_This article is part of a series The Conversation Africa is running on stem cell research and therapy. Read the rest of the series here._ Broadly speaking, stem cells are used to treat

disease or repair damaged tissue, to understand disease processes and for drug discovery. They are able to be used for these purposes because they belong to a special group of cells that are

capable of differentiation. This means that they can form any of the more than 200 different cell types found in our bodies. DIFFERENT KINDS OF STEM CELLS Although not a stem cell per se,

the fertilised egg creates all the cells that make up the embryo and the placenta. There are two types of stem cells: pluripotent and adult stem cells. Pluripotent stem cells are those that

laboratory in the presence of genes which are functional in the early embryo, the adult genes are switched off and the cells’ embryonic genes switched on. As our bodies develop, cells become

more restricted in their capacity to differentiate into other cells types, and are termed multipotent or unipotent. After we are born, adult stem cells replace cells lost through normal

wear-and-tear or disease. Adult stem cells are found throughout our bodies. There are several types. For example, hematopoietic (blood) stem cells are found in the bone marrow. They give

rise to red blood cells, white blood cells and platelets. Another example are neural stem cells found in the nervous system. Mesenchymal stem cells are found in fat (adipose) tissue, bone

marrow and the umbilical cord. WHAT CAN STEM CELLS BE USED FOR? Adult stem cells have been used for more than five decades to treat certain blood cancers and genetic or immunological

disorders. Known as bone marrow or hematopoietic stem cell transplantation, this procedure replaces the normal stem cells in a patient’s body that have been destroyed by high dose

chemotherapy. Stem cells are also used to replace skin in major burn injuries and to heal chronic wounds. A wide-range of other potential uses are being tested but at this stage are

considered to be experimental. These include treatment for heart disease, cerebral palsy, Alzheimer’s and Parkinson’s diseases, diabetes and spinal cord injury. Adult stem cells are

collected on a routine basis in many parts of the world. Current sources include the bone marrow, circulating blood, and cells harvested from umbilical cord blood. Adult stem cells, and in

particular those derived from umbilical cord blood, can be stored for future use either in a public or a private cord blood bank. Pluripotent stem cells are not yet used to treat patients

routinely but are being tested in clinical trials for a number of diseases including blindness. At present they are mainly used to understand disease processes and for drug discovery. WHY

ARE STEM CELLS CONTROVERSIAL? More than 90% of the work involving stem cells is not controversial. This includes the therapies that are legitimately administered every day as well as most of

the research work being done in many parts of the world. Nonetheless, there are a few areas in the stem cell field that have become controversial. These include: * embryonic stem cells,

whose preparation is seen to result in the destruction of a potential life. This is because ESCs are derived from a small group of cells in a five day embryo (called the inner cell mass)

that under normal circumstances would go on to form an entire living organism; * reproductive cloning in which a clone or copy of an organism is produced from a single cell removed from that

organism; this is the process that produced Dolly the sheep. Reproductive cloning is universally banned in humans; and * stem cell tourism, where patients pay large amounts for unproven

stem cells therapies, contravening ethical norms and safety standards. _This article based on a paper published in a special South African Medical Research Council Flagship edition of the

South African Journal of Bioethics and Law._