These stem cells come from embryos that are three to five days old. At this stage, an embryo is called a blastocyst and has about 150 cells. These are pluripotent stem cells, meaning they can divide into more stem cells or can become any type of cell in the body.

This versatility allows embryonic stem cells to be used to regenerate or repair diseased tissue and organs. These stem cells are the subject of much scientific research. However, since they must be derived from early human embryos their production and use in research have been a hotly debated topic.

These stem cells are found in small numbers in most adult tissues, such as bone marrow or fat. Compared with embryonic stem cells, adult stem cells have a more limited ability to give rise to various cells of the body. Until recently, researchers thought adult stem cells could create only similar types of cells. For instance, researchers thought that stem cells residing in the bone marrow could give rise only to blood cells.

However, emerging evidence suggests that adult stem cells may be able to create various types of cells. For instance, bone marrow stem cells may be able to create bone or heart muscle cells. This research has led to early-stage clinical trials to test usefulness and safety in people. For example, adult stem cells are currently being tested in people with neurological or heart disease.

Scientists have successfully transformed regular adult cells into stem cells using genetic reprogramming. By altering the genes in the adult cells, researchers can reprogram the cells to act similarly to embryonic stem cells.

This new technique may allow researchers to use reprogrammed cells instead of embryonic stem cells and prevent immune system rejection of the new stem cells. However, scientists don’t yet know whether using altered adult cells will cause adverse effects in humans.

Researchers have been able to take regular connective tissue cells and reprogram them to become functional heart cells. In studies, animals with heart failure that were injected with new heart cells experienced improved heart function and survival time.

Researchers have discovered stem cells in amniotic fluid as well as umbilical cord blood. These stem cells also have the ability to change into specialized cells. In the last decade, the list of putative human stem cell sources was amended to include human perinatal extraembryonic tissues, such as amniotic fluid, fetal membranes (amnion and chorion) and umbilical cord.

Due to their unique role in fetal development, perinatal stem cells represent an intermediate cell type which has recently been described to combine qualities of both their adult stem cell counterparts and embryonic stem cells and possess immunoprivileged characteristics, as well as a broad multipotent plasticity.

Most importantly, these cells, simply isolated from extraembryonic tissues which are normally discarded after birth, effectively avoid ethical issue involvement. All these attractive characteristics make perinatal stem cells a promising and noncontroversial source of stem cells.