Oocytes Activated in Youth Can Persist for a Lifetime, Mouse Study Reveals

In a discovery that reshapes how scientists understand the female biological clock, researchers in China have shown that oocytes awakened from their dormant state early in life can survive inside the ovary for nearly the entire reproductive lifespan — and still produce healthy offspring. The study, published in Nature Aging by Jing Liang, Xuebing Yang and colleagues at China Agricultural University, used an elegant genetic lineage-tracing system in mice to follow individual activated oocytes month by month, revealing a hidden reservoir of slow-moving follicles that acts as a buffer against reproductive exhaustion. The findings challenge the long-standing assumption that once an oocyte is activated for growth, its journey to ovulation is swift and irreversible.

Female mammals are born with a finite, nonrenewable pool of oocytes established before birth. Most of these cells remain sealed inside primordial follicles, dormant and protected, forming the ovarian reserve that must last a lifetime. A small fraction is continually recruited into growth, becoming what the researchers call activated oocytes. These enter growing follicles, where they are supported by surrounding somatic granulosa cells, and progress through well-defined stages: primary, secondary, preantral and antral. Because every activated oocyte represents a permanent withdrawal from the reserve, the pace at which these cells are consumed directly determines how long a female stays fertile. Yet the dynamics of this consumption — whether activated oocytes march forward at a uniform speed or linger in certain stages — had remained largely unknown, because conventional histology and short-term labeling techniques cannot follow individual follicles over months.

To solve this problem, the team engineered mice carrying a Zp3-CreER^T2 driver crossed with a dual-fluorescent mTmG reporter. When the drug tamoxifen was administered at postnatal day 35, corresponding to young adulthood, only oocytes actively expressing the zona pellucida protein ZP3 — that is, activated oocytes — switched on membrane-bound green fluorescent protein, while dormant oocytes stayed red and unlabeled. Labeling efficiency peaked at 63.12 ± 5.59 percent three days after induction, giving the researchers a large, precisely marked cohort to track. They then examined ovaries at intervals across a full 12-month tracing window, covering the entire reproductive lifespan of a laboratory mouse.

The results were striking. Two months after labeling, 47.97 ± 11.61 percent of the marked oocytes were still detectable; after six months, 15.46 ± 4.48 percent remained. Remarkably, even a full year later, 4.17 ± 1.11 percent of the originally labeled oocytes were still present in the ovaries. These were not cellular relics: when the researchers superovulated females that had been traced for nine months, they recovered green-labeled eggs that fertilized in vitro at rates comparable to unlabeled controls — 61.76 ± 18.11 percent versus 68.08 ± 16.60 percent — and yielded healthy two-cell embryos, blastocysts and live offspring with normal growth and fertility. Natural mating experiments at six and nine months after labeling produced GFP-positive pups, confirming that these long-lived oocytes contribute to real, physiological reproduction rather than merely surviving in tissue sections.

Having established that activated oocytes can persist, the team mapped where in the follicle pipeline they linger. Primary follicles proved fleeting: labeled oocytes disappeared from this stage within 30 days of tagging, meaning the primary phase lasts at most about a month. Secondary and antral follicles, by contrast, harbored labeled oocytes throughout the entire 12-month observation period. Because roughly 80 percent of the long-lived labeled oocytes were consistently found in secondary follicles — defined as follicles with multiple layers of granulosa cells but no visible antral cavity — the researchers concluded that the secondary stage is the principal site of long-term oocyte residence. Its maximum lifespan must therefore be at least 11 months, while its minimum can be estimated at roughly two weeks, based on transplantation studies showing that secondary follicles reach the antral stage within that time. In other words, secondary follicle lifespan spans an extraordinary range, from a few estrous cycles to nearly the whole reproductive life of the animal.

That variability turned out to be functional. BrdU incorporation assays revealed that some secondary follicles contained granulosa cells proliferating vigorously, while others showed far lower division rates — heterogeneity absent from antral follicles, which grew rapidly and uniformly. The secondary follicle population, the authors propose, functions as a developmental buffer zone: a secondary ovarian reserve that throttles the rate at which activated oocytes are spent, balancing immediate fertility against reproductive longevity.

The buffering capacity is not fixed but adapts to circumstances. When the team labeled activated oocytes in 10-month-old mice — late reproductive age — the marked cells vanished within three months, indicating accelerated utilization as the reserve dwindles. To separate the effect of age from that of reserve size, the researchers surgically removed one ovary from young females, halving the reserve without aging the animal. Labeled oocytes in these half-reserve mice were depleted in about seven months, roughly half the lifespan seen in intact controls. The primary follicle stage, notably, remained capped at about one month in all conditions, pinpointing the secondary stage as the adjustable regulatory window. The ovary, it appears, senses its remaining capacity and speeds up or slows down oocyte consumption accordingly — a homeostatic strategy the authors suggest may be governed by endocrine feedback reflecting reserve size.

