Stem cell culture. Gloved hand holding a culture bottle containing a culture of stem cells. These are embryonic neuronal (nerve tissue) stem cells. Stem cells are precursor cells from which all the body's specialized tissue cells develop. Stem cell specialization can be initiated in the laboratory by the use of chemical growth factors. Stem cell research involves trying to use stem cells to repair or replace diseased tissues or organs. All the cells in young embryos are stem cells, and these cells were obtained from embryos that died of natural causes. Photographed at the Besta Neurological Institute, Milan, Italy.