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 types of cells that do the jobs that keep people alive. Without stem cells like you, embryos couldn’t develop, and adults, unable to replenish blood and tissue, would soon die. Not bad for a generic little cell.
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, in a process called mitosis. This process will repeat over and over throughout embryonic development, infancy, and childhood. It will slow down a little for adults, but it will never stop. The end result is a human body made up of 10 to 100 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 4-foot-wide basketballs of undifferentiated flesh, and stem cells make it happen.
No one 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 cells like themselves. They just can’t differentiate into other cell types. They generally do contain a full set of DNA, coding a complete you. Yet specialized cells express only some of the genetic information they contain, just what they need to perform their specific job.
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 person. If one of these cells splits off from another, it can grow into a fully formed, separate, but genetically identical twin. Such cells exist only for a few short days after conception. After that, the totipotent cells will have divided into somewhat specialized cells that can’t produce a person on their own.
These somewhat specialized cells fall into the second stem cell category: pluripotent cells. 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. Yet like totipotent cells, pluripotent cells don’t last long. As cell differentiation continues, they give way to the last stem cell type: multipotent.
Multipotent stem cells are further specialized cells that can grow into only a few cell types. For example, multipotent cells in your bone marrow continually make red blood cells, white blood cells, and platelets. Although multipotent cells have lost most of their ability to differentiate into various cell types, they do have one distinct advantage. They exist throughout life, replenishing the cells you need to live.
Scientists have known about stem cells, and their role in embryonic development, for some time. But no one 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 a pluripotent state, preserving their ability to become many other cell types.
Scientists have been working to build on this breakthrough ever since. Some researchers 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. Still others are trying to implant stem cell-derived cells and tissues into patients in a way that allows them to regain lost abilities or lost organs.
All this research can involve 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 stem cells of embryonic development. Researchers obtain these from the inner layer of a blastocyst, in a process that out of necessity destroys the embryo. It’s this process that sparks most of the current debate on stem cells, since many people regard destroying an embryo as killing a human being.
Because embryonic stem cells can give rise to the greatest variety of cell types, most experts say they hold the greatest hope for medicine. Research into the benefits of adult stem cells has so far produced mixed results. Some studies indicate that adult stem cells may be induced to become something more than multipotent, expanding the range of cells they could produce. Other research contradicts that finding.
No one knows what future stem cell research will reveal. Yet many researchers have seen enough from the studies done so far to think that stem cells might revolutionize medicine. They hold out hope that paralyzed people might walk, that the blind might see, or that lifetime diabetics might never again need insulin needles. Stem cell science, and the ethical debate over it, is just getting started.