Rare Earth Elements in Testicular Function: Current Knowledge and Future Directions

Rare earth elements (REEs) are increasingly recognized as modulators of male reproductive health. This review presents the first comprehensive and critical analysis of the detrimental and potentially beneficial effects of REEs on male vertebrate reproductive function. In vivo and in vitro studies show that elements, particularly Gadolinium, Lanthanum, and Yttrium, share a toxicological profile characterized by oxidative stress, inflammation, mitochondrial dysfunction, endocrine disruption, and impairment of the blood–testis barrier. These mechanisms converge on both somatic and germ cell populations, ultimately compromising spermatogenesis and hormonal balance. Limited epidemiological data in humans are consistent with experimental findings, indicating inverse associations between seminal REE levels and sperm quality. CeO2 nanoparticles represent an exception, showing dose-dependent duality: although some murine studies describe toxic effects, a broader and more consistent body of evidence indicates their protective action by restoring testicular homeostasis in diabetic models, in rats exposed to pesticides, pharmacological agents or irradiation, and in in vitro-treated spermatozoa. Preliminary data on Yttrium (YO) and Gadolinium (GdVO4) nanoparticles similarly point to protective effects under pathological conditions, whereas Neodymium and Samarium show reproductive toxicity, including hormonal disruption and impaired sperm quality. Overall, REEs cannot be considered a homogeneous class: some pose reproductive hazards, others show potential biomedical utility. A systematic, mechanistic research framework is needed to resolve these dualities. It should contextualise reproductive risks in relation to the growing environmental and occupational exposure to REEs, while simultaneously exploring the biomedical potential of those elements that exhibit protective profiles.