(CNN) -- Here's a look at what you need to know about stem cells.
Scientists believe that stem cell research can be used to treat medical conditions including Parkinson's Disease, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.
About Stem Cells: Stem cell research focuses on embryonic stem cells and adult stem cells. It is very experimental in humans.
Embryonic stem cells are highly controversial. In order to harvest embryonic stem cells, the four to six day old embryo is destroyed. In 2005, researchers told a Congressional panel they would like to develop ways to remove only a single stem cell from an embryo without destroying it, but it is not known if something like this can succeed.
Stem cells have two characteristics that differentiate them from other types of cells: - Stem cells are unspecialized cells that replicate themselves for long periods through cell division. - Under certain physiologic or experimental conditions, stem cells can be induced to become mature cells with special functions, such as the beating cells of the heart muscle or insulin-producing cells of the pancreas.
There are four classes of stem cells: totipotent, multipotent, pluripotent, and unipotent. - Stem cells that develop into cells that make up all the cells in an embryo and fetus are called totipotent. (Ex: The zygote/fertilized egg and the cells at the very early stages following fertilization are considered totipotent) - Multipotent stem cells can give rise to multiple types of cells, but all within a particular tissue, organ, or physiological system. (Ex: blood-forming stem cells/bone marrow cells, most often referred to as adult stem cells) - Pluripotent stem cells (ex: embryonic stem cells) can give rise to any type of cell in the body. These cells are like blank slates, and they have the potential to turn into any type of cell. - Stem cells that can self-renew as well as give rise to a single mature cell type are called unipotent. (Ex: sperm producing cells)
Embryonic stem cells are harvested from 4 to 6 day-old embryos. These embryos are either leftover embryos in fertility clinics or embryos created specifically for harvesting stem cells by therapeutic cloning. Only South Korean scientists claim to have successfully created human embryos via therapeutic cloning and have harvested stem cells from them.
Adult stem cells are already designated for a certain organ or tissue. Some adult stem cells can be coaxed into or be reprogrammed into turning into a different type of specialized cell within the tissue type - for example, a heart stem cell can give rise to a functional heart muscle cell, but it is still unclear whether they can give rise to all different cell types of the body.
The primary role of adult stem cells is to maintain and repair the tissue in which they are found.
Uses of Stem Cell Research: Regenerative (reparative) medicine uses cell-based therapies to treat disease.
Scientists who research stem cells are trying to identify how undifferentiated stem cells become differentiated as serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation.
Scientists believe stem cells can be used to generate cells and tissues that could be used for cell-based therapies as the need for donated organs and tissues outweighs the supply.
Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases, including Parkinson's and Alzheimer's Diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.
Policy Debate: Cloning human embryos for stem cells is very controversial.
The goal of therapeutic cloning research is not to make babies, but to make embryonic stem cells which can be harvested and used for cell-based therapies.
Using fertilized eggs leftover at fertility clinics is also controversial because removing the stem cells destroys them.
Questions of ethics arise because embryos are destroyed as the cells are extracted, such as: When does human life begin? What is the moral status of the human embryo?
Timeline: 1998 - President Bill Clinton requests a National Bioethics Advisory Commission to study the question of stem cell research.
1999 - The National Bioethics Advisory Commission recommends that the government allow federal funds to be used to support research on human embryonic stem cells.
2000 - During his campaign, George W. Bush says he opposes any research that involves the destruction of embryos.
2000 - The National Institutes of Health (NIH) issues guidelines for the use of embryonic stem cells in research, specifying that scientists receiving federal funds can use only extra embryos that would otherwise be discarded. President Clinton approves federal funding for stem cell research but Congress does not fund it.
August 9, 2001 - President George W. Bush announces he will allow federal funding for about 60 existing stem cell lines created before this date.
January 18, 2002 - A panel of experts at the National Academy of Sciences (NAS) recommends a complete ban on human reproductive cloning, but supports so-called therapeutic cloning for medical purposes.
February 27, 2002 - For the second time in two years, the House passes a ban on all cloning of human embryos.
July 11, 2002 - The President's Council on Bioethics recommends a four-year ban on cloning for medical research to allow time for debate.
California and New Jersey announce they will raise state funding to support embryonic stem cell research, including research on stem cell lines created after August 9, 2001.
