The inconsistent superovulatory response to gonadotropins is a challenge in biological experiments using mice as a research model. In mice, limited superovulatory response to gonadotropin may be related to the initiation of gonadotropin treatment as suggested in domestic animals. Therefore, the objective of the study was to evaluate the effect of stage of estrous cycle on the superovulatory response and embryo quality after treating with FSH or eCG in in BALB/c mice.
In experiment 1, relative ratios of vaginal epithelial cells and leukocytes were used to characterize the stages of estrous cycle in mice (n=10). In experiment 2, effect of stage of estrous cycle on no. of oocytes/cleaved embryos at 36 h post hCG, no. of cells in blastocysts at 84 h post hCG and no. of somites in embryos at dpc 9.5 were determined by treating mice (n=62) with eCG or FSH at each stage of estrous cycle. A total of twenty eight mice (n= 7 per stage) were treated with single dose of eCG (5 IU; IP). A total of thirty four mice (n=7-10 mice per stage) were treated with FSH (2.5 IU per does; SC) in four equal doses at every 12 h. All mice were given hCG (5IU, I.P) 48h after eCG or 12h after last injection of FSH.
Results revealed that the relative ratios of parabasal cells, cornfield cells and leukocytes were 84, 13, 3% in proestrus, 2, 95, 3% in estrus, 6, 37, 57% in metestrus, and 4, 5, 91% in diestrus, respectively. The mean interval between two consecutive estrus stages was 4.9 ± 0.3 days. Despite the inconsistent transitional pattern among the stages, the duration of proestrus, estrus, metestrus and diestrus was 24.0±2.7, 28.6±3.7, 17.3±2.9 and 52.6 ±3.7 h, respectively.
In FSH-treated mice, stage of estrous cycle did not affect the total no. of oocytes/embryos. However, in eCG-treated mice, proestrus and estrus had higher (P< 0.05) number of oocytes/embryos. Similarly, the number of cleaved embryos were not different (P> 0.05) among
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stages in FSH-treated mice while eCG-treated mice had higher (P< 0.05) no. of cleaved embryos in proestrus. Differential staining 84 hours post hCG revealed that proestrus stage of eCG treatment has blastocysts of highest total number of cells, number of trophectoderm cells and number of inner cell mass cells. At 9.5 dpc there was no significant difference in number of somites in embryos of all groups.
In conclusion, superovulatoion by eCG at proestrus yielded higher number of good quality oocytes/embryos as compared to FSH.