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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.

Even non-embryonic stem cells might be able to produce many types of specialized cells, but researchers are still trying to work out the details.

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.

That the depletion of stem cells as we age may be a consequence of DNA damage has been hinted at. New studies now show that DNA repair is needed to keep our blood stem cells in good working order as we age.

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.

On what theological grounds would one protect the line between species?

Variation contributes to embryo loss.

A technique to transform specialized cells could pave the way to create patient-specific “embryonic” stem cells, without requiring eggs or embryos.

Sox2, a protein known to be necessary to keep embryonic stem cells from differentiating, does so by an unexpected mechanism.

A clinically approved kinase inhibitor can stop human ES cells from clumping, and should make them easier to grow.

Dolly the sheep came not from the union of sperm and egg but from the mammary cell of one sheep and the unfertilized egg of another. Her birth, more than 10 years ago showed that nuclei from specialized adult cells can be reprogrammed into all the cells of an organism.

Much of the work that had previously convinced scientists that only oocytes, and not zygotes, could be used for cloning came from very careful studies by McGrath and Solter in the 1980s^1,2. A few weeks before Kevin Eggan's paper³ was made public, Nature Reports Stem Cells tracked down Davor Solter to learn his thoughts.

Unfertilized eggs have long been the limiting resource for attempts to make genetically tailored human embryonic stem cells. If a new technique for cloning mice from fertilized eggs works in humans, they might not be necessary.