What Are Stem Cells? How They Grow, Divide, and Specialise

Most cells in your body already have a fairly specialised job. A red blood cell carries oxygen, a muscle cell contracts, and a neuron helps transmit information. Stem cells are different because they are less specialised and can divide to produce new cells. Some of their descendants remain stem cells, while others begin a process called differentiation and develop particular structures and functions.

The answer to what stem cells are also depends on the type of stem cell. Some have a very broad ability to produce different cell types, while others have a more limited range. Scientists describe this developmental potential with terms such as pluripotent and multipotent. The important idea is that not every stem cell can become every kind of cell.

During early development, stem cells provide the starting material for the many specialised tissues of the body. As development continues, cells receive chemical signals that switch particular genes on or off. These changes guide cell differentiation, helping one group of cells develop differently from another even though they contain essentially the same DNA.

Adult stem cells remain in several tissues after development. For example, stem cells in bone marrow regularly produce new blood cells. Other tissues have their own populations of cells that help replace cells lost through normal wear or injury. When you study stem cells, it helps to connect their ability to divide with this ongoing need for replacement.

Embryonic stem cells are pluripotent, meaning they can develop into cells from many different body tissues. Adult stem cells are usually more restricted. Hematopoietic stem cells in bone marrow, for example, produce the different types of blood cells rather than every cell type in the body. Scientists can also create induced pluripotent stem cells by reprogramming certain specialised adult cells.

Research on stem cells is useful because these cells can help scientists study development, genetic disease, and possible ways to replace damaged tissue. Stem cell treatments already have established uses in some blood and immune disorders, while many other proposed uses are still being tested. A promising laboratory idea is not automatically a proven treatment, so evidence from careful clinical research matters.

The oldest stem cell therapy is the bone marrow transplant, used since the 1960s to rebuild the blood and immune system after high-dose treatment for leukaemia, and it remains the clearest success. Skin grown from a patient's own stem cells is used for severe burns, and stem cells from the edge of the cornea can restore sight after chemical injury to the eye. Beyond those, most of the field is still in trials: retinal cells for macular degeneration, dopamine-producing neurons for Parkinson's disease and insulin-producing cells for type 1 diabetes have all reached early human studies with promising but unproven results.

The gap between trial and treatment matters because clinics around the world sell unproven stem cell injections for arthritis, autism and spinal injury at high prices, and regulators have recorded serious harm from some of them. A useful test is whether a treatment is offered inside a registered trial or a licensed hospital service. If it is sold on a website with a testimonial, it is not yet medicine.

For a long time many biologists assumed that once a cell had specialised, its fate was sealed. In 1962 John Gurdon challenged that idea. He took the nucleus from a specialised intestinal cell of a frog, placed it into an egg cell whose own nucleus had been removed, and some of these eggs developed into tadpoles. The nucleus of a specialised cell still held all the information needed to build a whole animal. Decades later, the same principle allowed scientists to clone Dolly the sheep in 1996.

In 2006 Shinya Yamanaka and his team showed something even more surprising. By adding just four genes that control development to ordinary skin cells from mice, they could reprogramme the cells into a pluripotent state, and in 2007 they and another group did the same with human cells. These induced pluripotent stem cells avoid using embryos and can be made from a patient's own tissue. Gurdon and Yamanaka shared the 2012 Nobel Prize in Physiology or Medicine for showing that specialisation can be reversed.

Stem cells are busiest where wear is heaviest. The lining of your small intestine faces constant chemical and mechanical stress, and its surface cells are replaced every few days by stem cells tucked into small pits called crypts. Red blood cells last about 120 days, so bone marrow must make millions of new ones every second just to keep pace. Skin renews its outer layer over a few weeks. Other tissues are far slower, and most of the neurons in your brain are as old as you are.

Measuring the slow cases needed an unusual clock. Nuclear weapons tests in the 1950s and early 1960s sharply raised the amount of carbon-14 in the atmosphere, and the level fell steadily once testing above ground largely stopped. Because a cell takes up carbon when it builds new DNA, the carbon-14 in its DNA records roughly when that cell was born. Using this method, researchers found that heart muscle cells in adults are replaced only very slowly, around one per cent a year or less, which helps explain why a damaged heart repairs itself so poorly.

One of the most productive recent uses of stem cells does not involve putting them into patients at all. In 2009, researchers in the Netherlands led by Hans Clevers showed that a single stem cell from a mouse intestine, grown in a gel with the right chemical signals, could build a tiny hollow structure with the folds and crypts of real gut lining. These organoids, usually no more than a few millimetres across, organise themselves by following the same developmental instructions they would use in the body. Similar methods now produce organoids resembling stomach, liver, kidney and, from pluripotent cells, simplified parts of the developing brain.

Because organoids can be grown from an individual patient's cells, they make it possible to test treatments on that person's tissue before trying them on the person. Cystic fibrosis provides a striking example. Organoids grown from a small rectal biopsy swell up when a test chemical activates the chloride channel that the disease disrupts, but only if the channel works. Measuring how much they swell with different drugs helps predict which medicines will help that patient. Brain organoids have been used to study why the Zika virus caused abnormally small heads in some babies, by showing how the virus damages developing neural stem cells.

Stem cells are cells with two important abilities: they can produce more stem cells and they can give rise to specialised cells. Different stem cells have different levels of developmental potential, so learning the type of stem cell is just as important as learning the general definition.