Heavy Metals in Follicular Fluid Affect the Ultrastructure of the Human Mature Cumulus-Oocyte Complex

HIGHLIGHTS: What are the main findings? For the first time, ultrastructural alterations were detected using transmission electron microscopy in human cumulus-oocyte complexes (COCs) sampled from follicles with Pb and Cd levels in the FF of infertile women subjected to assisted reproductive technologies. The intrafollicular presence of these metals could be responsible for morphological alterations in some cell organelles, which may lead to altered maturation and quality of oocytes, impairment of energetic metabolism, cellular dysfunction, and apoptosis of CCs. Since blood Cd levels were above the current reference values established by the Agency for Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA), whereas blood Pb levels were below the ATSDR reference values, we believe that these alterations could be due especially to Cd, even if we cannot exclude a possible additional effect of Pb. What is the implication of the main finding? Cd levels in FF may influence the oocyte directly and indirectly (CC-mediated), affecting its quality, as well as fertilization, early embryonic development, and pregnancy. ABSTRACT: It is known that exposure to heavy metal such as lead (Pb) and cadmium (Cd) has several adverse effects, particularly on the human reproductive system. Pb and Cd have been associated with infertility in both men and women. In pregnant women, they have been associated with spontaneous abortion, preterm birth, and impairment of the development of the fetus. Since these heavy metals come from both natural and anthropogenic activities and their harmful effects have been observed even at low levels of exposure, exposure to them remains a public health issue, especially for the reproductive system. Given this, the present study aimed to investigate the potential reproductive effects of Pb and Cd levels in the follicular fluid (FF) of infertile women and non-smokers exposed to heavy metals for professional reasons or as a result of living in rural areas near landfills and waste disposal areas in order to correlate the intrafollicular presence of these metals with possible alterations in the ultrastructure of human cumulus-oocyte complexes (COCs), which are probably responsible for infertility. Blood and FF metals were measured using atomic absorption spectrometry. COCs corresponding to each FF analyzed were subjected to ultrastructural analyses using transmission electron microscopy. We demonstrated for the first time that intrafollicular levels of Pb (0.66 µg/dL–0.85 µg/dL) and Cd (0.26 µg/L–0.41 µg/L) could be associated with morphological alterations of both the oocyte and cumulus cells’ (CCs) ultrastructure. Since blood Cd levels (0.54 µg/L–1.87 µg/L) were above the current reference values established by the guidelines of the Agency for Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA) (0.4 µg/L), whereas blood Pb levels (1.28 µg/dL–3.98 µg/dL) were below the ATSDR reference values (≤5 µg/dL), we believe that these alterations could be due especially to Cd, even if we cannot exclude a possible additional effect of Pb. Our results highlighted that oocytes were affected in maturation and quality, whereas CCs showed scarcely active steroidogenic elements. Regressing CCs, with cytoplasmic alterations, were also numerous. According to Cd’s endocrine-disrupting activity, the poor steroidogenic activity of CCs might correlate with delayed oocyte cytoplasmic maturation. So, we conclude that levels of heavy metals in the blood and the FF might negatively affect fertilization, embryo development, and pregnancy, compromising oocyte competence in fertilization both directly and indirectly, impairing CC steroidogenic activity, and inducing CC apoptosis.