Abstract
Background:
Zinc oxide nanoparticles (ZnO NPs) are widely used in industrial and biomedical applications; however, concerns remain regarding their potential reproductive toxicity.

Aim:
This study investigated the size-dependent effects of ZnO NPs (20 nm and 40 nm) on male reproductive function in adult rats, focusing on sperm characteristics, hormonal profile, testicular histology, and immunohistochemical markers.

Methods:
Forty-five adult male albino rats were randomly divided into three groups (n=15): control, 20 nm ZnO NPs, and 40 nm ZnO NPs. Nanoparticles were administered orally for 21 days. Sperm morphology and motility were evaluated, and serum testosterone was measured using Cobas 6000. Body and testicular weights were recorded. Testicular tissues were examined histologically (H&E) and immunohistochemically for Ki-67, Bcl-2, and von Willebrand factor (vWF). Data were analyzed using one-way ANOVA followed by Tukey’s post hoc test.

Results:
ZnO NPs exposure significantly impaired reproductive parameters in a size-dependent manner. Treated groups showed a significant reduction in normal sperm morphology and progressive motility, with increased sperm abnormalities, particularly tail defects (P<0.05). Serum testosterone levels were significantly decreased in exposed groups compared to controls (P<0.001). Body weight decreased significantly, while testicular weight showed no significant change. Histopathological analysis revealed degeneration of seminiferous tubules, germ cell loss, and disruption of spermatogenesis, more pronounced in the 20 nm group. Immunohistochemistry showed decreased Ki-67 expression and increased Bcl-2 expression in treated groups, indicating altered proliferation and apoptotic regulation. vWF expression was significantly elevated in the low-dose group and moderately increased in the high-dose group, suggesting endothelial alterations.

Conclusion:
Zinc oxide nanoparticles induce significant, size-dependent reproductive toxicity in male rats by impairing spermatogenesis, reducing testosterone levels, and disrupting testicular cellular proliferation, apoptosis, and vascular integrity.

Key words: Rats; Reproductive toxicity; Sperm motility; Testosterone; Zinc oxide nanoparticles.