Medicine's Holy Grail
Page 2: What they are
Saturday, August 18, 2001
When a sperm fertilizes an egg, the resulting single cell begins to divide and multiply at a staggering rate.
Among these are what scientists call "totipotent" stem cells. These primordial, embryonic cells have the potential to grow into a complete human being and all its parts.
Within days of fertilization, these new and dividing cells form a hollow sphere. Those on the outer layer give rise to things such as the placenta and other membranes to surround the embryo. Stem cells arise in the inner mass of the young embryo, which scientists call a blastocyst.
The blastocyst is a barely visible dot, having no features of an embryo or fetus. The stem cells within are considered "pluripotent" - they can divide indefinitely and blossom into all the various tissue types of the human body, but they have the lost the totipotent ability to grow into a separate being.
After roughly 14 days, the first stirrings of a nervous system develops as a primitive streak appears down the length of the new being that will eventually grow into the spine. At this stage, the stem cells within the embryo are considered "multipotent." They can grow into some tissues, but not all. Those poised to become bone or blood, for example, cannot form stomach or skin.
There are thought to be three ways to extract stem cells from human embryos: embryos can be created for medical research, as Britain will allow; used after a fertility clinic deems them surplus, which Canada is expected to sanction with new legislation; or they might be cloned, the way scientists created Dolly the sheep. No one is known to have tried this last method.
Scientists suspected that stem cells also existed in adult tissues, since muscles, skin, hair and blood regenerate and work as the body's natural repair mechanism.
But conventional wisdom suggested that these stem cells in a mature body were not multipotent, lacking the versatility of embryonic stem cells. If stem cells were found in muscle, for example, they had already received chemical cues and were destined to become only muscle cells.
But in April of 1999, researchers at a Baltimore biotechnology company called Osiris Therapeutics reported that they had collected stem cells from adult human bone marrow, the spongy tissue inside bones that produces the body's blood cells. They said they had chemically coaxed the stem cells to become not only blood cells, but also fat, cartilage and bone cells.
The discovery meant that it might be possible to reprogram adult stem cells, an appealing option given the ethical controversy of experimenting on human embryos. Over the next two years, scientists learned that adult stem cells are more multipotent - having more plasticity - than anyone had ever imagined.
"The more data we get that says stem cells from bone, brain and skin can do the job, the less you will need embryonic stem cells and then the controversy goes away," said Ronald Worton, head of the Stem Cell Network in Canada and scientific director of the Ottawa Research Institute.
What's more, adult stem cells have been found in unusual places: in the fat people have liposuctioned from their bellies and the layer of skin that lies beneath the scalp.
Adult stem cells in bone marrow have been manipulated to grow into kidney, liver and nerve cells. Blood has morphed into brain cells and brain into blood cells. This week, researchers at McGill University in Montreal revealed that they had harvested stem cells from the skin of mice and humans that are capable of growing into complex brain cells, smooth muscle and fat.
As the possibility of regenerating tissue moves out of the realm of science fiction and into the lab, scientists have barely been able to contain their excitement.
Next page: The breakthrough