Developmental potential of oocytes with smooth endoplasmic reticulum clusters and their behaviors observed by time-lapse recording system

Abstract

We previously reported that oocyte morphology was associated with embryo quality and viability, suggesting that controlled assessment of oocyte morphology is useful to predict human embryo quality.During intracytoplasmic sperm injection (ICSI), we occasionally observe oocytes with smooth endoplasmic reticulum clusters (sERC) (Otsuki et al, 2004), which are thought to disappear after fertilization and be closely associated with poor prognosis in achieving pregnancy. This study aimed to investigate the developmental potential of oocytes with sERC and their behaviors using time-lapse recording system. Research study We collected the data of 6,765 matured oocytes retrieved after controlled ovarian hyper-stimulation from August 2010 to September 2015. During ICSI, three experienced embryologists noticed oocytes containing vacuole-like structures, which were defined as sERC by their disappearance at the zygote stage. These embryos were incubated and imaged by either high-resolution time-lapse cinematography or EmbryoScope®. Developmental velocity, rate of clinical use in embryo transfer or cryopreservation, and achieving pregnancy were compared between oocytes with [sERC(+)] and without sERC [sERC(-)]. Of the 6,765 mature oocytes, 46 (0.7%) showed sERC(+). The developmental velocity of embryos derived from sERC(+) oocytes was significantly higher than that of sERC(-) oocytes, including the time of 2nd polar body (PB) extrusion, pronuclei (PN) formation, and syngamy from the ICSI procedure (in h: 2.5±0.6 vs. 2.9±1.0, 7.6±2.4 vs. 9.9±3.9, and 20.7±2.2 vs. 23.7±4.5, respectively; P<0.05). The sERC all disappeared by 4.3±1.2 h after ICSI in the period from 2nd PB extrusion to PN formation. The rate of clinical use of sERC(+) oocytes was 80.4% (37/46), of which 6 underwent fresh embryo transfer and 31 were cryopreserved. In total, 15 embryos were transferred (6 fresh embryos, 9 frozen/thawed embryos), the pregnancy rate was 40% (6/15), and miscarriage rate was 33.3% (2/6). Moreover, the live birth rate was 50.0% (3/6) and one is still ongoing. Healthy babies were born from oocytes with sERC(+). This study demonstrated that oocytes with sERC could develop into good quality embryos and achieve successful pregnancy resulting in healthy babies. Further follow-up of the children are clearly needed, and mechanisms of the emergence of sERC and its impact on embryonic development are still under investigation. Detailed studies with molecular biological methods are necessary.