Simple SummaryExposure to endocrine-disrupting pollutants, such as the fungicide mancozeb, is linked to various environmental health hazards, including female fertility. Although the dithiocarbamate mancozeb has low reported toxicity in mammals, it impairs female reproductive functions in exposed animals and humans. The specific mechanism of action of mancozeb and the damage to cell structures in the female reproductive system are still unclear. This study aims to describe the ultrastructure of mouse oocytes exposed in vitro to increasing concentrations of mancozeb (0.001–1 μg/mL) by light and transmission electron microscopy and to perform a morphometric analysis over significant organelles. While from 0.001 to 0.1 μg/mL, oocyte ultrastructure was comparable to controls, at the highest concentration (1 µg/mL), a decrease in the numerical density of mitochondria and cortical granules, an altered organelle distribution, and flattening of microvilli were observed. These results could be responsible for the adverse effect of this fungicide on mammalian reproductive performance.AbstractMancozeb is a widely used fungicide, considered to be an endocrine disruptor. In vivo and in vitro studies evidenced its reproductive toxicity on mouse oocytes by altering spindle morphology, impairing oocyte maturation, fertilization, and embryo implantation. Mancozeb also induces dose-dependent toxicity on the ultrastructure of mouse granulosa cells, including chromatin condensation, membrane blebbing, and vacuolization. We evaluated the effects on the ultrastructure of mouse oocytes isolated from cumulus-oocyte complexes (COCs), exposed in vitro to increasing concentrations of mancozeb. COCs were matured in vitro with or without (control) low fungicide concentrations (0.001–1 μg/mL). All mature oocytes were collected and prepared for light and transmission electron microscopy. Results showed a preserved ultrastructure at the lowest doses (0.001–0.01 μg/mL), with evident clusters of round-to-ovoid mitochondria, visible electron-dense round cortical granules, and thin microvilli. Mancozeb concentration of 1 μg/mL affected organelle density concerning controls, with a reduction of mitochondria, appearing moderately vacuolated, cortical granules, and microvilli, short and less abundant. In summary, ultrastructural data revealed changes mainly at the highest concentration of mancozeb on mouse oocytes. This could be responsible for the previously described impaired capability in oocyte maturation, fertilization, and embryo implantation, demonstrating its impact on the reproductive health and fertility.