Embryonic stem cells : (ES cells) are stem cells derived from the inner cell mass of an early stage embryo known as a blastocyst. Human embryos reach the blastocyst stage 4-5 days post fertilization, at which time they consist of 50-150 cells.
ES cells are pluripotent. This means they are able to differentiate into all derivatives of the three primary germ layers: ectoderm, endoderm, and mesoderm. These include each of the more than 220 cell types in the adult body. Pluripotency distinguishes ES cells from multipotent progenitor cells found in the adult; these only form a limited number of cell types. When given no stimuli for differentiation, (i.e. when grown in vitro), ES cells maintain pluripotency through multiple cell divisions.
Somatic stem cells : Adult (or somatic) stem cells are found, for example, in bone marrow, blood, the eye, the brain and skeletal muscle. Their purpose is to replace and replenish cells that are continually lost by depletion and damage, such as blood cells.
Because they are already partially specialised, adult stem cells do not appear to have the same capabilities as embryonic stem cells. Under laboratory conditions, they have been manipulated to form other cell types. It may be possible, eventually, to direct these cells to function in other areas of the body to replenish damaged or diseased body tissue. However, adult stem cells are rare within the body and it is not known whether they are present in some organs, such as the heart. They are also difficult to extract and grow using the techniques that are currently available.
Bone marrow transplants are a type of adult stem cell therapy. For more than 20 years, patients with blood disorders such as leukaemia have been treated by introducing haematopoietic (blood-forming) stem cells into their bodies through bone marrow transplants. This has been possible because haematopoietic stem cells are readily accessible, unlike many other adult stem cell types found in our bodies, and they are able to replenish blood cells continuously at high rates.
SCNT : In genetics and developmental biology, somatic cell nuclear transfer (SCNT) is a laboratory technique for creating an ovum with a donor nucleus
In SCNT the nucleus, which contains the organism's DNA, of a somatic cell (a body cell other than a sperm or egg cell) is removed and the rest of the cell discarded. At the same time, the nucleus of an egg cell is removed. The nucleus of the somatic cell is then inserted into the enucleated egg cell. After being inserted into the egg, the somatic cell nucleus is reprogrammed by the host cell. The egg, now containing the nucleus of a somatic cell, is stimulated with a shock and will begin to divide. After many mitotic divisions in culture, this single cell forms a blastocyst (an early stage embryo with about 100 cells) with almost identical DNA to the original organism.