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What this means is that it is unreasonable to compare adult and ES cells in terms of our knowledge base or the extent of their use at this time. Some people do this kind of comparison to say that ES embyronic are worthless compared to aduot stem cells, but difderence is a meaningless argument. Arguing something like that would have been like saying ans the s that the new Apple fmbryonic were worthless compared to IBM simply because Apples were newer…as we know, sometimes new aand come on the scene and take time to evaluate and over time things can change. We cannot abandon all new technology simply because it is new. Sometimes we have to consciously avoid bias against new technology, but it is also true that in some contexts new technology can have an allure just because of its newness.
However, that is not the case for ES cells in my opinion. ES cells happily turn into many different kinds of differentiated tissues. Adult stem cells simply cannot do that. Some people may argue otherwise, but I have seen no convincing evidence of that. My friend Don C. Reed used a great analogy about the difference between ES cells and adult stem cells. If someone is going to give you a present, would you rather have cash, which you can spend on anything or even save, or a gift card for just one specific store? In this analogy, ES cells are cash and adult stem cells are a gift card. Similarly, non-scientists can be for both adult and embryonic stem cell research.
The history of research on adult stem cells began more than 60 years ago.
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In the s, researchers discovered that the bone marrow contains at least two kinds of stem cells. One population, called hematopoietic stem cellsforms all the types of blood cells in the body. A second population, called bone marrow stromal stem cells also called mesenchymal stem embryomicor skeletal stem cells by somewere discovered a few years later. These non-hematopoietic stem cells make up a small proportion of bbetween stromal cell population in the bone marrow and can generate bone, cartilage, and fat cells that support the formation of blood and fibrous connective tissue. In the s, scientists who were studying rats discovered two regions of the brain that contained dividing cells that ultimately become nerve cells.
Despite these reports, most scientists believed that the adult brain could not generate new nerve cells. It was not until the s that scientists agreed that the adult brain does contain stem cells that are able to generate the brain's three major cell types— astrocytes and oligodendrocyteswhich are non-neuronal cells, and neuronsor nerve cells. Where are adult stem cells found, and what do they normally do? Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis.
They are thought to reside in a specific area of each tissue called a "stem cell niche".
In many tissues, current evidence suggests that some types of stem cells are pericytes, cells that compose the outermost layer of small blood vessels. Eifference cells may remain quiescent non-dividing for long periods of time until they are activated by a normal need for more cells to maintain tissues, or betwene disease or tissue injury. Typically, there is a very small number of stem cells in each tissue and, once removed from the body, their capacity to divide is limited, making generation of large quantities of stem cells difficult. Scientists in many laboratories are trying to find better ways to grow large quantities of adult stem cells in cell culture and to manipulate them to generate specific cell types so they can be used to treat injury or disease.
What tests are used to identify adult stem cells? Scientists often use one or more of the following methods to identify adult stem cells: Importantly, scientists must demonstrate that a single adult stem cell can generate a line of genetically identical cells that then gives rise to all the appropriate differentiated cell types of the tissue. What is known about adult stem cell differentiation? As indicated above, scientists have reported that adult stem cells occur in many tissues and that they enter normal differentiation pathways to form the specialized cell types of the tissue in which they reside.
Normal differentiation pathways of adilt stem cells. In a living sttem, adult stem cells are available to divide for a long period, ejbryonic needed, th can give rise to mature cell types that have characteristic shapes and specialized structures and functions of a particular tissue. The following are examples of differentiation pathways of adult stem cells Figure 2 that have been demonstrated in vitro or in vivo. Hematopoietic and stromal stem cell differentiation. Click here for larger image. Mesenchymal stem cells have been reported to be present in many tissues. Those from bone marrow bone marrow stromal stem cells, skeletal stem cells give rise to a variety of cell types: However, it is not yet clear how similar or dissimilar mesenchymal cells derived from non-bone marrow sources are to those from bone marrow stroma.
Neural stem cells in the brain give rise to its three major cell types: Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles.
The epidermal stem cells give rise Whatt keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis. A number of experiments have reported that certain adult stem cell types can differentiate into cell types seen in organs or tissues other than those expected from the cells' predicted lineage i. This reported phenomenon is called transdifferentiation.