Mechanistic foundations of the metaphase II spindle of human oocytes matured in vivo and in vitro

Abstract
Study question: Are morphometric and morphological parameters of the metaphase II (MII) spindle of human oocytes matured in vivo or in vitro predictive of chromosome alignment on the metaphase plate?
Summary answer: Morphometric spindle parameters were very comparable between oocytes matured in vivo and in vitro and were unable to predict chromosome alignment, while a flattened shape of both poles was positively associated with chromosome displacement from the metaphase plate.
Main results and the role of chance:In ivo-MII oocytes, the sizes (mean ± SD) of major and minor axes were 11.8 ± 2.6 and 8.9 ± 1.7 μm, respectively, while maximum projection was 88.8 ± 29.5 μm(2). Very comparable values of these parameters were found in lgv-MII and ivm-MII oocytes. Double-focused spindles were rarely found (3.1%), unlike those with a double-flattened conformation (47.7%). Spindles with both focused and flattened poles amounted to almost half of the sample set (49.2%), but in this subgroup it was very infrequent (4.6%) to observe the flattened pole oriented towards the oolemma. Overall, differences in the relative proportions of pole morphology categories in ivo-MII, lgv-MII and ivm-MII oocytes were not statistically significant. For both the distal and proximal spindle hemidomains, optical intensity profiles were also comparable between ivo-MII, lgv-MII and ivm-MII oocytes. None of the morphometric parameters (major and minor axes, their ratio, maximum projection, distances of the metaphase plate from the poles) was associated with chromosome alignment on the metaphase plate or arrangement inside and outside the spindle. Importantly, a double-flattened outline of pole morphology was positively associated with the displacement of one or more chromosomes from the metaphase plate. Moreover, when a flattened pole was oriented towards the oolemma, a higher rate of chromosome displacement was observed.

Autori: Giovanni Coticchio, Maria Cristina Guglielmo, Mariabeatrice Dal Canto, Rubens Fadini, Mario Mignini Renzini, Elena De Ponti, Fausta Brambillasca, David F Albertini