After 3 - 4 divisions of totipotent cells, there follows a series of stages in which the cells become increasingly specialized. The next stage of division results in pluripotent cells, which are highly versatile and can give rise to any cell type except the cells of the placenta. At the next stage, cells become multipotent, meaning they can give rise to several other cell types, but those types are limited in number. An example of multipotent cells is hematopoietic cells - blood stem cells that can develop into several types of blood cells, but cannot develop into brain cells. At the end of the long chain of cell divisions that make up the embryo are "terminally differentiated" cells - cells that are permanently committed to a specific function. Figure 10-13a shows the pathway from embryonic stem cell to multipotent stem cell and on to the different type of specialized cells.
Multipotent stem cell is self-renewing. When the stem cell divides, one of the two daughter cells may go on to give rise
| ||to other types of cell, whereas the other daughter cell
remains a stem cell, capable of dividing again and always giving one daughter to diversification. (See Figure 10-14b)
Blood cells in human body are replaced every 120 days. The replacement comes from the multipotent stem cell in the bone marrow. Skin cells are shed every few weeks. The replacement comes from the multipotent stem cell at the base of the skin (the basal layer). Scientists are still looking for the the neural stem cell. Its identity, location and potential remain unclear.
It has been long held that differentiated cells cannot be altered or caused to behave in any way other than the way in which they have been naturally committed. New research, however, has called that assumption into question. In recent stem cell experiments, scientists have been able to persuade blood cells to behave like neurons, or brain cells.