Testicular morphology of Skp2-deficient mice. (A) Macroscopic comparison of the testes of Skp2+/+, Skp2+/-, and Skp2-/- mice at 2 months of age. Scale bar, 1 mm. B, Ratio of the weight of testes to body weight in male mice of the three genotypes at 2 months of age. Data are means ± SD for 6–8 animals of each genotype. *P < 0.05 versus Skp2+/+, †P < 0.05 versus Skp2+/-. (C-F) Representative light (C, E) and electron (D, F) micrographs of testicular sections of wild-type (C, D) and Skp2-/- (E, F) males at 2–4 months of age. Note the pronounced loss of spermatogenic cells (asterisks in F), leaving only Sertoli cells, and the hyperplasia of the interstitial cellular population (asterisks in E) apparent adjacent to severely degenerated tubules in Skp2-/- testis. Degenerating spermatocytes and clusters of round or elongated spermatids had detached from the seminiferous epithelium and been sloughed off into the tubule lumen of Skp2-/- males (open arrowheads in E). Multinucleated giant spermatogenic cells were also present throughout the seminiferous epithelium of Skp2-/- testis (closed arrowheads in E). Scale bars, 100 μm (C, E) or 5 μm (D, F).