title: The effect of cryopreservation on the genome of gametes and embryos: principles of cryobiology and critical appraisal of the evidence.
abstract: BACKGROUND
  Cryopreservation has been extensively used in assisted reproductive technology, agriculture and conservation programmes for endangered species. The literature reports largely positive results regarding the survival of frozen-thawed cells and clinical outcomes. Nonetheless, it is unclear whether or not cryopreservation of sperm, oocytes and embryos causes any disruption in their genetic integrity. Drawing on the available published evidence, this review paper describes in detail the physical and biochemical factors of cryopreservation that could potentially affect genomic integrity.
  
  
  METHODS
  A critical review of the published literature using PubMed with particular emphasis on studies which include assessment of genetic stability after cryopreservation of oocyte, sperm and embryos. The search was performed in 2014 and covered the period from the beginning of electronic records until July 2014. No language restrictions were applied.
  
  
  RESULTS
  Cryopreservation is associated with extensive damage to cell membranes, and results in alteration of the functional and metabolic status of the cells and mitochondria. Some evidence suggests an increase in DNA single-strand breaks, and degree of DNA condensation or fragmentation in sperm after cryopreservation. The extent of these changes may vary between different individuals and different techniques. The addition of antioxidants to the cryopreservation media and the use of well-controlled cooling regimes could potentially improve such outcomes. Limited numbers of studies on oocytes provide controversial results regarding the effect on DNA fragmentation, sister chromatid exchange (SCE) and aneuploidy. The only study on human embryos suggested that vitrification affects DNA integrity to a much lesser extent than slow freezing. Animal studies show increases in mitochondrial DNA mutations in embryos after cryopreservation. The limited numbers of long-term follow-up studies in humans provide reassurance that derives mostly from retrospective studies with some methodological weaknesses.
  
  
  CONCLUSIONS
  This review provides an overview of studies performed to date on the effect of cryopreservation on the oocyte, sperm and embryos. Controversy of the reported data has highlighted the gaps in our knowledge not only for clinical studies, but also for basic research in human embryos. New perspectives for future research are proposed.
authors.affiliation: Assisted Conception Unit, Guy's and St Thomas NHS Trust, Great Maze Pond, London SE1 9RT, UK julia_kop@yahoo.com.
authors.forename: Julia
authors.initials: J
authors.lastname: Kopeika
authors.affiliation: Department of Biosciences, University of Kent, Canterbury, Kent CT2 7NZ, UK.
authors.forename: Alan
authors.initials: A
authors.lastname: Thornhill
authors.affiliation: Assisted Conception Unit, Guy's and St Thomas NHS Trust, Great Maze Pond, London SE1 9RT, UK.
authors.forename: Yacoub
authors.initials: Y
authors.lastname: Khalaf
mesh_terms: D000818:Animals; D015925:Cryopreservation / Q000379:methods*; D004622:Embryo, Mammalian; D005260:Female; D016678:Genome; D042822:Genomic Instability; D006801:Humans; D008297:Male; D009865:Oocytes; D012189:Retrospective Studies; D013094:Spermatozoa; D058989:Vitrification
keywords: cryopreservation; embryos; genome; oocytes; sperm