Stem cell research. Computer artwork of the stages (anticlockwise from upper left) involved in stem cell therapy. 1: An egg cell is fertilised by a sperm cell. 2-4: The resulting embryo divides 3 times over the next 3 days. 5: By around day five, the blastocyst embryo has developed and consists of an inner cell mass of stem cells and an outer layer that will form the placenta. The stem cells would differentiate into the various tissues of a foetus. 6: Here, the stem cells are removed and cultured. 7: Different tissue types may be grown, including bone marrow and skin. 8: Heart muscle cells are injected into a heart to treat disease.