Читать книгу Cell Biology - Stephen R. Bolsover - Страница 21

Cells multiply by division. In the human body an estimated 25 million cell divisions occur every second! These provide new cells for the blood and immune systems, for the repair of wounds and the replacement of dead cells. In complex tissues such as those described above, division is restricted to a small number of undifferentiated stem cells that are capable of dividing many times; some of their daughter cells then differentiate to become all of the other cells of the tissue. In the case of the intestine, folds in the surface epithelium form crypts, each of which contains ~250 cells (Figure 1.6). Mature cells at the top die and must be replaced by the division of between four and six stem cells near the base of the crypt. Each stem cell divides roughly twice a day, the resulting cells moving up the crypt to replace those lost at the surface. Benign (non‐cancerous) polyps can be formed in the intestine if this normal balance between birth and death is disturbed.

As in the intestine, stem cells in other tissues exist in specific locales, called niches, with environments that support their special and vital functions. In many tissues the requirement to replace dead cells is much less than it is in the intestine and in such cases the stem cell niche must maintain its occupants in a quiescent (nondividing) state until needed (for more on stem cells see In Depth 14.1 on page 234).