title: Chromosomal stability of second polar bodies in mouse embryos.
abstract: OBJECTIVE
  Incorporation of a second polar body (PB2) into one of the blastomeres has been considered as a causal mechanism underlying diploid/triploid mixoploidy in humans. Using a mouse model, we examined whether PB2s can participate in the formation of mixoploidy.
  
  
  METHODS
  Uptake of BrdU was examined to determine DNA synthesis in PB2s up to 28 h after fertilization. PB2s from embryos at 4-6 (1-cell), 24 (2-cell), 48 (4-cell), and 72 h (morula) were fused with MII oocytes to induce premature chromosome condensation. Caspase and TUNEL assays were used to detect apoptotic PB2s at 24, 48, and 72 h. PB2s were fused with one of the blastomeres of the 2-cell embryos to produce mixoploid embryos.
  
  
  RESULTS
  DNA synthesis in the PB2s continued until 22 h after fertilization. At 4-6 h, nearly all of the PB2s showed G1-type chromosomes and there was no significant increase in chromosome damage. At 24, 48, and 72 h, S-type chromatin predominated. Few PB2s showed apoptotic response until 72 h. Regardless of the fusion with the PB2, more than 90 % of the embryos developed to 4-cell stage, and over 80 % of the resultant 4-cell embryos had daughter blastomeres with a morphologically normal nucleus. Some of the daughter blastomeres displayed triploidy.
  
  
  CONCLUSIONS
  The PB2 is viable for at least 72 h after fertilization, with slow progression through the cell cycle. Once the PB2 has been incorporated into a blastomere, the cell cycle of the PB2 might be synchronized with that of the host resulting in diploid/triploid mixoploidy.
authors.affiliation: Department of Biological Sciences, Asahikawa Medical University, 2-1-1-1 Midorigaoka-higashi, Asahikawa, 078-8510, Japan. hino@asahikawa-med.ac.jp
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Hino
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Kusakabe
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Tateno
mesh_terms: D000818:Animals; D017209:Apoptosis; D001757:Blastomeres / Q000166:cytology; D001973:Bromodeoxyuridine; D002453:Cell Cycle; D002467:Cell Nucleus / Q000235:genetics; D053144:Cell Nucleus Shape; D002470:Cell Survival; D002843:Chromatin / Q000235:genetics; D043171:Chromosomal Instability; D033481:Chromosomes, Mammalian / Q000235:genetics; D004622:Embryo, Mammalian / Q000166:cytology*; D047108:Embryonic Development; D005260:Female; D005306:Fertilization; D020287:In Situ Nick-End Labeling; D051379:Mice; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D011003:Ploidies; D059705:Polar Bodies / Q000166:cytology*; D020554:Sperm Injections, Intracytoplasmic; D013997:Time Factors
chemical_list: D002843:Chromatin; D001973:Bromodeoxyuridine