Chromatin organization and timing of polar body I extrusion identify developmentally competent mouse oocytes
Published: 16 April 2019
Federica Cavalera1,2, Mario Zanoni1,2, Valeria Merico1,2, Lucia Sacchi2,3, Riccardo Bellazzi2,3, Silvia Garagna*,1,2 and Maurizio Zuccotti*,1,2
1Laboratorio di Biologia dello Sviluppo, Dipartimento di Biologia e Biotecnologie “Lazzaro Spallanzani”, 2Centre for Health Technologies (C.H.T.) and 3Dipartimento di Ingegneria Industriale e dell’Informazione, University of Pavia, Pavia, Italy
In the mouse, the use of the DNA-binding fluorochrome Hoechst 33342 allows the classification of fully-grown antral oocytes into two categories distinguished by their chromatin conformation: surrounding nucleolus (SN) and not-surrounding nucleolus (NSN) oocytes, the former capable of completing development, the latter unable to proceed beyond the 2-cell stage. In the present study, time-lapse observation of SN and NSN oocyte GV-to-MII transition highlighted differences in the timing of germinal vesicle breakdown (GVBD) and polar body I (PB-I) extrusion. PB-I extrusion, but not GVBD, revealed the presence of three main groups of significantly different oocytes: Group A (456-576 min) comprising mainly SN oocytes (91.4%), group B (584-728 min) entailing an almost equivalent percentage of SN (52.7%) and NSN (47.3%) oocytes, whereas group C (736-896 min) consisting of almost all NSN (94.4%) oocytes. In a further set of time-lapse experiments, GV oocytes were in vitro matured without Hoechst staining and, depending on the timing of PB-I extrusion, sorted into group A, B or C, inseminated with sperm and observed throughout preimplantation. The results show that 26.2 ± 12.3% of group A, 2.4 ± 5.0% of group B and none of group C MII oocytes developed to blastocyst. Overall, this study shows that SN oocytes that complete MI earlier are those with a better developmental competence. The possibility to avoid the use of the invasive DNA-binding fluorochrome Hoechst is relevant for future applications in human and domestic animal reproductive technologies.
oocyte, chromatin organization, in vitro maturation, polar body I, preimplantation development