figured out how to grow them without using a
feeder layer of mouse fibroblasts. Feeder layers are commonly needed for picky cells. The feeder layer produces some substance that the cells need to grow as
desired. Various labs are working on what these fibroblasts produce that the stem cells need. However, you cannot use cells grown on mouse cells in humans.
The promise of ESCs is in their ability to differentiate into different tissues. Normal ESC when injected into mice will form teratomas, tumors that contain a
variety of tissue types (these are immunosuppressed mice). Mouse ESC have been used to produce offspring mice that appeared normal
and were viable and fertile. However, prolonged culture of the cells reduced this ability (was only 15-25 passages). The ESCs lifespan in culture has not yet been defined,
but mouse ESCs have lived apparently unaltered for up to 250 cell divisions. For laymen, that's a LOT! That is as assayed by things like morphology,
chromosome karyotype, and cell survival, which are fairly rough measures. But all other cell lines that I am aware of will peter out much sooner; they simply
stop growing. There is a debate going on in the field as to whether these cells acquire mutations. I think we need to work with the longer to know that answer,
particularly in human ESCs.
Tiny word about cell culture: all cells, including stem cells, change properties when they think they are forming a tissue, that is, when the cells touch each
other. Cell division slows down when there are too many in a dish. So, in order to keep cells growing in culture dishes, you have to dilute them every few days.
Each time you dilute the cells it is called a "passage" and it stimulates cell division, so is a marker of lifespan. Since most cell lines can only last
so long, vials of young cells from each line are kept frozen in liquid nitrogen. A small scraping can start a brand new fresh passage. At the next first passage, some cells are
kept and frozen down again. This is how cell lines are kept going indefinitely.
So back to the promise of stem cells. We are hearing a lot about Parkinson's,
Alzheimer's and spinal cord injury because the most data has been gathered by labs looking at the neuronal differentiation of stem cells. They can form all types of neural cells and in mice, when injected into the brain, they even migrate to
the proper part of the brain, grow there and from synapses. This is very exciting data! Other labs have induced stem cells to form pancreatic islet-like cells that
make insulin, form structures that look like cardiac vessels, generate liver enzymes and form hepatoctye-like cells, and form all types of blood cells. Think about
that last one: these cells could eliminate the need for blood donation drives and the infectious risks of blood transfusions. And the field is only in its infancy.
Looking down the road to human trials, there are concerns about potential immunogencity of stem cells, which are derived from particular
genetic backgrounds. Like organ transplants, they may trigger rejection when used in a disparate background (although nobody knows this for sure). Researchers are working on
immune-neutral cells, but that requires genetic manipulation, which is technically difficult, and which opens a whole different kettle of fish. The NIH has strict
criteria for use of genetically altered cells in humans.
That is one of the reasons that hundreds, if not thousands, of cell lines will eventually be needed. Another is that every ESC line is derived from a clonal
population. So even though they are totipotent, they may still have special properties, and different lines may have different special properties. Most cells
tested do not end up forming stable cell lines. For example, James Thompson, who was the first to report on establishment of human ESC lines in 1998, started
with 36 donated embryos, from which he cultured 20 blastocysts, of which 14 were selectable, of which 5 formed viable cell lines. (I assume these are 5 of the
60 that Bush mentioned.)
A brief word about the alternative to embryonic stem cells, adult derived stem cells. These are even more difficult. They are rare and hard to isolate. They are
also picky and don't last nearly as many passages. They have a limited ability to differentiate. They very well may prove to be
useful though, and research should certainly proceed in this area.
I have not been able to find any tally of available cell lines in the scientific
literature, but I read that GW Bush might have been privy to proprietary information.