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Updated: Aug 24, 2026

Human Egg Maturity Assessment and Its Clinical Application
Published on: August 19, 2019
The relationship between pregnancy outcome and smooth endoplasmic reticulum clusters in MII human oocytes
J Otsuki1, A Okada, K Morimoto
1Nagai Clinic, Misato, Saitama 341-0004, Japan. midori@nagai-cl.com
Background:
During ICSI, we occasionally observe pronucleus sized translucent vacuoles. We investigated why these vacuoles occur and determined the effect on pregnancy outcome.
Methods:
Translucent vacuole-positive oocytes and the corresponding cohort were examined by transmission electron microscopy (TEM) and histochemical staining with DiI and ER-Tracker. Stimulation methods, hormonal levels, patients' condition and grade of transferred embryos were compared between vacuole-positive and vacuole-negative cycles.
Results:
By TEM, we confirmed that the vacuoles were tubular-type smooth endoplasmic reticulum clusters (sERCs). Numerous small sERCs were also observed in the oocytes from the same cohort. Veeck's grades of transferred embryos were higher in sERC-positive cycles and fertilization rate was similar to those of sERC-negative cycles. However, in sERC-positive cycles, significantly lower pregnancy and higher biochemical pregnancy rates were shown. Serum estradiol levels on the day of hCG administration were significantly higher in sERC-positive cycles.
Conclusions:
The presence of sERCs is associated with lower chances of successful pregnancy, even in sERC-negative oocytes from the same cohort that are transferred along with the sERC-positive oocytes. High estradiol levels could be one of the causes of sERC formation.
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