Stem cells have only recently been isolated and used in experimentation so there still exist many aspects of stem cell characteristics and specializations that scientists are trying to understand. Stem cell research has many different implications. In the body, adult stem cells largely maintain and repair the tissue in which they are found. As of 2009, there are reports of stem cells in the following adult tissues: brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver. Neuronal stem cells have been isolated from fetal tissue and suggested neuronal stem cells have been removed from brain tissue isolated for treatment of epilepsy. Though experimentation with rat and mouse models were more easily facilitated, this kind of work in humans is much more limited. This belief was dispelled when scientists finally isolated neuronal stem cells from both the rat and mouse nervous systems. To begin with, NIH points out that until the turn of the 21st century, stem cells were believed to be non-existent in the adult nervous system. Isolating adult stem cells has been more difficult than isolating embryonic stem cells. SCNT would be a way to overcome the incompatibility problem by using the patient’s own somatic cells. One problem faced in tissue transplants is immune rejection, where the host body refuses and attacks the introduced tissue. The resulting cell is believed to be totipotent, with the full ability to differentiate into specialized cells and develop into an entire animal. Then a somatic cell is fused with the egg. In this method, the nucleus, which contains genetic material, is removed from a normal animal egg. The final technique NIH mentions for deriving pluripotent stem cells is Though Thomson and Gearhart obtained tissue from differing sources, according to NIH, the resultant cells seem to be the same. These pluripotent stem cells were obtained from terminated pregnancies. ![]() After also obtaining informed consent from the donors, Gearhart removed fetal tissue that was destined to become testes or ovaries. In vitro fertilization (IVF) clinics and used after obtaining informed consent. The embryos Thomson worked with were acquired from James Thomson, who first isolated pluripotent stem cells from the inner cell mass of human blastocysts. In 2000, NIH listed several methods for obtaining human pluripotent stem cells, two of which were based on experiments reported in 1998. The majority of experiments involving animal stem cells have been performed using mice. This phenomemon–stem cells of one tissue giving rise to cell types in a completely different tissue–is known as plasticity. Though this fairly rigid definition was accepted by NIH in 2000, in the most recent 2009 report on stem cell basics, NIH notes that recent experiments have resulted in contrary evidence, such as hematopoietic stem cells giving rise to neurons. For example, hematopoietic stem cells in the bone marrow give rise to red blood cells, white blood cells, and platelets whereas skin stem cells give rise to the different skin cells. Multipotent stem cells give rise to cells with more specialized functions. After pluripotent stem cells further specialize, they can become multipotent stem cells, which can develop into more than one type of cell. Thus, unlike totipotent cells, pluripotent stem cells cannot develop into an entire organism. Pluripotency is the ability to differentiate into all cell types except those involved in the extra-embryonic tissues. The cells clustered inside the sphere are known as the inner cell mass and are pluripotent. ![]() ![]() The totipotent cell begins to divide and after several divisions, the cells starts to specialize and form a hollow sphere of cells, known as a blastocyst. When an egg is first fertilized, it is totipotent, which means it has the ability to develop into an entire organism. They are usually removed from the inner cell mass of aīlastocyst. As the name suggests, embryonic stem cells are derived fromĮmbryos. Scientists predominantly work with two types of stem cells from animals and humans:Īdult somatic stem cells. As long as the stem cells remain unspecialized, meaning they lack tissue-specific structures, they are able to sustain long-term self-renewal. This ability to proliferate can yield millions of stem cells over several months. When allowed to grow in particular environments, stem cells divide many times. National Institutes of Health (NIH), the standard American source on stem cell research, three characteristics of stem cells differentiate them from other cell types: (1) they are unspecialized cells that (2) divide for long periods, renewing themselves and (3) can give rise to specialized cells, such as muscle and skin cells, under particular physiological and experimental conditions.
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