Meiotic spindle configuration is differentially influenced by FSH and epidermal growth factor during in vitro maturation of mouse oocytes

BACKGROUND: To ascertain whether different hormonal treatment protocols could affect metaphase II (MII) spindle morphology, meiotic spindle organization was detected in prepubertal mouse oocytes matured under conditions allowing spontaneous, FSH- or epidermal growth factor (EGF)-dependent meiotic maturation. METHODS: Oocyte-cumulus complexes (OCCs) were matured either spontaneously (control; n = 270) or in the presence of hypoxanthine (Hx) plus FSH (n = 400) or EGF (n = 370). Spindles were detected by immunofluorescence analysis. In vivo ovulated (IVO) oocytes were processed similarly. RESULTS: IVO oocytes displayed spindles underlying the oolemma and with focused poles marked by spots of gamma-tubulin, whereas the majority (89%) of control oocytes had barrel-shaped spindles, positioned away from the oolemma, and with gamma-tubulin distributed along microtubules. Similar configuration/localization was found in 85% of the oocytes matured in vitro in the presence of Hx and FSH. In the presence of Hx-EGF, 35% of the oocytes showed spindles with an IVO-like configuration, although gamma-tubulin was homogeneously distributed throughout microtubules. Independently of spindle shape, 52% of EGF-stimulated oocytes had spindles positioned near the oolemma, in comparison to just 24% of FSH-treated and 13% of control oocytes. CONCLUSIONS: These results indicate that FSH and EGF can differently affect meiotic spindle morphology, and that EGF might be a stronger contributor than FSH to the acquisition of oocyte competence.