176 EFFECT OF HYPERTHERMIA ON DEVELOPMENTAL COMPETENCE OF GERMINAL-STAGE OOCYTES: IN VIVO AND EX VIVO STUDIES IN MICE

Abstract

Mammalian oocytes are susceptible to thermal stress at various stages of follicular development. The present study was performed to determine the effects of maternal hyperthermia on germinal vesicle (GV)-stage oocytes in mice. Evaluation parameters included (a) the oocyte's ability to become fertilized, (b) ex vivo embryonic development, (c) pregnancy outcome, and (d) quality of offspring. Female mice (CB₆F₁) were synchronized with pregnant mare serum gonadotropin (PMSG) followed by hCG 48 h later. One hour after hCG administration, control mice were kept under normo-thermal conditions (22°C, 45% RH), whereas the treated mice were exposed to thermal stress (40°C, 70% RH) for 1.5 to 2 h to induce a rise of 1.8°C in rectal temperature. After a 1-h recovery period, both treated and control mice were paired with stud males overnight. In Exp. 1, mated mice were sacrificed 20 h after hCG administration and embryos were recovered and cultured in vitro. The number of putative zygotes recovered was counted, and cleavage and blastocyst-formation rates were recorded on Days 1 and 5 post-fertilization, respectively. Examination of oocyte developmental stage during hyperthermia induction revealed that oocytes were at the GV stage. The percentage of putative zygotes that cleaved into the two-cell stage was higher (P < 0.05) in the control (91.2 ± 1.3) than in the treated group (84.5 ± 1.7). In addition, blastocyst-formation rate was higher in the control (P < 0.05; 84.3 ± 1.8%) than in the treated group (57 ± 2.6%). In Exp. 2, females were left with the stud males until litter delivery. The date of delivery and the number of pups were documented. Eight-week-old offspring were submitted to behavioral (locomotor activity and episodic memory) tests. The one-way ANOVA procedure of JAMPIN (SAS Institute, Inc., Cary, NC, USA, 2004) was used for statistical analysis. The percentage of females that gave birth during the first and second estrous cycles after hyperthermia induction was 59% and 71% for treated and control groups, respectively. The average number of pups in the first estrous cycle was lower (P < 0.05) for the treated group, and only became equal to that in the control group in the third estrous cycle. Behavioral tests did not reveal any differences between pups from the two experimental groups. In summary, GV-stage oocytes from stimulated ovaries are highly sensitive to thermal stress, since induction of maternal hyperthermia during this stage disrupts oocyte developmental competence both in vivo and ex vivo. However, oocytes that survive thermal stress exhibit normal developmental potential, since heat-stressed mice produced normal pups, at least as reflected by offspring behavioral tests. Thus, strategies designed to protect the ovarian pool of oocytes from thermal stress are likely to improve fertility in heat-stressed females.