title: Heat stress during in vitro fertilization decreases fertilization success by disrupting anti-polyspermy systems of the oocytes.
abstract: Low pregnancy rates during the summer are due, in part, to reduced fertilization. Given that elevated temperature is associated with this season, we investigated the effect of heat stress during fertilization using an in vitro model. Three experiments were performed to determine the mechanism by which exposure to elevated temperature disrupts fertilization. Oocytes were fertilized for 6 hr at 38.5°C or 41.0°C or 40.0°C with non-pre-incubated sperm, or for 6 hr at 38.5°C with sperm that had been pre-incubated at 38.5°C or 41.0°C for 4 hr. In each experiment, zygotes were cultured at 38.5°C in 5% CO(2) and 5% O(2). Rates of cleavage and blasocyst formation were reduced when fertilization occurs at elevated temperatures. The percent of sperm classified as alive, using fluorescein diacetate labeling, was decreased by pre-incubation and fertilization at 40.0°C. Although no difference was observed in sperm penetration rate, polyspermy tended to be increased by heat stress during fertilization. The zona pellucidae of zygotes formed following fertilization at 40.0°C for 6 hr were more sensitive to digestion with pronase. Furthermore, these zygotes exhibited higher hydrogen peroxide levels, measured by 2,7-dihydrodichlorofluorescein diacetate staining, and showed increased transcript abundance for HSPA1A, a gene involved in the heat-shock response, but decreased transcript abundance for UCHL1, a gene involved in preventing polyspermy. Results indicate that heat stress during fertilization is lethal to sperm, and causes oxidative stress, altered transcript abundance, and a defective block to polyspermy in the zygote. Thus, an increase in polyspermy is likely one cause of the reduced competency of zygotes fertilized under elevated temperatures to develop to the blastocyst stage.
authors.affiliation: Livestock and Grassland Research Division, Kyushu Okinawa Agricultural Research Center, NARO, Kumamoto, Japan.
authors.forename: Miki
authors.initials: M
authors.lastname: Sakatani
authors.forename: Kenichi
authors.initials: K
authors.lastname: Yamanaka
authors.forename: Ahmed Z
authors.initials: AZ
authors.lastname: Balboula
authors.forename: Naoki
authors.initials: N
authors.lastname: Takenouchi
authors.forename: Masashi
authors.initials: M
authors.lastname: Takahashi
mesh_terms: D000818:Animals; D001757:Blastomeres / Q000166:cytology / Q000378:metabolism*; D002417:Cattle; D004622:Embryo, Mammalian / Q000166:cytology / Q000378:metabolism*; D005260:Female; D005306:Fertilization; D005307:Fertilization in Vitro; D018869:Heat-Shock Response; D009865:Oocytes / Q000166:cytology / Q000378:metabolism*; D011247:Pregnancy; D015044:Zona Pellucida / Q000378:metabolism; D015053:Zygote / Q000166:cytology / Q000378:metabolism*