Cells are the structural and functional units of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms are multicellular and may have many cells. Humans have an estimated 100,000,000,000,000 (one hundred trillion) cells and more than 200 different types of cells (liver cells, skin cells, muscle cells, etc.).

Stem cells have the remarkable potential to develop into many different cell types in the body. They can divide without limit to replenish other cells, serving as a sort of repair system for the body. When a stem cell divides, each new cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

No. Stem cells isolated from different sources and tissues are distinct in that they have varying degrees of potency (see next question) and give rise to differing mature cell types. Additionally, as each person differs slightly at the genetic level (their DNA sequence), the stem cells derived from each individual are likewise different.

Totipotent cells can form all the cell types in a body, plus the extraembryonic, or placental, cells. Embryonic cells within the first couple of cell divisions after fertilization are the only cells that are totipotent. Pluripotent cells can give rise to all of the cell types that make up the body; embryonic stem cells are considered pluripotent. Multipotent cells can develop into more than one cell type, but are more limited than pluripotent cells; adult stem cells and cord blood stem cells are considered multipotent.

Embryonic stem cells (ESCs) are derived from the embryo and have the potential to become all the different cell types of the body (pluripotency). Somatic stem cells, sometimes called adult stem cells, are found in organs or tissues, can self-renew and yield the differentiated cell types comprising that organ or tissue (multipotency), and are important for maintenance and repair of the organ or tissue. Cord blood stem cells can be isolated from the umbilical cord of newborn infants and are less mature than adult stem cells. Cord blood stem cells are a type of somatic stem cell. Somatic stem cells are restricted in the types of cells they can produce in the lab.

Embryonic stem cells are usually derived from the inner cell mass of preimplantation embryos, corresponding to 5-9 days after fertilization in humans and 3-4 days in mice. Embryos used to generate human ESCs come from several sources. The first human ESCs were derived from donated embryos left after in vitro fertilization (IVF). IVF embryos analyzed by preimplantation genetic diagnosis can also be used to generate ESCs. An alteration of this technique allows generation of ESCs from single cells removed from embryos in a process similar to preimplantation genetic testing. ESCs can be derived from eggs that have been parthenogenetically activated; that is, the eggs are induced to divide without being fertilized by sperm. Somatic cell nuclear transfer (SCNT) can be used to produce embryos from somatic or adult cells using donated enucleated eggs, and then ESCs can be generated from the resulting embryos.

When embryos are used to generate human ESC lines, they come from donations after in vitro fertilization cycles by individuals who have given written informed written consent. Alternatively, hESC lines can be derived from donated eggs that are activated to begin development without fertilization by sperm, or from SCNT embryos.

iPS cells are somatic cells that were manipulated to exhibit properties of embryonic stem cells. Introduction of a set of four factors into somatic cells, along with specific culture conditions, alters each cell's epigenetic signature, resetting the cell to a pluripotent ESC-like state. This process is termed "reprogramming." Like ES cells, iPS cells can be differentiated into many different cell types in the lab, and mouse iPS cells have passed even the most stringent tests for pluripotency.

So-called cancer stem cells are cancer cells that have stem cell-like properties, i.e., they can self-renew and differentiate into other cell types. They are associated with some, but not all, types of cancers. Data suggest that recurrence of some cancers is caused by a failure of current therapies to target and kill these cancer stem cells. However, the relationship between cancer stem cells and somatic stem cells is unclear. Somatic stem cells can become cancerous, but cancer stem cells do not necessarily come from somatic stem cells.

Somatic stem cells, such as blood-forming stem cells in bone marrow (called hematopoietic stem cells, or HSCs), are currently the only type of stem cell commonly used to treat human diseases. Doctors have been transferring HSCs in bone marrow transplants for over 40 years. More advanced techniques for collecting, or "harvesting," HSCs are now used in order to treat leukemia, lymphoma and several inherited blood disorders.

The National Institutes of Health indicates that approximately 1.1 million Americans suffer a heart attack each year, and together cardiovascular diseases and cancers are the top two causes of death according to the CDC, with each killing over half a million Americans each year. Regenerative medicine holds the promise of new ways to repair cardiovascular damage and of improved cancer treatment.

As with any treatment, there are certain risks to stem cell therapy, including immune rejection of the cells used in treatment. Stem cells have the potential to divide many times and differentiate into many cell types, which is their great promise. Paradoxically, because of these abilities, stem cells also have the potential to form tumors. These potential risks dictate that both doctors and patients proceed with caution, and thus it is critically important that further research is conducted.

Reproductive cloning involves creating an animal that is genetically identical to a donor animal through somatic cell nuclear transfer. In reproductive cloning, the newly created embryo is placed back into the uterine environment where it can implant and develop. Dolly the sheep is perhaps the most well known example.

The National Institutes of Health maintains a registry of current clinical trials, including trials that are recruiting new volunteers. The FDA recently approved the first clinical trial in the US using hESC-derived cells. However, this trial uses cells derived from hESCs; hESCs themselves have not yet been approved for use in clinical trials.