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FIGURE 3.8 Cyclin D1 -deficient mice have reduced cell proliferation in the nervous system. A. The normal
retina is a laminated structure consisting of alternating layers of cells (stained purple) and dendrites (lighter
stained layers). B. In mice that have had the cyclinD1 gene deleted by homologous recombination, the layers
are much thinner, and there are considerably fewer cells in the retina. This is due to the reduced amount of
proliferation of the progenitor cells. On the other hand, if the gene coding for the negative regulator of the
cell cycle, p27kip , is deleted in mice (C), the progenitor cells keep proliferating past the normal time when
they would cease mitosis, and produce extra cells that abnormally spill out of their normal layer (arrows).
D. When both cyclinD1 and p27kip are deleted in mice, however, the retina returns to a remarkably normal
appearance, indicating that the balance in proliferation has been restored when both the “gas” and the
“brakes” of the cycle have been removed. Clearly, other regulators must take over when the cyclinD1/p27kip
system is removed, but it is not clear yet what these are. (From Geng et al., 2001)
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