Fig. 4. Granulosa cell proliferation is reduced in Cx43 null mutant follicles as revealed by PCNA staining. (A,B) In wild-type ovaries of the CD1 strain, virtually every granulosa cell is positive for PCNA. (C) In Gja1^–/Gja1^– ovaries of the C57BL/6 strain, many granulosa cells of the arrested primary follicles lacked PCNA immunoreactivity. (D) Gdf9^–/Gdf9^– follicles had very little PCNA immunoreactivity. (E) In Gja1^–/Gja1^– ovaries of the CD1 strain, some primary follicles (single arrow) stained intensely for PCNA whereas many granulosa cells of secondary follicles (double arrows) stained weakly or were negative for PCNA. (F) Quantitative differences in the frequency of PCNA staining between mutant lines are evident. Bars represent the % of PCNA-positive cells in all follicles and error bars indicate the s.e. Different letters above the bars indicate significant differences between mutant lines within the same genetic background (P<0.01 in all cases). The number of follicles is indicated in parentheses above each bar. Bar, 50 µm.