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Some Stem Cell Science
Stem cells under the microscope
Last week, a team of South Korean scientists announced that they have used cloning techniques to create human embryos in the lab--embryos that, for the first time ever, survived long enough in the petri dish for scientists to harvest their stem cells. It's the latest breakthrough in stem cell science, and unlike the "science" behind several recent cockamamie cloning claims, experts say the work looks real.
The research might even set the stage for a revolution in medicine, one in which your doctor cures what ails you by designing cells and tissues tailored to your genetic identity. How would that work? It's complicated, but you'll get the idea just by learning some stem cell science.
Generic Little Stem Cells
Imagine you're a stem cell. As cells go, you're not very sexy. Your cellular chums even call you "generic." You don't secrete hormones, form protective layers, digest food, or otherwise perform an immediately productive role in the body. But that doesn't mean you're a freeloader.
Like little factories, you and the rest of the stem cell clan produce all of the 220 other types of cells that do the jobs that keep folks alive. Without stem cells like you, embryos couldn't develop, and adults, lacking the ability to replenish tissues like blood and skin, would soon die. Not bad for a generic little cell.
Cellular Destiny
In fact, generic little stem cells shape everyone's cellular destiny. Every human begins as a single cell, the zygote. Almost immediately after its creation, the zygote begins to divide itself, in a process of cellular division called mitosis. This process will repeat over and over throughout embryonic development, infancy, and into childhood. It will slow down a little for adults, but it will never stop. The end result is a human body made up of roughly 10 trillion cells.
This amazing cell division usually works like a biological Xerox, making copy after copy of the original cell. Yet in some cases, a biological fine-tuning occurs, whereby the dividing cell--a stem cell--doesn't make copies of itself but instead gives rise to a different-looking cell that performs some specialized function. This differentiation is what keeps us from growing into four-foot-wide basketballs of undifferentiated flesh, and what distinguishes each species from another. And stem cells make it happen.
Nobody knows exactly how stem cells pull off this neat trick. When specialized cells--skin cells, muscle cells, bone cells, and others--divide, they give rise only to other cells of the same kind. They just can't differentiate into other cell types. They generally do contain a full set of DNA, coding a complete you (though a few types, like red blood cells, contain no DNA at all). Yet specialized cells express only some of the genetic information they contain, just what they need to perform their specific role.
Potent Stuff
Scientists recognize three different types of stem cells: totipotent, pluripotent, and multipotent. Totipotent stem cells, as the name suggests, have total potential: the ability, given the right conditions, to grow into a complete embryo. They exist only during the first few days following conception. If one of these cells splits off from the others, it can grow into a fully formed separate but genetically identical twin. After those first few days in embryonic development, though, these cells have divided into more specialized cells that just can't produce an entire adult on their own.
These somewhat specialized cells fall into the second stem cell category: pluripotent cells. These cells can produce the many kinds of different cells in the body, but can't make the placenta and other supportive cells necessary to grow a complete embryo. The pluripotent stem cells exist in the inner layer of a small ball of about 100 cells called a blastocyst. They can grow to become the hands, feet, digestive system, and other complex parts of the human body, each comprising many cell types. Yet like totipotent cells, pluripotent stem cells don't last long. As cell differentiation occurs during fetal development, they give way to the last stem cell type, multipotent.
Multipotent stem cells are further specialized cells that can grow into only a few types of cells. For example, the blood stem cells that reside in your bone marrow continually produce red blood cells, white blood cells, and platelets. Although multipotent stem cells have lost most of their ability to differentiate into various cell types, they do have one distinct advantage. They exist throughout life, continually producing cells even into late adulthood.
Medical Miracles
Scientists have known about stem cells, and their role in embryonic development, for some time. But nobody realized how useful they might be until 1998, when a team of researchers led by James Thomson at the University of Wisconsin successfully cultured human embryonic stem cells in the lab. Not only that, Thomson's team maintained the cultured cells in their pluripotential state, preserving their ability to become other cell types.
Ever since, scientists have been working to build on this breakthrough. Some are trying to harvest different types of stem cells and culture them in large, medically useful numbers. Others are trying to discover the various hormones and developmental signals that cause stem cells to differentiate into specific cell types. By perfecting these procedures, scientists hope to make replacement cells and tissues available for patients at a moment's notice, perhaps even culturing entire organs and eliminating the long wait many transplant patients now endure.
Still other scientists have been trying to implant stem cell-derived cells or tissue into recipients in a way that allows them to regain lost abilities. Early research has been impressive. Stem cell therapy for patients with Parkinson's disease has led to partial recovery of lost function. Blood stem cell transplants have caused a complete remission in an otherwise fatal case of lupus. And neural stem cells have partially restored motor function lost through spinal cord injury and Lou Gehrig's disease--in rodents, at least.
Controversial Research
Blastocyst blast-off
All of this research involves a variety of sources and subjects. It can involve either human or animal cells and either embryonic or adult stem cells. Adult stem cell research uses multipotent stem cells that researchers can extract from practically anyone. Embryonic stem cell research uses the pluripotent cells of early fetal development. Researchers obtain these from the inner layer of a blastocyst, in a process that out of necessity kills the embryo. It is this process that sparks current debate on the stem cell issue.
Most scientists believe that the greatest promise lies in research with embryonic stem cells. Because of their pluripotent abilities, they can give rise to the greatest variety of cell types, and therefore hold the greatest hope for medicine. Understanding how these stem cells work could also lead to earlier diagnosis, better treatment, and possible cures of congenital diseases and developmental disorders, a benefit that adult stem cells cannot provide.
Research exploring the benefits of adult stem cells has produced mixed results. Some studies indicate that adult stem cells may be induced to become something more than multipotent, expanding the range of cells that they could produce. Yet other research contradicts that finding. Scientists do know that if adult stem cells were taken from an individual, then tissue cultivated from them could be used on that individual without the possibility of immune rejection, a major complication in many transplant procedures. They also know that many key adult stem cell types, like cardiac muscle and insulin-producing pancreatic stem cells, have yet to be found in humans, and that others, like neural stem cells in the brain, could prove impossible to extract in an undamaging way.
No one knows what future stem cell research might reveal. Yet many researchers have seen enough from the studies done so far to think that stem cells might revolutionize medicine, holding out hope, for example, that paralyzed people might walk, that the blind might see, or that lifetime diabetics might never again need an insulin needle. The most amazing stem cell discoveries, and perhaps the most aggressive ethical debates about them, are yet to come.
Christopher Call
updated February 16, 2004
Information via KnowledgeNews
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02-17-2004 01:01 PM
