Summery Most of the cells in the human body are specialized, meaning they perform a specific function in a certain kind of tissue. Skin cells, blood cells, heart cells, nerve cells, liver cells, and others are among the hundreds of different types of specialized cells. Stem cells are cells that have not differentiated, or become specialized. They are found in plants and animals, including humans. Stem cells can renew themselves (make more stem cells) again and again through cell division. They also give rise to the different types of specialized cells that make up the body’s tissues and organs. Scientists are working to develop therapies that use stem cells to replace defective or damaged cells in the body. For instance, in a laboratory stem cells could be influenced to develop into a particular type of specialized cell, such as liver cells. If enough liver cells were grown to produce liver tissue that worked normally, they could be transplanted into a person whose own liver had failed. Stem cell research has become an important part of regenerative medicine, in which treatments are devised to replace tissues damaged by injury or disease. Stem cell therapies potentially could be used to treat a variety of physical disorders and injuries, such as Alzheimer disease, Parkinson disease, heart disease, diabetes, and damage to the spinal cord. Researchers also use stem cells in experiments to study diseases, genes, and cell functioning and to test possible new medicines.
Scientists obtain embryonic stem cells, or ES cells, from embryos of humans and certain other mammals. An embryo is an unborn animal in the early stages of development. Embryonic stem cells are taken from the embryos at a very early point. The cells are kept in a laboratory dish filled with a nutrient-rich liquid.
The most-studied embryonic stem cells are mouse embryonic stem cells. This is done by introducing new genes into mouse embryonic stem cells in the laboratory. Scientists can then breed the mice to create a line of mice with the same genetic makeup as the embryonic stem cells. Such mice may be used in experiments to study genetic diseases or the functioning of specific genes. Extensive experience with mouse embryonic stem cells made it possible for scientists to grow human embryonic stem cells from early human embryos. A stem cell line consists of generations of cells that originated from the same group of parent cells.
Human embryonic stem cells have the potential to become any type of body cell. They are thus thought to be a possible source of specialized cells for cell therapy—the replacement of a patient’s defective cells with healthy cells. Large quantities of cells, such as neurons for the treatment of Parkinson disease and insulin-secreting pancreatic cells for the treatment of diabetes, could be produced from embryonic stem cells.
Human embryonic stems cells are obtained from an embryo when it is about five days old and has about 150 cells.
Embryonic stem cells are not obtained from embryos that develop in the mother’s body. In a typical IVF procedure, multiple eggs are removed from a woman’s ovaries and fertilized outside the body with sperm from a man. The embryos are cultured in the laboratory for a few days. The procedure to obtain embryonic stem cells results in the destruction of the embryo. . Other people regard the blastocysts as simply a collection of human cells. There is a great difference of views on this issue. The use of human embryonic stem cells is allowed in some countries and banned in others.
In adult humans, some tissues of the body continuously form new cells to replace old ones. These tissues contain a small number of stem cells, which persist indefinitely. When these adult stem cells divide, they renew themselves. These stem cells give rise to cells that become new skin cells, to replace old skin cells that have died. Other tissues with adult stem cells that continuously give rise to replacement cells include the lining of the small intestine, parts of the brain, and bone marrow. The different types of cells of the blood and immune systems are derived from adult stem cells in bone marrow. Adult stem cells are also found in many other tissues.

Before the transfer, scientists remove the egg cell’s own nucleus. After the transfer, the egg cell is grown in the lab until it reaches the blastocyst stage. Cells taken from the embryo can be used to develop a line of embryonic stem cells. These stem cells can then be stimulated to specialize into various types of cells needed for transplantation. The cells would be genetically identical to the donor of the somatic cell. They could thus be used to treat the donor with no problems of immune rejection. Researchers have also used SCNT to clone animals