Disorganized eggs hinder fertility in obese women

By Sally Robertson, B.Sc.Sep 13 2012

Severely obese women are at an increased risk for producing eggs that have multiple spindles and disorganized chromosomes, possibly explaining the reduced fertility in this population, report researchers.

"Our observations indicate a high prevalence of cytoskeletal abnormalities in failed fertilized oocytes from severely obese patients compared with those from normal BMI [body mass index] patients," say Catherine Racowsky (Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA) and colleagues.

Development of the haploid oocyte is dependent on normal formation of the meiotic spindle, which, in turn, controls precise chromosomal segregation, explain the researchers. The oocyte should be "mature" - with one spindle on which an organized set of chromosomes are aligned.

As reported in Human Reproduction, in an analysis of failed fertilized eggs obtained from women undergoing in vitro fertilization, the likelihood for an oocyte having more than one spindle was significantly greater among 105 severely obese (body mass index [BMI] 35.0-50.1 kg/m²) women than among 90 women who had a normal BMI (18.5-24.9 kg/m²), at an odds ratio of 2.68.

Furthermore, when only one spindle was present, the severely obese group demonstrated a significantly higher prevalence of spindles with nonaligned chromosomes, compared with the normal BMI group, at 28.6% versus 8.6%.

These findings were independent of the effect of potential confounders including age at cycle start, intracytoplasmic sperm injection, and polycystic ovary syndrome.

The increased prevalence of spindle abnormalities in the severely obese women raises the concern of impaired quality and possible cytoplasmic immaturity of oocytes that would otherwise have been deemed mature based on nuclear maturation stage, writes the team.

"Such an immature cytoplasmic environment is associated with abnormal sperm processing and formation of premature chromosome condensation."

Racowsky et al say their observations provide a novel insight into a possible cause for the reduced fertility in severely obese patients.

"Further work is warranted to investigate the mechanism underlying the spindle abnormalities we have observed in this population," the authors conclude.

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