The effects of blast-induced traumatic brain injury on brain cellular mechanics and differentiation.

Male reproductive health has deteriorated at an alarming pace over the past several decades, emerging as a pressing public health concern that cuts across industrialized and developing nations alike. A landmark meta-analysis drawing on 244 estimates from 7,518 men across 53 countries documented a 51.6% decline in sperm concentration and a 62.3% reduction in total sperm count between 1973 and 2018, with the slope of decline accelerating after the year 2000 (). Parallel epidemiological surveillance has flagged rising incidences of testicular dysgenesis syndrome, cryptorchidism, and testicular germ cell tumors in young men, pointing to insults operating during fetal programming windows (). Environmental chemical exposure—spanning phthalates, bisphenols, perfluoroalkyl substances, pesticide residues, and heavy metals—has been consistently linked to these trends through both human cohort data and mechanistic investigations (). The biological plausibility is compelling: many of these compounds disrupt androgen signaling, perturb Sertoli–germ cell crosstalk, or induce oxidative damage within the seminiferous epithelium (). Yet causal attribution at the individual compound level remains elusive, largely because our assessment toolkit lags behind the complexity of real-world exposures. Traditional reproductive toxicology has leaned heavily on rodent bioassays, which, while informative, suffer from a stubborn species-gap problem. Murine spermatogenesis unfolds over roughly 35 days versus approximately 74 days in humans, and the transcriptional choreography of spermatogonial stem cell maintenance diverges markedly between species (). Compounds such as 2,5-hexanedione and certain glycol ethers exhibit dramatically different testicular toxicity profiles in rats than in primates or humans, leading to both false positives and missed hazards when rodent data are extrapolated to human risk assessment (). Ethical pressure, encoded in the 3Rs principle and formalized through regulatory initiatives like REACH and the U.S. EPA’s push toward new approach methodologies, has further constrained the scope of permissible testing. Two-dimensional primary testicular cell cultures were developed as a partial remedy, yet they fail to preserve the blood-testis barrier, lose Leydig cell steroidogenic competence within days, and cannot sustain meiotic progression—meaning that the very endpoints most relevant to human fertility simply cannot be read out (). Testicular organoid technology has moved into this gap. Early work by Baert and colleagues demonstrated that dissociated human testicular cells could self-organize into tubule-like structures retaining Sertoli cell polarity and tight junction formation (). Subsequent efforts using rodent models achieved, for the first time, meiosis and limited haploid cell production within three-dimensional constructs (). Chinese research groups have contributed substantially through hydrogel optimization and the incorporation of induced pluripotent stem cell-derived germ cell precursors, broadening the donor base beyond biopsy-dependent primary cells (). Despite these advances, three challenges continue to constrain the field. Construction protocols differ considerably between laboratories—varying in cell ratios, matrix composition, culture duration, and hormonal supplementation—which complicates direct cross-study comparison, a limitation examined further in the Discussion. Functional maturity also remains incomplete; most reported organoids plateau at pachytene or round spermatid stages, with elongated spermatozoa rarely observed. And, perhaps most importantly, a coherent toxicological evaluation framework tailored to organoid-specific readouts has yet to be codified, leaving investigators to borrow endpoints from conventional assays ().