(CNN) Here is some background information about stem cells.

Scientists believe that stem cell research can be used to treat medical conditions including Parkinsons 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.

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.- Totipotent stem cells that develop into cells that make up all the cells in an embryo and fetus. (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.- Unipotent stem cells can self-renew as well as give rise to a single mature cell type. (Ex: sperm producing cells)

Embryonic stem cells are harvested from four to six-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 Parkinsons and Alzheimers 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 left over 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 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 Presidents Council on Bioethics recommends a four-year ban on cloning for medical research to allow time for debate.

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 Hwang of faking some of his research. Hwang asks to have his paper withdrawn while his work is being investigated and resigns his post.

January 10, 2006 An investigative panel from Seoul National University accuses Hwang of faking his research.

July 18, 2006 The Senate votes 63-37 to loosen President Bushs 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 worlds 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 President Barack Obama signs an executive order overturning an order signed by President Bush in August 2001 that barred the NIH from funding research on embryonic stem cells beyond using 60 cell lines that existed at that time.

August 23, 2010 US 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 US Court of Appeals for the D.C. Circuit grants a request from the Justice Department to lift a temporary injunction that blocked federal funding of stem cell research.

September 28, 2010 The US 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 judges 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 US 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 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 patients 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.

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 For the first time scientists are able to 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.

October 2014 Researchers say that human embryonic stem cells have restored the sight of several nearly blind patients and that their latest study shows the cells are safe to use long-term. According to a report published in The Lancet, the researchers transplanted stem cells into 18 patients with severe vision loss as a result of two types of macular degeneration.

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