Relationship between meiotic spindle location with regard to the polar body position and oocyte developmental potential after ICSI

Abstract

BACKGROUND:The recent development of a computer-assisted polarization microscopy system (Polscope) with which the meiotic spindle can be visualized in living oocytes on the basis of its birefringence permits analysis of the meiotic spindles of oocytes subjected to ICSI. Previous studies have shown that the meiotic spindle is not always aligned with the ®rst polar body (PB) in metaphase II human oocytes prepared for ICSI. In the present study, the relationship between the degree of meiotic spindle deviation from the ®rst PB location and ICSI outcome was analysed. METHODS: Oocytes were divided into four groups according to the angle of meiotic spindle deviation from the PB position. The angle of deviation was 0±5°, 6±45°, 46±90°and >90°for groups I to IV respectively. RESULTS: The rates of normal [2 pronuclei (PN)] and abnormal (1PN or >2PN) fertilization did not differ between groups I, II and III. However, the rate of normal fertilization was lower among oocytes in which the meiotic spindle deviation angle was >90°; this led to an increased proportion of tripronucleated zygotes that failed to extrude the second PB. When embryos developed from normally fertilized oocytes were evaluated on day 3 after ICSI, no relationship was found between the angle of meiotic spindle deviation and embryo quality. The meiotic spindle was not detected in only 9% of oocytes, and these showed a higher incidence of fertilization and cleavage abnormalities than did oocytes in which the spindle was detected. When oocytes at metaphase I after cumulus oophorus and corona radiata removal were matured in vitro, the meiotic spindle was detected in 53.8% of those that reached metaphase II. In these in-vitro-matured oocytes the meiotic spindle was always aligned with the ®rst PB, suggesting that misalignment seen in those oocytes matured in vivo resulted from PB displacement during manipulations for cumulus and corona removal. CONCLUSION: High degrees of misalignment between the meiotic spindle and the ®rst PB predict an increased risk of fertilization abnormalities. However, when normal fertilization had occurred, the cleavage potential of embryos developing from such oocytes was not impaired. These ®ndings facilitate the selection of oocytes for ICSI in situations when the creation of supernumerary embryos is to be avoided.Key words: ICSI/meiotic spindle/oocyte quality/polar body/Polscope Introduction The orientation of the injection channel and the sperm deposition site during ICSI are usually chosen with regard to the position of the ®rst polar body, assuming that the meiotic spindle with metaphase chromosomes is located in a nearby region of the oocyte cytoplasm. However, recent observations made with the use of a novel orientation-independent polarized microscopy system (Polscope), coupled with image processing software enabling the visualization of meiotic spindles in living oocytes, have shown that the ®rst polar body position does not predict accurately the location of the meiotic spindle in

Visualization and Examination of the Meiotic Spindle in Human Oocytes with Polscope

“…Rienzi et al found, however, that in case of normal fertilization the cleavage potential of embryos developing from such oocytes was not impaired (13).…”

“…Studies have shown that the spindle is not, as had been assumed, always adjacent to the polar body (11,13,22). Generally, the proportion of oocytes in which the spindle is significantly displaced is low and the displacement is not necessarily associated with poor oocyte and embryo development.…”

“…Previous studies have shown that the presence of birefringent spindles predicts higher fertilization rates and that the position of the polar body is not an accurate predictor of spindle position in mammalian oocytes (8,(11)(12)(13). In this study we examined 1) whether spindles in living human oocytes could safely be imaged/examined by the polscope during clinical application of IVF/ICSI-ET and 2) the effect of detectability and location of meiotic spindle to fertilization of human oocytes with ICSI.…”

Sperm retention site and its influence on cleavage rate and early development following intracytoplasmic sperm injection

“…Recently, the meiotic spindle location with regards to polar body position has been studied (6)(7)(8). In clinical practice, instrument calibration and the condition of the oocyte cytoplasm influence sperm injection.…”

“…Blake et al reported that injection at the seven or eleven o'clock position in relation to the polar body resulted in the greatest number of high-quality embryos (5). Moreover, a high degree of misalignment between the meiotic spindle and the first polar body results in an increased risk of fertilization abnormalities (6). The injection procedure is an extremely important step in instituting fertilization; however, it is technically difficult to precisely inject a sperm at a desired location during ICSI.…”