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Stem cells are broadly classified as either adult or embryonic. Technically, even stem cells that come from fetal tissue or umbilical cord blood are classified as adult stem cells, and so most researchers prefer the term tissue stem cells for all stem cells other than those from embryos.
Embryonic stem cells can, in theory, produce any type of tissue in large quantities. Researchers use these cells to study development and disease and, hopefully, to find treatments.
In the near term, stem cells that are genetically identical to someone with, say, diabetes or a neurodegenerative disease can be used to study that disease. Eventually, genetically matched cells could mean that patients receiving cell therapies wouldn't need antirejection drugs.
Scientists want to have a variety of stem cell lines so that they can pick the best ones for their experiments. Some lines are better suited for becoming pancreatic cells, others for neurons (no one is sure why). Moreover, older cell lines are harder to grow, and many contain mutations and chromosomal abnormalities.
Difficulties in growing stem cells stymie research. Consistent conditions for growing stem cells will mean that more informative experiments can be performed in less time, and also that experiments done by scientists in different laboratories can be compared more easily.
Scientists are working on ways to make embryonic stem cells without destroying embryos and to make cells that behave like embryonic stem cells. Even if these techniques work, many scientific and ethical issues remain unresolved.
An adult human contains trillions of cells of more than 200 types. All these cells (plus the many, many more cells that are shed throughout life) can be traced back to the fertilized egg, the one cell that can, ultimately, create every type of cell in the body.
Stem cells know when and how to act through 'conversations' with other cells. An ecosystem of surrounding cells — a stem-cell niche — both keeps stem cells in their self-renewing state and guides their differentiation into specialized cells.
Human embryonic stem cells are made from 4- to 6-day-old embryos that have been created in laboratories, usually fertility clinics. The inner cells from the ball-shaped embryos are isolated and placed in a dish along with the nutrients they need to grow.
Therapeutic cloning creates a line of embryonic stem cells genetically identical to an individual. Reproductive cloning creates a new organism genetically identical to an individual.
Stem cells could help medicine in three general ways: cell-based therapies, drug discovery and basic knowledge. Cell therapies would use stem cells, or cells grown from stem cells, to replace or rejuvenate damaged tissue. Scientists also want to use stem cells to understand disease and find drugs that might treat it.
Like some stem cells, cancer cells can grow without pause. Some cancers use stem cells' tricks to do this, and so some cancer researchers study stem cells.
Some opponents of stem cell research argue that it offends human dignity or harms or destroys human life. Proponents argue that easing suffering and disease promotes human dignity and happiness, and that destroying a blastocyst is not the same as taking a human life.
Policies vary by country, by region and even by university. They govern whether research can be performed or funded on human embryos themselves and cells made from human embryos. Other policies also require researchers to show that certain stem cell therapies are likely to be safe before testing them in people.
Bone marrow transplants containing blood stem cells are used routinely for blood diseases such as leukemia. Almost all stem cells currently in clinical trials are from the blood and bone marrow.
