Gene expression; stem cells

Like plant cells, animals cells formed during the early stages of embryonic developemnt are totipotent. These embryonic stem cells exist in humans embryos for only a few weaks, during which time they can give rise to any type of tissue. When the stem cell divides by mitosis after this intial stage, it produces two daughter cells. One gives rise to differentiated cells which lose their totipotency; the other gives rise to cells called adult stem cells that retain some totipotency . Such cells are callled pluripotent. Adult stem cells normally give rise to only a small range of cell types; Adult stem cells have two important properties; They can continually reproduce themselves, under approriate conditions, they differentiate into specilased types if given give the appropriate
Adult stem cells normally have a low level of totipotency, but they can be induced to differentiate into a wider range of cells types if given the appropriate enviroment
Theoretically, stem cells can be induced to make any tissue types. Stem cells with normal genes could therefore be used to treat tissue affected by genetic disorder
Embryonic stem cells, being the most totipotent, are the easiest to induce to form specialised cells. However, genetic differences between the donor and host could cause unforeseen problems in the long term and trigger rejection in the short term
One possible way of overcoming these problems is to take adult stem cells from the patient, replace the defective allele with a normal allele, implant the gentically manipulated cells into eggs which have had their nuclei removed, and simulate the eggs to grow cloned embryos of the patient. The embryo would then be a source of stem cells that could be used to make cells that are gentically the same as those of the patient, except that they will rather than a defective one

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Gene expression; stem cells

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