2005 - Connecticut and Illinois designate state funds to support stem cell research in their states.
February 2005 - South Korean scientist Hwang Woo Suk publishes a study in Science announcing he has successfully created stem cell lines using therapeutic cloning.
December 2005 - Experts from Seoul National University accuse Hwang Woo Suk of having faked some of his research. Hwang Woo Suk asks to have his paper withdrawn while his work is being investigated and resigns his post.
July 18, 2006 - The Senate votes 63-37 to loosen President Bush's limits on federal funding for embryonic stem-cell research.
July 19, 2006 - President Bush vetoes the embryonic stem-cell research bill passed by the Senate (the Stem Cell Research Enhancement Act of 2005), his first veto since taking office.
June 20, 2007 - President Bush vetoes the Stem Cell Research Enhancement Act of 2007, his third veto of his presidency.
January 23, 2009 - The FDA approves a request from Geron Corp. to test embryonic stem cells on eight to 10 patients with severe spinal cord injuries. This will be the world's first test in humans of a therapy derived from human embryonic stem cells. The tests will use stem cells cultured from embryos left over in fertility clinics.
March 9, 2009 - U.S. President Barack Obama signs an executive order overturning an order signed by President George W. Bush in August 2001 that barred the National Institutes of Health from funding research on embryonic stem cells beyond using 60 cell lines that existed at that time. - President Obama says, "In recent years, when it comes to stem cell research, rather than furthering discovery, our government has forced what I believe is a false choice between sound science and moral values. In this case, I believe the two are not inconsistent. As a person of faith, I believe we are called to care for each other and work to ease human suffering. I believe we have been given the capacity and will to pursue this research -- and the humanity and conscience to do so responsibly."
August 23, 2010 - U.S. District Judge Royce C. Lamberth issues a preliminary injunction that prohibits the federal funding of embryonic stem cell research.
September 9, 2010 - A three-judge panel of the U.S. Court of Appeals for the D.C. Circuit grants a request from the Justice Department to lift a temporary injunction issued August 23, 2010, that blocked federal funding of stem cell research.
September 28, 2010 - The U.S. Court of Appeals for the District of Columbia Circuit lifts an injunction imposed by a federal judge, thereby allowing federally funded embryonic stem-cell research to continue while the Obama administration appeals the judge's original ruling against use of public funds in such research.
October 8, 2010 - The first human is injected with cells from human embryonic stem cells in a clinical trial sponsored by Geron Corp.
November 22, 2010 - William Caldwell, CEO of Advanced Cell Technology, tells CNN that the FDA has granted approval for his company to start a clinical trial using cells grown from human embryonic stem cells. The treatment will be for an inherited degenerative eye disease.
April 29, 2011 - The U.S. Court of Appeals for the District of Columbia lifts an injunction, imposed last year by a federal judge, banning the Obama administration from funding embryonic stem-cell research.
May 11, 2011 - Stem cell therapy in sports medicine is spotlighted after New York Yankee pitcher Bartolo Colon is revealed to have had fat and bone marrow stem cells injected into his injured elbow and shoulder while in the Dominican Republic.
July 27, 2011 - Judge Royce Lamberth dismisses a lawsuit that tried to block funding of stem cell research on human embryos.
February 13, 2012 - Early research published by scientists at Cedars-Sinai Medical Center and Johns Hopkins University show that a patient's own stem cells can be used to regenerate heart tissue and help undo damage caused by a heart attack. It is the first instance of therapeutic regeneration.
April 4, 2012 - According to Michael Waalkes, Ph.D., and his team at the National Toxicology Program and Laboratory, National Institute of Environmental Health Sciences, part of NIH, arsenic can turn normal stem cells into cancer stem cells and develop tumor growth.
May 2013 - Scientists make the first embryonic stem cell from human skin cells by reprogramming human skin cells back to their embryonic state, according to a study published in the journal Cell.
April 2014 - Scientists, for the first time, use cloning technologies to generate stem cells that are genetically matched to adult patients, according to a study published in the journal Cell Stem Cell.
July 2, 2014 - Researchers at the Riken Center for Developmental Biology in Japan retract their results regarding the embryonic stem cell findings. They state that "errors were found in the figures, parts of the methods descriptions were found to be plagiarized, and early attempts to replicate the work failed."