Stem Cells

A stem cell is a special kind of cell with two amazing abilities: it can divide and make more copies of itself, and it can turn into different types of cell when needed. Stem cells are how your body grew from a single fertilised egg into all the different cell types you now have. They are also how you heal a cut, grow new hair, and constantly replace the cells of your skin, blood and gut. Modern medicine is just starting to use stem cells to treat illness, and the field is one of the most exciting areas of biology today.

  • Two main typesEmbryonic, adultPlus newer induced pluripotent (iPS) cells
  • Most powerfulEmbryonic stem cellsCan become any cell in the body
  • Most common adult sourceBone marrowSource of all blood cells
  • Discovered1961In bone marrow of mice
  • First human bone marrow transplant1956Treated a leukaemia patient
  • Nobel Prize for iPS cells2012Shinya Yamanaka (Japan)

What makes stem cells special

Most cells in your body are specialised: a muscle cell can only ever be a muscle cell, a skin cell can only ever be a skin cell. They cannot change job, and most of them stop dividing. Stem cells are different. They keep all their options open. A stem cell can divide many times without committing to a particular job, and when the body needs a new specialised cell (a fresh skin cell, a new red blood cell, a replacement gut lining cell), nearby stem cells differentiate, turning into exactly the right type of cell to do the job.

The two main types

  • Embryonic stem cells: found in very early embryos (just a few days after a fertilised egg starts dividing). They are pluripotent, meaning a single one can grow into any of the 200+ cell types in the human body. Every cell in your body is descended from these embryonic stem cells.
  • Adult stem cells: found in small numbers in many parts of the adult body (bone marrow, skin, gut, brain and more). They are more limited: each kind can only turn into a few related cell types. The stem cells in your bone marrow, for example, can become any kind of blood cell, but they cannot turn into a brain cell or a muscle cell.

Stem cells in everyday life

Stem cells are working hard in your body right now. The lining of your gut wears out and is completely replaced about every 4 days; the new cells come from gut stem cells. The outer layer of your skin is completely replaced about every month. The cells in your bone marrow are constantly producing new red blood cells (about 2 million per second), new white blood cells and new platelets. Without stem cells, your body could not maintain itself, and you would not last a week.

Stem cells in medicine

Doctors have actually been using stem cells for a long time without always realising it.

  • Bone marrow transplants (first done in 1956 to treat leukaemia) replace a patient's blood-making system with healthy donor stem cells.
  • Skin grafts use the patient's own skin stem cells to grow new skin for burn victims.
  • Cord blood banking: blood saved from a newborn's umbilical cord is rich in stem cells that can be used later for medical treatments.

Newer techniques are even more exciting. Scientists have already grown small "organoids": tiny lab-grown versions of organs (mini-livers, mini-brains, mini-kidneys) from stem cells. These are being used to study diseases and test new drugs without needing animal experiments.

Fact In 2006, a Japanese scientist called Shinya Yamanaka discovered how to turn ordinary adult skin cells back into stem cells by adding just four chemical factors. These reprogrammed cells, called induced pluripotent stem cells (iPS cells), behave like embryonic stem cells but can be made from a tiny skin sample. It was a revolutionary discovery: it solved many of the ethical concerns about embryonic stem cells, and it earned Yamanaka the 2012 Nobel Prize in Physiology or Medicine.

The future of stem cells

Stem cell research is one of the most active areas of modern medicine. Researchers are working to use stem cells to:

  • Regrow damaged nerves after spinal cord injuries.
  • Replace dying brain cells in Parkinson's disease.
  • Build new heart muscle after a heart attack.
  • Restore eyesight by replacing retinal cells.
  • Grow whole organs for transplant, removing the need for donor waiting lists.

Many of these treatments are still in clinical trials, but a few are already routine. Stem-cell-based skin grafts have been saving the lives of severe burn victims for over 40 years.

Did you know? When you give a small amount of blood, the stem cells in your bone marrow start working extra hard to replace the missing red blood cells. Within a few weeks the donation is fully restored. You can safely give blood every 3 to 4 months because your stem cells are so efficient at making more.
Deeper dive: the ethics of stem cell research

Stem cell research has been one of the most ethically debated areas of modern science. The reason is that the most powerful stem cells (embryonic stem cells) historically had to be taken from a very early human embryo, usually a few days old, which destroys the embryo in the process.

Most embryos used for stem cell research are surplus embryos from IVF (in-vitro fertilisation) treatment, which would have been thrown away anyway. But many people (especially some religious groups) believe that destroying a human embryo at any stage is morally equivalent to ending a human life. Others see early embryos (which are just a small cluster of cells, with no nervous system, organs or awareness) as fundamentally different from a born baby. The debate has been intense, has shaped laws in many countries, and continues today.

Yamanaka's 2006 discovery of induced pluripotent stem cells (iPS cells) eased much of the controversy. iPS cells are reprogrammed from a small sample of an adult's own skin, so no embryos are needed at all. They have the same power as embryonic stem cells but raise none of the same ethical questions. Today, most cutting-edge stem cell research uses iPS cells. Yamanaka's technique is one of the most important biological discoveries of this century.

For more on cells, see what is a cell. For how ordinary cells divide, see mitosis.