Correlated evolution of core skin structures in freshwater fishes and challenges to epidermal club cells as the material basis of alarm responses.

The European Society of Human Reproduction and Embryology (ESHRE) reports that nearly four million assisted reproductive technology (ART) cycles are performed annually, resulting in more than one million births worldwide. Increasing evidence indicates that ART procedures may influence the epigenetic landscape of offspring. A key component of ART is ovulation induction, during which high doses of gonadotropins are administered to promote the development of multiple follicles. In mouse and human oocytes, CpG methylation accumulates during growth and is largely established by the germinal vesicle (GV) stage as a bimodal landscape, with the MI/MII stages contributing to locus-specific refinement. Histone modifications play an essential role in directing this process; for example, H3K36me2 and H3K36me3 guide DNMT3A-mediated de novo DNA methylation, whereas the removal of H3K4me2/3 facilitates CpG island methylation. Although dispensable for oocyte maturation and fertilization competence, proper DNA methylation remains critical for normal embryonic development. Previous studies have suggested that ovarian stimulation may alter DNA methylation patterns and interfere with the acquisition of methylation imprints during oocyte growth. However, whether ovulation induction in ART perturbs oocyte DNA methylation and, in turn, alters epigenetic states in the ensuing embryo remains unresolved. Over the past decade, multiple prospective cohort studies have reported that ovulation induction may increase the risk of neurodevelopmental abnormalities in offspring, prompting concern that epigenetic changes induced during follicular stimulation could underlie these effects. In parallel, accumulating evidence links superovulation to epigenetic perturbations in oocytes and embryos, which are associated with alterations in the development of numerous mammalian species. Given the multistep nature of clinical ART, existing epigenetic studies have focused predominantly on medications that support follicular development (e.g., FSH and hMG) during superovulation, while the potential impact of trigger-day medications remains unclear. We therefore tested whether trigger-day exposure alters DNA methylation patterns in mice and evaluated potential neurodevelopmental outcomes. In a prospective birth cohort of 365 families, we found that ART-conceived offspring had lower global DNA methylation levels than naturally conceived children, and this reduction was primarily attributable to hCG exposure on the trigger day. Importantly, this hCG-associated reduction in DNA methylation was linked to impaired neurodevelopment in offspring, as higher hCG doses were associated with an increased risk of suboptimal receptive communication in a cohort of 1333 ART-conceived singleton families. These findings suggest that hCG exposure may affect neurodevelopment in offspring by influencing DNA methylation. We tested this hypothesis by establishing a mouse model in which transient hCG exposure impaired neurocognitive behaviors in offspring and suppressed neurogenesis in the hippocampal dorsal dentate gyrus (dDG). We further demonstrated that transient hCG exposure led to the downregulation of expression and a concomitant loss of DNA methylation in oocytes. Importantly, microinjection of the mRNA into hCG-exposed zygotes restored neurogenesis and improved behavioral outcomes in offspring. Collectively, these findings identify trigger-day hCG as a modifiable epigenetic risk factor that compromises neurodevelopment in offspring through a disruption of gametic DNA methylation patterns, underscoring the need for evidence-based ART protocols that minimize epigenetic perturbations and protect the neurodevelopment of offspring.