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What are stem cells? Discuss how these cells play a role in normal development and turnover of tissues. How might stem cells be used to treat disease?
Stem cells are undifferentiated cells that are capable of limitless division. There are two types division which these cells can undergo; symmetric division forms two daughter cells that both remain as stem cells whereas in asymmetric division one of the two daughter cells becomes a transit amplifying cell which is committed to a specific fate and becomes terminally differentiated to a specific type of cell. The number of different cell types the transit amplifying cells can differentiate into determines the potency of the stem cell. Totipotent stem cells are able to differentiate into all types of cells including the extra embryonic structures such as the placenta. In comparison, pluripotent cells can differentiate into all the cells found in the embryo but can't differentiate to form the placenta. Multipotent stem cells can differentiate into multiple lineages of cells found in the embryo whereas a unipotent stem cell can only differentiate into one type of stem cells. Stem cells can be classified into categories depending on where they are found. Embryonic stem cells arise from the inner cell mass during in vitro culture of blastocyts removed from a fertilised egg and these cells are totipotent. The other main type of stem cells is adult stem cells which are either multipotent or unipotent and are found in the body after development. These stem cells play a key role in replacing differentiated cells that cannot themselves divide such as the epithelium, blood, muscle, liver and brain. The continual renewal of these cells is essential in the development and turnover of tissues. In many tissues there are multipotent stem cells and when under homeostatic conditions they remain quiescent but when they receive a particular signal they enter the cell cycle where they often divide asymmetrically. One of the two daughter cells form progenitor cells that undergo further cellular proliferation, progressive differentiation and this leads to the expansion of committed progenitor cell populations. The other daughter cell remains as a stem cell. Epidermal stem cells One example of adult stem cells are the epidermal stem cells which are found in the basal layer of the epidermis. The main function of the epidermis, which is a stratified squamous keratinised epithelium, is to resist mechanical stresses and to protect the underlying tissues from bacteria, UV light and excessive wetting. In the basal layer of the epidermis there are few true stem cells which undergo asymmetric division to form transit amplifying cells. The transit amplifying cells continue to proliferate but once they become committed to differentiation the cell leaves the cell cycle and migrates through the basal, prickle and granular layers of the epidermis where they progressively differentiate. As these cells travel through these layers they move through states of gene expression and express a series of different keratin proteins. Once they reach the stratum corneum, the outermost
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