Epidermal growth factor combined with recombinant human chorionic gonadotrophin improves meiotic progression in mouse follicle-enclosed oocyte culture

Using a mouse early preantral follicle culture system, mature full grown oocytes, arrested in prophase I of meiosis, were produced after 12 days using a recombinant gonadotrophin-supplemented medium. This culture medium does not mimic the normal extracellular environment of the oocyte and might therefore modify meiotic regulation and more particularly progression to metaphase II (MII). The aim of this study was to optimize the treatment using recombinant stimulatory ligands which were known to induce germinal vesicle breakdown (GVBD) and completion of meiosis I, metaphase II (MII), namely recombinant follicle stimulating hormone (r-FSH), chorionic gonadotrophin (r-HCG) and epidermal growth factor (EGF). Full-grown intrafollicular oocytes could not resume meiosis when the 'ovulatory' stimulus was r-FSH, used at a 100 times higher dose than during culture. r-FSH did not increase progesterone production. When 1.5 IU/ml r-HCG was used as meiotic trigger, germinal vesicle breakdown was obtained in 95% of the oocytes 64% of which extruded a first polar body. r-HCG induced a dramatic increase in progesterone production. When EGF was administered as sole stimulus on day 12 to the attached follicle-enclosed oocytes, only doses ≥5 ng/ml could cause GVBD, although less effectively than r-HCG (45 versus 95%; P < 0.0001). Oocytes undergoing GVBD by the EGF pulse reached metaphase II at a rate of 54% (not significant versus r-HCG). EGF did not stimulate progesterone production. Addition of increasing doses of EGF (0.5; 5; 10; 50 ng/ml) to r-HCG did not increase the GVBD-rate, but EGF doses ≥5 ng/ml improved MI to MII transition (P = 0.027), thereby improving the final yield of MII oocytes by 12.5%. These data show that up to a dose of 50 ng/ml, EGF on its own could only override the somatic inhibitory stimuli in less than half of the cultured follicles. However, in addition to HCG, EGF (≥5 ng/ml) had a stimulatory effect on completing the first meiotic division. It was concluded that, under the present culture conditions, EGF in combination with HCG provided optimal nuclear maturation.