A footballer suffers a serious knee injury that destroys cartilage cells. A doctor explains that adult stem cells from the patient's own body could be used to repair the damaged tissue. Explain why adult stem cells from the patient's own body are used in this treatment rather than embryonic stem cells, and describe the process by which the stem cells would produce new cartilage cells.

Adult stem cells are used because they are genetically identical to the patient, so the immune system is unlikely to recognise them as foreign and reject them. In contrast, embryonic stem cells would be rejected and their use also raises ethical problems, as embryos must be destroyed to obtain the cells.

The adult stem cells are isolated from the patient’s own tissue and then introduced into the damaged knee. They first divide by mitosis to increase their number. After division, the cells undergo differentiation, during which they become specialised chondrocytes – the cells that produce cartilage matrix. The differentiated cells then synthesise the collagen and proteoglycan components of cartilage, restoring the damaged tissue.