In the hands of a trained craftsman, a lump of soft clay can be formed into practical any shape. Embryonic stem cells are the living equivalent of that piece of moist clay; they have the prospective to give rise to virtually all of the over 200 cell types making up the human body. How do they do this? Consider what happens to a newly fertilized egg cell. 
          Soon after fertilization an egg cell begins to divide. In humans about five days of cell division results in a minute ball of cells called a BLASTOCYST. It is essentially a hollow sphere that is composed of a shell-like outer cell layer and a small cluster of about 30 cells called the inner cell mass, which is attached to the inside wall of the sphere. The outer cell layer becomes the PLACENTA; the inner cell mass, the human embryo.
         At the blastocyst stage, though, the cells of the inner cell mass have not yet begun to specialize into specific cell types, such as nerve, kidney, or muscle cells. Hence, they are designated stem cells. And because they give rise to virtually all the different cell types in the body, they are said to be PLURIPOTENT. To make sense of the excitement and mystery surrounding stem cells, let us see what the researchers have done thus far and what their goals are, beginning with embryonic stem cells.
                                            [1]           EMBRYONIC STEM CELLS
            The report Stem Cells and the Future of Regenerative Medicine states: “In the last three years, it has become possible to remove these [human embryonic] stem cells from the BLASTOCYST and maintain them in an undifferentiated state in cell culture lines in the laboratory.” Simply put, embryonic stem cells can be cultured so as to produce an unlimited number of identical copies of themselves. Embryonic stem cells extracted from mice, first cultured in 1981, have produced billions of duplicate cells in the laboratory.
          Because all these cells remain undifferentiated, scientists hope that with the right biochemical triggers, stem cells could be directed to develop into virtually all the cell kinds that may be needed for tissue replacement therapy. Simply put, stem cells are seen as a potential source of unlimited ‘spare parts,’
          In two animal studies, researchers coaxed embryonic stem cells into becoming insulin-producing cells, which were then transplanted into diabetic mice. In one study the symptoms of diabetes were reversed, but in the other the new cells failed to produce enough insulin. In similar studies, scientists have had partial success in restoring neural function in spinal cord injuries and in correcting Parkinson’s disease symptoms. Those studies provide promise but not definitive evidence, that similar treatment could be effective in humans.
         The main focus of concern is that the process of extracting embryonic stem cells essentially destroys the embryo. This deprives a human embryo of any further potential to develop into a complete human being. For those who believe that the life of a human being begins at the moment of conception, ESC [embryonic stem cell] research violates tenets that prohibit the destruction of human life and the treatment of human life as a means to some other end, no matter how noble that end might be.
                 Where do laboratories get the embryo from which stem cells are extracted? Generally from in vitro fertilization clinics, where women have provided eggs for in vitro fertilization. Leftover embryos are usually either frozen or discarded.
                                                        [2]           ADULT STEM CELLS
               The adult stem cells is an undifferentiated [specialized] tissue, such as bone marrow, blood and blood vessels, the skin, the spinal cord, the liver, the gastrointestinal tract, and the pancreas. Initial research suggested that adult stem cells were much more limited in scope than their embryonic counterparts. However, later findings in animal studies suggest that certain kinds of adult stem cells may be able to differentiate into tissues other than those which they came.
         Adult stem cells isolated from blood and bone marrow, called HEMATOPOIETIC STEM CELLS [HSCs], have the ability to self-renew continuously in the marrow and to differentiate into the full complement of cell types found in blood. This type of stem cell has already been used to treat leukemia and a number of other blood disorders. Now some scientists also claim that HSCs appear to give rise to nonblood cells such as liver cells and cells that resemble neurons and other cell types found in the brain.
      Using another type of stem cell derived from the bone marrow of mice, researchers in the United States appear to have made another break through. Their study, published in the journal Nature, showed that these cells seem to have all the versatility of embryonic stem cells. In principle these adult stem cells could do everything expected of embryonic stem cells but these cells are rare and difficult to identify. On the other hand, any medical benefits they may yield will not involve the destruction of human embryos.
                                                          [3]   EMBRYONIC GERM CELLS
       Besides adult and embryonic stem cells, embryonic germ cells have also been isolated. Embryonic germ cells are derived from the cells in the GONADAL ridge of an embryo or a fetus, which give rise to eggs or sperm. [The gonadal ridge becomes the ovaries or testes.] Although embryonic germ cells are different in many ways from embryonic stem cells, both are pluripotent, or able to give rise to virtually all cell types. This potential makes pluripotent cells very attractive candidates for the development of unprecedented medical treatments. However, the excitement over such potential therapies is tempered by the controversy centering on the source of these cells. They are derived either from aborted fetuses or from embryos. Thus, obtaining these cells involves fetal and embryo destruction.
             One of the major obstacles is the rejection of foreign tissue by the recipient’s immune system. The present solution is to administer potent drugs that suppress the immune system, but such drugs carry serious side effects. Genetic engineering may avoid this problem if stem cells can be altered so that tissues derived from them do not appear foreign to their new host.
     Embryonic stem cell transplantation also carries the risk of tumor formation, in particular a tumor called a teratoma, meaning ‘monster tumor.’ This growth may comprise a variety of tissues, such as skin, hair, muscle, cartilage, and bone. During normal growth, cell division and specialization follow a strict genetic program. But these processes can run awry when stem cells are severed from the blastocyst, cultured in viro, and later injected into a living creature. Learning to master artificially the enormously complex processes of cell division and specialization is yet another major hurdle facing researchers
                                                           HOW A CLONE CAN BE MADE
          In recent years scientists have cloned a variety of animals. In 2001 a laboratory in the United States even attempted, albeit unsuccessfully, to clone a human. One way that scientists make clones is through a process called NUCLEAR TRANSFER.
         First, they extract an unfertilized egg cell from a female and enucleate the cell, or remove its nucleus, which contains the DNA. From the body of the animal to be cloned, they obtain a suitable cell, such as a skin cell, the nucleus of which contains its owner’s genetic blueprint. They insert this cell [or just its nucleus] into the enucleated egg and pass an electric through it. This fuses the cell with egg cytoplasm. With its new nucleus, the egg now divides and grows as if it were fertilized, and a clone of the creature from which the body cell was taken begins to develop.
      The embryo can then be implanted in the womb of a surrogate mother, where, in the rare instance all goes well. It will grow to term. Alternatively, the embryo may be kept only until the inner cell mass can be used to obtain embryonic stem cells that can be kept in culture. Scientists believe that this basic process should work with humans. In fact, the above-mentioned attempt to clone as human was performed with a view to acquiring embryonic stem cells. Cloning for this purpose is called THERAPEUTIC CLONING.