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A culture can be established of cells taken from normal tissue – for example, fibroblasts from a skin biopsy. Given a nutrient-rich medium, the cells proliferate until they reach confluence, i.e. have covered the surface of the dish; they then stop dividing. Division is inhibited when the cells make contact with each other – a process known as ‘contact inhibition’. This is illustrated by the photographs: Cells were incubated with 3H-labelled thymidine, so that cells in S-phase were labelled in the DNA. The cell layers were coated with photographic emulsion, and decay of 3H over S-phase nuclei produced silver grains (black dots). In autoradiograph 1, cells are confluent, and only one labelled nucleus is seen. In autoradiograph 2, where there is still space for cells to grow into, many are labelled.