The ovaries are among the first organs to age, and this affects not only fertility but also overall health. To study genes in adulthood, scientists often use tamoxifen-inducible Cre systems: the drug triggers recombination at the desired moment, avoiding side effects from developmental interference. However, tamoxifen itself is a modulator of estrogen receptors, and the ovary is extremely sensitive to hormonal signals. Alongside this work, the archive contains studies on cell aging and drug dose control, but the long-term effects of tamoxifen on the ovaries remained unclear.
Female Cx3cr1 Cre/ERT2 NuTRAP mice at 3 months of age were randomly divided into two groups: one received tamoxifen in sunflower oil for five consecutive days, the other received only oil. The estrous cycle was monitored by vaginal smears at 3 days, 2, 3, and 9 months after injections. At 6 and 12 months, ovaries were collected for histology and RNA sequencing. Fibrosis was assessed by picrosirius red staining, and accumulation of multinucleated giant cells by Sudan black.
Tamoxifen caused a sharp but temporary disruption of the estrous cycle: immediately after the course, mice were stuck in metestrus, but after 3 months the cycle fully recovered and thereafter did not differ from controls. Histological signs of aging, fibrosis and accumulation of multinucleated giant cells, increased with age equally in both groups, with no effect of tamoxifen. Transcriptomic analysis revealed only three genes with altered expression in young mice and three in old mice upon direct treatment comparison, and these genes are not related to ovarian aging. Age-related expression changes were virtually identical in mice receiving tamoxifen and vehicle: the correlation of log fold changes was 0.771.
The work shows that tamoxifen-inducible models are suitable for studying ovarian aging, provided a recovery period is observed. Unlike neighboring studies that focus on cellular mechanisms of aging or pharmacokinetics, this is the first systematic test of the long-term effects of the inducer itself at the whole-organ level. This fills an important gap in interpreting many reproductive aging experiments.
It would be promising to test other Cre lines, dosages, ages of administration, and longer observation periods. Of particular interest is the effect of tamoxifen on ovarian cell subpopulations, especially tissue macrophages, which can be labeled in this model. It is also important to assess effects on the fallopian tubes, since that is where ovarian cancer often originates.
The results will directly influence the planning of experiments in reproductive biology, geroscience, and oncology, where inducible genetic mouse models are used.
Conduct similar tests with other inducible systems and across a wider range of ages. Study the cellular composition of ovaries after tamoxifen using single-cell RNA-seq.
Ovarian aging still lacks a unified model explaining the contribution of hormonal, immune, and stromal changes. This work clarifies the boundaries of applicability of a popular tool, allowing artifacts of induction to be separated from true aging mechanisms.
In humans, a 6-month-old mouse roughly corresponds to a 20-year-old woman, and a 12-month-old to a 40-year-old. So the experiment spans the period from peak fertility to the onset of age-related decline.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.