Stem cells — AQA GCSE Combined Science

Stem cells have different roles depending on where they are found. In an embryo, stem cells are pluripotent: they can differentiate into almost any cell type, so they build all the tissues and organs of the developing organism. In adult animals, stem cells are found in tissues such as bone marrow and are used to replace cells that are worn out or damaged, for example making new blood cells throughout life. In plants, stem cells sit in meristems at growing tips of shoots and roots and in cambium; they divide throughout life to produce new plant tissues, allowing roots and shoots to grow and damaged parts to be replaced. A useful comparison is that embryonic stem cells build the body once, adult stem cells maintain and repair it, and meristem cells keep a plant growing for its whole life.

Embryonic stem cells are unspecialised cells taken from a very early human embryo, when it is a ball of cells. Because they have not yet switched on the genes that make a cell specialised, they are pluripotent: they can differentiate into almost any cell type, such as nerve, muscle, skin or blood cells. They can also be cloned, meaning the embryo is allowed to divide in the laboratory to produce many genetically identical stem cells, giving a large supply for research or treatment. Scientists can direct differentiation by controlling the conditions and chemical signals the cells receive. This makes embryonic stem cells medically valuable, but their use raises ethical issues because obtaining them involves destroying an early embryo.

Adult stem cells are found in tissues such as bone marrow. They are unspecialised cells that can divide to replace worn-out or damaged cells, but they are more limited than embryonic stem cells: they can form many cell types, including blood cells, rather than most cell types. In bone marrow, stem cells can differentiate into red blood cells, which carry oxygen, and white blood cells, which defend against pathogens. They can also produce platelets, but platelets are cell fragments, not whole cells, so they are not counted as blood cells. This is why bone marrow transplants can restore blood cell production in patients whose marrow is damaged or diseased. Using a patient's own adult stem cells avoids the ethical concerns linked to embryos and reduces the risk of immune rejection, but adult stem cells are harder to grow in large numbers and cannot form as many cell types.

Meristem tissue is plant stem cell tissue found in regions of active growth, such as root and shoot tips and the cambium. Its cells are unspecialised and retain the ability to divide by mitosis and then differentiate into any plant cell type, including xylem, phloem, palisade mesophyll, epidermal and root hair cells. Unlike most animal cells, which lose this potency after early development, plant meristems remain active throughout the plant's life, so growth and repair continue for years. For example, cells left behind at a shoot tip can become new leaves, while cambium cells can become xylem or phloem as the stem thickens. This lifelong potency is why gardeners can grow new plants from cuttings.

Stem cells are unspecialised cells that can divide and differentiate into many cell types. In diabetes, the insulin-secreting pancreatic cells are damaged or destroyed, so a patient cannot control blood glucose normally. In paralysis, nerve cells of the spinal cord are damaged and cannot carry impulses to muscles. Stem cell treatment aims to replace these lost cells: stem cells could be guided to become new pancreatic cells that secrete insulin, or new neurones that reconnect damaged pathways. The word 'may' is important: much of this work is still experimental, with risks such as immune rejection and uncontrolled cell division, so stem cell therapy is not yet a routine cure for either condition.

Therapeutic cloning begins by taking the diploid nucleus from one of the patient's body cells and placing it into an egg cell whose own nucleus has been removed. The resulting cell divides to form an early embryo that is genetically identical to the patient, so its stem cells carry the patient's own antigens. Because the immune system recognises these cells as 'self', it should not attack them, avoiding the rejection that can occur with donor tissue. The stem cells can then be cultured and stimulated to differentiate into the needed cell type, such as pancreatic cells for diabetes or nerve cells for paralysis. The technique is still experimental and raises ethical questions about creating and destroying embryos.

Stem cells can divide to form many cell types, so they are useful in medicine, but their use is not risk-free. Because stem cells are living cells, any virus present in donor tissue could be passed to the patient, especially if the patient's immune system is suppressed. There are also ethical and religious concerns: some people believe an embryo should not be used or destroyed for research, because they regard it as a potential human life. Others accept adult stem cells, which can be donated with consent, but object to embryonic sources. Scientists therefore weigh possible treatments against these risks and beliefs, and follow consent and safety rules.

In plants, stem cells are found in meristems, such as the tips of roots and shoots. These cells can divide and differentiate into any plant cell type. A small piece of meristem tissue can be grown in sterile conditions on a nutrient medium, where it forms many genetically identical plantlets. Because one parent plant supplies all the cells, the offspring are clones. This method is quick, because many plantlets form at once, and economical, because only a small amount of parent tissue is needed and the plants can be grown in controlled conditions. It is used to produce large numbers of identical crop plants with useful features, such as disease resistance.

Cloning produces genetically identical offspring from a single parent, so it can preserve the genes of a rare species. In conservation, adult cell cloning or embryo cloning may be used when few individuals remain. For example, a tissue sample from a rare animal can provide a nucleus that is placed into an egg cell with its nucleus removed; the resulting embryo is implanted into a surrogate. This creates a new individual without needing a mate. The method protects the species from extinction because it maintains a breeding population and preserves genetic material. However, cloned populations have low genetic variation, so they may be vulnerable to the same disease or environmental change. Cloning is therefore one conservation tool among others, such as habitat protection and captive breeding.

Cloning crop plants allows farmers to obtain many genetically identical plants that all carry a desirable feature, such as disease resistance. A common method is taking cuttings: a piece of stem or leaf is removed from the parent plant and grown into a new plant. Another method is tissue culture, where small groups of cells are grown on a sterile nutrient medium to produce many plantlets. Because the offspring are clones, they inherit the same alleles for disease resistance as the parent. This gives uniform, predictable crops and can increase yield. However, genetic uniformity means that if a new disease or pest can overcome the resistance, the whole crop may be lost. Farmers may also need to manage soil nutrients and pests carefully because all plants have the same requirements.