1. Story map
3. Latest news
4. All grown up
5. Making sense
A cell that can change into other types.
Embryonic stem (ES)
Cell from embryo, can become any cell
Adult stem cell: Stem cell from mature animal, often from
of becoming more specialized.
Biologists have long known that some cells must
be changing form -- dividing into more specialized cells -- as a
multi-cellular organism grows from a fertilized egg to the billions
of cells in a mature naked mole rat or developmental biologist.
Some of the first stem cells, discovered in bone marrow, differentiate
into all kinds of blood cells. Though they struggle with an unpronounceable
handle, these "hematopoietic" stem cells are the active ingredient
in a popular cancer treatment, the bone marrow transplant.
Hemapoietic stem cells, derived from the bone
marrow, give rise to all types of blood cell.
(Improved techniques now allow bone marrow transplant doctors to isolate hematopoietic stem cells from blood. That's much easier on the donor than the older procedure, poking dozens of holes in bones to extract marrow. In other words, adult stem cells already have a clinical use.)
In the years since the 1998 announcement on human ES cells, researchers have described startling results from two other types of adult stem cells, also originating in bone marrow. Mesenchymal stem cells seem able to form specialized cells in the nervous system, muscle, liver, kidney and elsewhere. (We'll return to the third type on the next page.)
The idea that adult stem cells were so flexible
flouted biology's conventional wisdom, since healthy cells were
considered to be trapped on a one-way path toward greater differentiation.
Healthy cells, in other words, don't get less specialized -- they
get more specialized. (Cancer cells, however, do get more primitive
as they reproduce. For more on the issue, see "Flexible Arrangement
..." in the bibliography).
A second reason for the widespread skepticism
about adult stem cells is the political backdrop: Many who oppose
embryonic stem cell research on ethical or religious grounds would
love to find a less-objectionable replacement cell. ("Politicians
are already keenly interested in results from Catherine Verfaillie,
director of the Stem Cell Institute at the University of Minnesota --
as a way to put ES cells out of business," reported Science last
year.) Verfaillie, who, as we'll see shortly, has assembled good
evidence for change, or plasticity, in adult stem cells, told the
legislators it was "too soon to draw any conclusions" (see "Plasticity..."
in the bibliography).
cord neurons developed when ES cells were cultured for five days.
The differentiation into neurons was monitored using antibodies,
and by profiling the expression of genes and proteins. Neurons also
may be derived from adult stem cells. Photo:
K. Murashov, East Carolina University School of Medicine.
At any rate, as the adult cell wave crested, researchers discovered that some of the stem cells were not becoming specialized cells but rather fusing with them. That meant the markers on the outside of specialized cells, which supposedly proved their descent from adult stem cells, actually indicate that the cell was a hybrid, with two nuclei.
Fusion was just one source of confusion. Researchers have also attributed some erroneous markers erroneous markers to mutations. And when a cell takes up DNA from another cell, as some immune cells do while "eating" foreign cells, they will contain foreign DNA, at least temporarily.
Can anybody make sense
of human stem cell research?