Superovulated mature mice produce more heterogeneous oocyte populations compared to superovulated prepuberal mice
Date
1992
Authors
Hammer, Mary-Anne Agnes
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Abstract
Superovulation of prepuberal and mature mice has become an established procedure. Pregnant mare's serum gonadotrophin (PMSG) and human chorionic gonadotrophin (hCG) are injected to induce growth of Graafian follicles and the expulsion of secondary oocytes into the oviduct. Superovulated ova are considered normal - arrested in metaphase of meiosis II. Mouse age and hormone dose are known to affect the number of eggs ovulated, but are not considered determinants of their meiotic maturity. The current study provides evidence against this assumption, and the indiscriminate superovulation of mice to obtain metaphase II oocytes. Ovulated oocytes from individual mice were analyzed using indirect immunofluorescence of microtubules and Hoechst 33258 staining of chromosomes. Almost all of the superovulated ova from prepuberal mice were arrested in metaphase, as expected. Variation in spindle shape was the only detected heterogeneity. However, stimulated mature mice produced mixed oocyte populations 13 - 16 h post-hCG, which included non-metaphase eggs and metaphase eggs with cytoplasmic microtubule asters. The conditions of superovulation affected the type of abnormal egg observed. Putative primary oocytes were recovered after 10 IU hormone doses, and significantly more oocytes in anaphase were observed after 5 IU hormone doses, or recovery times of 15 - 16 h post-hCG. The percentage of abnormal oocytes recovered from mature mice was positively correlated with the number of eggs ovulated. Past indications of heterogeneity of superovulated mouse oocytes were not thoroughly investigated and evidence may not have been detected due to: mouse age, hormone dose, ovulation rate, or parameter measured. Pellicer et al. (1988) proposed exogenous gonadotrophins stimulate a more heterogeneous group of oocytes compared to normal cycling, by recruitment of younger and older follicles. The current data supports this suggestion, and extends the model to include the effects of animal age. The greater heterogeneity of superovulated ova from mature mice, compared to prepuberal mice, may reflect the inherent differences in ovarian environments at the time of hormone injection.
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UN SDG 16: Peace, Justice, and Strong Institutions