Fig. 5. Proliferation and apoptosis in cerebella lacking Rb or Rb/p107 at P15. Proliferation (assessed by incorporation of BrdU) in the EGL (A) and IGL (C) is increased in the Rb- and p107-deficient vermis, but is counterweighted by an increased apoptosis rate in the vermis deficient of Rb and Rb/p107 (B,D). Proliferation and apoptosis were most pronounced if both Rb and p107 alleles were lost. Asterisks indicate that the value is significantly different from control value (P<0.01 for all values). (D) There is no statistically significant difference between En2cre; Rb^LoxP/LoxP; p107^+/- (purple bar) and En2cre; Rb^LoxP/LoxP (red bar). Standard deviations (±1 s.d.) are indicated by error bars. (E,H) BrdU labeling of P15 wild-type (E; arrows indicate proliferating cells in the EGL) and Rb/p107 double mutant (H) illustrated the markedly increased proliferation in the broadened EGL and in the IGL. The cell layers in the double mutant (H) are indicated (EGL, external granular layer; PCL, Purkinje cell layer; IGL internal granular layer). Red circles indicate the inner border of the presumed IGL, which would normally form the cerebellar white matter. The cell cycle inhibitor p27 is expressed in cells of the inner EGL, in most migrating and IGL neurons (F). The outer p27-negative EGL region is broadened in Rb/p107 double mutant cells (red arrow in F indicates the layer that is one or two cells thick; arrows in I indicate the broadening of the p27-negative outer EGL). Likewise, a substantial proportion of migrating and IGL neurons are p27 negative (arrows, I). As indicated by the graphs (B,D), there is also significantly increased apoptosis in double mutant (J) compared with wild-type (G) vermis. Arrow in G indicates a single TUNEL-positive cell. Scale bar: 100 µm.

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