Impairment of the ultrastructure of mammalian oocytes under different in vitro culture conditions

In vitro culture systems of cumulus-oocyte-complexes (COCs) allow for the precise control of environmental factors and provide a valuable tool for studying the ultrastructure of oocytes under various experimental culture conditions. Endocrine disruptors (EDs) can interfere with the normal functioning of the endocrine system, also affecting reproduction. We previously demonstrated a dose-dependent detrimental effect exerted by EDs, such as the dithiocarbamate Mancozeb and the organochlorine Lindane, on the ultrastructure of granulosa cells cultured in vitro. Since exposure to EDs can hamper crucial processes for the success of fertilisation and embryonic development, in this study we aimed to assess the effect of Mancozeb and Lindane on the ultrastructure of mouse MII-stage oocytes retrieved by COCs cultured in vitro with or without increasing concentrations of Mancozeb (0.001-1 µg/ml) and Lindane (1-100 µM). TransMsion electron microscopy was used to investigate the effects of the selected EDs. The results showed a preserved ultrastructure at the lowest doses, with an ooplasm rich in clusters of round-to-ovoid mitochondria, cytoplasmic lattice, elements of the smooth endoplasmic reticulum and electron-dense round cortical granules. Lipid droplets, multivesicular aggregates and lamellar bodies were numerous. The ooplasm was continuous and contained thin microvilli. A narrow perivitelline space was delimited by an intact zona pellucida. The highest concentrations of EDs affected organelle density, with a reduction of mitochondria, also appearing moderately vacuolated, cortical granules and microvilli. The latter were short and less abundant. These results contribute to understanding the mechanisms by which EDs can impair oocyte development and maturation, and to the identification of potential strategies for minimising their impact on reproductive health.