What molecular machinery allows an oocyte to idle for months? Quantitative single-oocyte proteomics on individual oocytes isolated from secondary follicles after five months of tracing identified more than 5,962 protein groups, with 5,240 passing quality control. Unsupervised clustering cleanly separated long-lived labeled oocytes from their shorter-lived unlabeled counterparts: the long-lived group showed 826 significantly downregulated proteins and only four upregulated ones. The suppressed proteins clustered in developmental and metabolic pathways, including the translation regulator LIN28A, the amino-acid transporter SLC7A5, fatty acid synthase, glutaminase, the mitochondrial fusion protein MFN2 and the pyruvate carrier MPC1. Direct measurements confirmed the picture: EU staining showed reduced nascent RNA synthesis and HPG staining showed reduced protein synthesis in long-lived oocytes, while the granulosa cells surrounding them proliferated more slowly. Longevity, in short, is achieved through metabolic quiescence — a coordinated shutdown of biosynthesis, mitochondrial activity and energy production.

The linchpin connecting oocyte state to follicle pace is the proprotein convertase FURIN. Long-lived oocytes expressed markedly less FURIN than their rapidly developing neighbors. When the team inhibited FURIN with the drug DRVC in cultured secondary follicles, growth was dramatically suppressed; conversely, mice lacking Furin specifically in oocytes accumulated secondary follicles and lacked antral ones. Xenotransplantation experiments sealed the case: secondary follicles from knockout mice transplanted into recipient ovaries remained stuck at the secondary stage even after four weeks, while control follicles had progressed to ovulation. FURIN’s substrate appears to be GDF9, an oocyte-secreted factor of the transforming growth factor beta family that carries a canonical FURIN cleavage motif. Western blotting showed that knockout oocytes contained normal amounts of GDF9 precursor but drastically reduced mature GDF9, and supplementing cultures with recombinant GDF9 partially rescued the follicle growth defect. The authors note that BMP15 and other factors likely contribute as well, since GDF9 rescue was incomplete. The model that emerges is one of coordinated pacing: oocytes with high FURIN release abundant mature GDF9, driving granulosa cell proliferation and rapid follicle growth, while oocytes with low FURIN hold their follicles in slow motion, extending their own lifespan.

The implications reach well beyond mice. If human folliculogenesis follows similar dynamics — and recent comparative work suggests substantial similarity between human and mouse ovaries — then eggs ovulated in a woman’s thirties may have been activated years or even decades earlier, overturning the textbook estimate that the full journey from activation to ovulation takes about a year. Because activated oocytes are metabolically active and in constant dialogue with their environment, prolonged residence could allow the gradual accumulation of damage from endocrine disruptors, metabolic disease and lifestyle exposures long before ovulation, offering a mechanistic explanation for the age-related decline in oocyte quality. The authors caution that their experimental mice lived in specific pathogen-free conditions that may have protected long-lived oocytes from the stresses of the outside world, and that human-focused studies are still needed. Even so, the identification of a tunable secondary-follicle buffer — and of FURIN-mediated GDF9 maturation as its molecular lever — points toward concrete strategies for fertility preservation, from modulating oocyte metabolic activity to adjusting follicular dynamics, in the effort to extend reproductive health.

Subject of Research: Developmental dynamics and lifespan regulation of activated oocytes and secondary follicles in the mouse ovary

Article Title: Heterogeneity in secondary follicle development buffers the lifespan of activated oocytes to optimize female fertility in mice

Article References: Liang, J., Yang, X., Li, Q., Li, L., Wang, G., Mu, L., Sun, N., Geng, K., Hu, X., Niu, S., Jia, L., Wang, Y., Xu, X., Li, Z., Wang, F., Zhang, Y., & Zhang, H. (2026). Heterogeneity in secondary follicle development buffers the lifespan of activated oocytes to optimize female fertility in mice. Nature Aging. https://doi.org/10.1038/s43587-026-01237-5

Image Credits: AI Generated

DOI: 10.1038/s43587-026-01237-5

Keywords: oocytes, ovarian reserve, folliculogenesis, secondary follicles, FURIN, GDF9, lineage tracing, reproductive aging, fertility, metabolic quiescence, granulosa cells, Nature Aging