Growing Oocytes Need Zinc: Zinc Deficiency in the Preantral Ovarian Follicle
Authors: Hester James M. , Diaz Francisco 
Published: June 2021

Objective

To evaluate the effect of zinc deficiency on oocyte development in the preantral ovarian follicle – a novel developmental stage for micronutrient investigation.

Methods

Oocyte‐granulosa cell complexes were collected from preantral ovarian follicles of 14 day old CD‐1 mice and cultured on collagen membranes under control, zinc‐deficient, or rescue conditions. Oocyte diameter was measured from brightfield images taken with an Olympus DP20 microscope digital camera. Chromatin arrangement and nucleolar development was observed after DAPI staining. Gene expression was measured with qPCR. Meiotic competence was measured by exposing cultured oocyte‐granulosa cell complexes to Epidermal Growth Factor (EGF) to mimic the in‐vivo ovulatory signal (maturation media). The ability of the oocytes to undergo meiotic division was assessed 24 hours after EGF treatment.

Results

Zinc deficiency disrupted oocyte growth in vitro. Zinc deficient oocytes were smaller than control or rescue on day 4 (p<0.001) and 6 (p<0.05), but not day 10 of cell culture. Zinc deficiency impacted nuclear development; zinc deficient oocytes were more likely to have multiple nucleoli on day 6 (p<0.001), but not day 10 of cell culture. Differentiation of somatic cells was also impaired; zinc‐deficient granulosa cell gene expression was consistent with a preantral, undifferentiated phenotype, even after 10 days of culture. Preantral somatic cell markers lhcgr, ar, pcsk6, and fosb were elevated compared with control (p<0.05 for all). Preantral zinc deficiency reduced the meiotic potential of cultured oocytes. After EGF treatment, oocytes matured in zinc deficient media were less likely to resume meiosis and reach the meiotic stage required for fertilization (chi square p<0.001). The meiotic defect was seen in zinc‐deficient oocytes even when transferred to maturation media containing zinc.

Conclusions

Prior to entering the pool of antral ovulatory follicles, oocytes and the surrounding somatic cells undergo a period of growth and differentiation. This phase is known as preantral oocyte development and may last 3–5 months in humans. This study shows that zinc deficiency impairs multiple preantral developmental milestones including oocyte growth, oocyte nuclear maturation, and somatic cell differentiation. Most notably, the acquisition of meiotic competence – a benchmark of oocyte fertility and viability – was severely reduced by zinc deficiency. The meiotic defect was noted in zinc‐deficient cultured oocytes even when they were transferred to maturation media containing zinc, indicating a zinc requirement during development, rather than during meiotic resumption. These results demonstrate that zinc is required in oocyte development earlier than previously reported, and that even short periods of zinc deficiency may impact fertility months later. These results may impact our understanding of prenatal micronutrient supplementation, as well as optimal methods for in‐vitro oocyte maturation.

Support or Funding Information

NIH ‐HD074831‐01

This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.