Stem cells exist beyond the embryo, in both foetal and adult tissues, though their proportional number and, it’s thought, their multipotency diminish with age. Foetal tissues are therefore a richer source of qualitatively better stem cells than adult tissue. But they’re less multipotent than ES cells. One significant advantage foetal cells do have over ES cells is that their use provokes less ethical debate since they can be isolated from foetuses whose development has been aborted for medical reasons, or indeed by miscarriage.

Stem cells from human foetal brain have been used to treat patients with Parkinson’s disease and in some cases have conferred sustained clinical improvement. Despite this apparent level of success, foetal stem cells have one fundamental drawback: the source material is extremely limited. An additional limitation is that foetal cells have virtually no potential for patient-identical therapies (unless in utero techniques develop allowing the foetus itself to be treated). For these reasons foetal cells are restricted in both the range of diseases and the number of patients they can treat. As such they’re unlikely to make a significant contribution to the armoury of cell-based therapies used to battle against disease.