apply the technology to developing treatments for diabetes in humans. Animal studies can also reveal how human cells differentiate during normal development. For example, scientists may implant human stem cells into a developing mouse to observe the processes involved in building and organizing the different tissue types that make up the human body. Scientists can also trace the development and progression of certain diseases within an animal. By implanting human stem cells that lead to a particular disease into a mouse blastocyst, scientists can observe when and how the afflicted cells begin to show signs of disease and can test drugs that might prevent that process. Organisms that contain cells or tissues from another individual of the same or a different species are called chimeras. A common example of a chimera is a mouse that has been injected with some human cells so that it can be used for studying a human disease or testing a new drug. A person who has had a blood transfusion or a person who has received a heart valve transplant from a pig is technically a chimera, as well. The making of chimeras for research has unique ethical implications that have been the topic of discussions among scientists, ethicists and the public, especially when the chimeras contain both human and animal cells. 11 WORKING WITH STEM CELLS Many research mice are chimeras because they contain both human and mouse cells. Courtesy of Advanced Cell Technology, Inc., Alameda, CA. Alternatives to Using Embryos in Stem Cell Research To address ethical concerns about the destruction of blastocysts, scientists are trying to find new ways of obtaining stem cells that behave like embryonic stem cells but that don’t require harming a blastocyst. As the science progresses, ethical issues surrounding these alternatives may also arise. Some possible alternatives include: • Cells collected from the morula (MOR-yoo-la), the developmental stage prior to the blastocyst. The morula, a solid ball of about 16–30 cells, seems able to sustain the loss of a few cells without developmental damage so that the remaining cells can continue to develop. Cell extraction from the morula is already being used in some clinics to screen for genetic disorders in embryos produced by in vitro fertilization. Researchers have recently shown that cells isolated from a mouse morula can give rise to embryonic stem cells while the remaining morula cells develop into a healthy mouse. However, this process may still be morally objectionable to some because of the chance of harm to the morula, and because the long-term effects of removing cells from a morula are not yet known. • The creation of embryonic stem cells through a process called altered nuclear transfer (ANT). In this variation of the nuclear transfer technique, scientists create a blastocyst whose genetic material has been changed so that further development and implantation into the uterus is not possible. It aims to create embryo-like entities that are not truly embryos but that can be a source of pluripotent stem cells. ANT, so far only tested with mouse blastocysts, could allow the creation of embryonic stem cells without destroying a viable human blastocyst. Some who object to embryonic stem cell research support ANT because the resulting blastocyst could never develop into a full human being and therefore would not have the moral status of a human embryo. However, this procedure is objectionable to some because they believe that it involves the creation of an imperfect blastocyst that is designed to be destroyed. • Causing an adult cell to act like an embryonic stem cell. During development, as cells become more and more specialized, they gradually lose the ability to turn on the genes that allow embryonic stem cells to be so versatile. The silencing of these genes seems to be responsible for keeping specialized cells specialized and limiting the differentiation capacities of adult stem cells. By “reprogramming” adult stem cells so that they can turn on the genes that allow versatility, scientists hope to cause them to revert to a more flexible state. It is even possible that scientists could one day “reprogram” any cell, not only stem cells. However, research in this area is in the early stages and scientists may be many years away from making an adult cell as versatile as an embryonic stem cell. 12 WORKING WITH STEM CELLS Courtesy of Leonard I. Zon 13 Right now, only a few diseases are treatable with stem cell therapies because scientists can only regenerate a few types of tissues. However, the success of the most established stem cell-based therapies—blood and skin transplants—gives hope that someday stem cells will allow scientists to develop therapies for a variety of diseases previously thought to be incurable. Many major diseases are caused by the loss of a single type of cell or tissue. For example, type I diabetes (juvenile-onset) is caused by the loss of the insulin-producing cells of the pancreas, and its treatment is limited to merely alleviating the symptoms. Finding a cure for such diseases would be much easier if scientists could simply re-grow the missing or damaged cells and implant them into patients. Blood Stem Cells After scraping a knee or donating blood, the body replenishes the blood cells that are lost by drawing on a small number of semi-specialized hematopoietic (heem-AT-oh-poh-EH-tik) stem