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Stem cells & therapeutic cloning   page 4
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3. Stem cells Link to the Medical Research Council web site
 
What are stem cells?
A stem cell is a fairly undifferentiated form of cell that can divide and eventually give rise to more specialised cells. There are various types of stem cells. It is easiest to understand what these are if we start by considering a human zygote.

 

Figure 5. By about five days after fertilisation, a fertilised human egg has given rise to a hollow mass of cells known as a blastocyst. The inner 50 or so cells of the blastocyst develop to form the tissues of the developing embryo. Because of the ability of each of these cells to give rise to most cell types, they are known as pluripotent embryonic stem cells.
Totipotent cells
A human zygote is a singe diploid cell. After it has undergone three generations of mitotic cell divisions, it consists of eight identical diploid cells. Each of these cells is said to be totipotent as it can develop into a complete, healthy human being. This is what happens when identical twins (or triplets) occur. (These cells are called 'totipotent' as they have the 'potential' to develop into a 'total' individual.)
The blastocyst
By five days after conception, a hollow ball of cells called the blastocyst has formed (Figure 5). The outer blastocyst cell layer goes on to form the placenta. The inner 50 or so cells go on to form the tissues of the developing embryo. These 50 cells are known as pluripotent embryonic stem cells. Each of these cells can give rise to most cell types though they cannot each give rise to all the 216 different cell types that make up an adult human body. (The 'pluri' of 'pluripotent' is like the word 'plural'. It means that the cell can give rise to many cell types but, unlike the earlier totipotent cells, not all of them.)
More cell differentiation
As the embryo develops, the cells of which it is made become increasingly differentiated. Most of its cells lose the capacity to develop into a wide range of cells. Instead they become increasingly specialised, functioning as a red blood cell, one of the cell types in bone, or whatever. However, even in adults some cells retain a certain capacity to give rise to various different cell types. These cells are known as multipotent stem cells. For example, neural stem cells can develop into the various types of cell found in the nervous system while blood stem cells, located in bone marrow, can develop into red blood cells, platelets and the various sorts of white blood cells (macrophages, lymphocytes and so on).
 
Question 3
How many chromosomes are there in a normal human zygote?
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