Name: __________________________________________________ Date: ___________ Period______ Stem Cells Reading - Adapted from Science News for Students In recent years, scientists have made an amazing discovery. When most other cells divide, the daughter cells look and act exactly like their parents. For example, a skin cell can’t make anything but another skin cell. The same is true for cells in the intestine or liver that have specific jobs in the body. But a special family of cells — called stem cells — can become many different types of cells in the body. Adult Stem Cells Consider red blood cells: Inside your body, red blood cells are constantly on the move. They deliver oxygen to every tissue in every part of your body. These blood cells also cart away waste. So their work is crucial to your survival. But all that squeezing through tiny vessels is tough on red blood cells. That’s why they last only about four months. Where do their replacements come from? Stem cells. Stem cells can replace worn-out or damaged cells — including red blood cells. Adult stem cells are inside your body right now. For example, blood stem cells live inside your bones, in what is called marrow. There, they divide over and over. Some of the new cells remain stem cells. Others form red blood cells. Still others morph into any of the five types of white blood cells that will fight infections. Although blood stem cells can become any one of these specialized blood cells, they cannot become muscle, nerve or other types of cells. These multipotent stem cells are too specialized to do that. Embryonic Stem Cells Another type of stem cell is more generalized. These can mature into any type of cell in the body. Such stem cells are called pluripotent. The word means having many possibilities. Until recently, all pluripotent cells came from embryos. That’s why scientists called them embryonic stem cells. In the first few days of this embryo’s development, each of its cells is identical to all the others. Yet each cell has the potential to develop into any specialized cell type. Think about it this way: For maybe a day, about 9 months before you were born, you were just one cell. Then you were two identical cells. Then you were four. Since then, you've grown into a complicated organism with many trillions of cells grouped into specialized tissues and organs. The cells in your brain do the thinking. The cells in your heart pump blood. The cells in your tongue let you taste food. And so on. Pluripotent stem cells can mature into any type of cell in the body. Embryonic stem cells can become many different types of cells. That is how an embryo grows from a single fertilized egg into a fetus with trillions of specialized cells. They need to specialize to make up tissues that function very differently, including those in the brain, skin, muscle and other organs. When the human embryo reaches three to five days old, its cells start to realize their potential. They Name: __________________________________________________ Date: ___________ Period______ specialize. Some will develop into muscle cells or bone cells. Others will form lung cells — or maybe the cells lining the stomach. Once cells specialize, their “many possibilities” suddenly become limited. By birth, almost all of a baby’s cells will have specialized. Each cell type will have its own distinctive shape and function. For example, muscle cells will be long and able to contract, or shorten. Red blood cells will be small and plate-shaped, so they can slip through blood vessels with ease. Hidden among all of these specialized cells are pockets of adult stem cells. (Yes, even newborns have “adult” stem cells.) Unlike embryonic stem cells, adult stem cells cannot transform into any and every cell type. However, adult stem cells can replace several different types of specialized cells as they wear out. One type of adult stem cell is found in your marrow, making new blood cells. More types are found in other tissues, including the brain, heart and gut. Stem Cell Research and Medicine The search is on for ways to use stem cells to treat injuries and cure diseases. For example, stem cells could be extracted, turned into new bone cells, and then injected into weak or broken bones. Or, they could become nerve cells that could heal spinal cord injuries, skin cells that could replace badly burnt skin, or brain cells that could help people who have suffered brain damage. The possibilities are endless. Stem Cell Ethics As promising as the research may seem, discussions about stem cells often involve more than just science. Ethics is also involved, along with politics and religion, especially when it comes to stem cells taken from embryos. To repair heart muscle in a mouse, researchers inject adult stem cells into the muscle of the damaged wall of a mouse heart. So far, embryonic stem cells appear to be more useful than stem cells that come from adults. Because an embryo's cells are still dividing and specializing anyway, its stem cells can still become almost anything. By the time we grow up, however, our stem cells have a more limited ability to diversify. The problem with embryonic stem cells, for some people, is that they originally come from destroyed embryos. Many scientists argue that stem cells are our best hope for curing a huge number of diseases. Nevertheless, critics think it’s wrong to use cells from embryos. It's a very complicated issue that involves basic beliefs about when life begins, and these are the types of beliefs about which people tend to feel passionate. Scientists are researching numerous stem cell techniques as alternatives to using embryonic stem cells. Thees include using umbilical cord blood and a new technique called induced pluripotent stem cells (iPS) in which adult cells are “tricked” into acting like embryonic stem cells. All this may sound a bit confusing, but it's worth learning more. Stem cells are big news in medicine right now. "I don't think a day goes by when there aren't articles or something on the Web about it," Friedman says. As you get older, you're bound to hear more and more about stem cells. Name: __________________________________________________ Date: ___________ Period______ Stem Cells Reading Questions 1. What are stem cells? How are they different from body cells like your skin cells or liver cells? 2. What is different about multipotent adult stem cells and pluripotent embryonic stem cells? 3. Explain in at least two complete sentences how stem cells help explain how a single fertilized egg can develop into a complex human baby? 4. How do you think stem cells, gene expression, and cell differentiation are related? Answer in at least two complete sentences. 5. What do you think “regenerative medicine” is? How might it relate to stem cells? 6. Summarize the ethical issues surrounding the use of stem cells in research and medicine. 7. List at least two questions you have about stem cells.
