title: Time-dependent capability of human oocytes for activation and pronuclear formation during metaphase II arrest.
abstract: BACKGROUND
  The purpose of this study was to investigate the fertilization rate and developmental potential of human oocytes in relation to the duration of their metaphase II (MII) arrest stage following the extrusion of the first polar body (1PB).
  
  
  METHODS
  Immature metaphase I oocytes (MI; study oocytes, n = 468) that underwent meiotic maturation during brief in vitro culture and their matured in vivo, MII siblings (control oocytes, n = 3293) were subjected to ICSI. Fertilization and early cleavage were evaluated in both study and control groups.
  
  
  RESULTS
  The overall fertilization rate was significantly lower in the oocytes matured in vitro than in those matured in vivo (42 versus 77%, P < 0.0001). A significant relationship was observed between oocyte activation potential and the length of MII arrest. The majority of study oocytes injected soon after PB extrusion remained unfertilized (64%; 98/154 oocytes). The proportion of normally activated oocytes that contained two pronuclei and two PBs gradually increased with prolonged time of MII arrest (43 and 61% at 2 and 3-6 h after 1PB extrusion). Significantly more embryos originating from the study than control oocytes were arrested soon after the first two cleavage divisions (39 and 17%; P < 0.0001) and exhibited multinucleated blastomeres (23 and 13%; P < 0.0001), which suggests the existence of chromosomal abnormalities.
  
  
  CONCLUSIONS
  Human oocytes progressively develop the ability for full activation and normal development during the MII arrest stage.
authors.affiliation: CReATe Program Inc. Sunnybrook and Women's College Health Sciences Center, University of Toronto, Toronto, Ontario, Canada. hbalakier@sympatico.ca
authors.forename: Hanna
authors.initials: H
authors.lastname: Balakier
authors.forename: Agata
authors.initials: A
authors.lastname: Sojecki
authors.forename: Gelareh
authors.initials: G
authors.lastname: Motamedi
authors.forename: Cifford
authors.initials: C
authors.lastname: Librach
mesh_terms: D001757:Blastomeres / Q000648:ultrastructure; D002467:Cell Nucleus / Q000648:ultrastructure; D002478:Cells, Cultured; D016922:Cellular Senescence; D002970:Cleavage Stage, Ovum; D003593:Cytoplasm / Q000502:physiology; D005260:Female; D005306:Fertilization; D006801:Humans; D008540:Meiosis; D008677:Metaphase / Q000502:physiology*; D009865:Oocytes / Q000502:physiology* / Q000648:ultrastructure; D020554:Sperm Injections, Intracytoplasmic; D013997:Time Factors