Spindle positions and their distributions in in vivo and in vitro matured mouse oocytes

Abstract

This study was carried out to compare spindle locations and their developmental competencies both in vivo and in vitro in matured mouse oocytes. Spindle locations were identified using a polscope. Since meiotic spindles in living oocytes are highly birefringent, their structures can be viewed non-invasively by using a polscope. In vivo matured metaphase II oocytes were collected from the oviducts of mice. Immature oocytes were collected from mouse ovaries, and then cultured in YS medium until the first polar body (PB) extrusion. In vitro and in vivo matured oocytes were classified into four categories according to their spindle positions relative to the first PB (0 degrees , 0-90 degrees , 90-180 degrees and without a spindle image), and rates of fertilization and blastocyst formation were assessed. In vivo matured oocytes with a 0 degrees spindle disposition relative to PB were cultured in vitro for 24 h, and then their spindle positions were re-assessed. Most in vivo matured oocytes (89.1%) had a 0 degrees spindle position. Only 6 and 3% of oocytes had spindle positions of 0-90 degrees and 90-180 degrees , respectively. No spindle image was observed in 2%. However, most in vitro matured oocytes (83.1%) had a 0-90 degrees spindle position and, in contrast, only 6.5% of these oocytes had a 0 degrees spindle position. The rate of fertilization and blastocyst rate were significantly higher for in vivo matured oocytes than in vitro matured oocytes (87.1 versus 64.9% and 76.1% versus 66.0%, respectively, P<0.05 for each). We also observed that 71.7% of the in vivo matured oocytes with the 0 degrees spindle position showed a spindle position change to 0-90 degrees after 24 h of culture. These oocytes had a poor fertilization rate (43%) and a zero blastocyst rate. In vitro matured mouse oocytes showed quite different spindle positions compared with in vivo matured oocytes. Moreover, in vivo matured oocytes cultured for 24 h had a spindle position distribution that was similar to that of in vitro matured oocytes. The different spindle positions observed in in vivo and in vitro matured oocytes may reflect differences in their cytoplasmic maturation processes. These findings have implications regarding the lower developmental competency of in vitro matured oocytes.