EPIDERMAL GROWTH-FACTOR COMBINED WITH RECOMBINANT HUMAN CHORIONIC-GONADOTROPIN IMPROVES MEIOTIC PROGRESSION IN MOUSE FOLLICLE-ENCLOSED OOCYTE CULTURE

Using a mouse early preantral follicle culture system, mature full gro wn oocytes, arrested in prophase I of meiosis, were produced after 12 days using a recombinant gonadotrophin-supplemented medium. This cultu re medium does not mimic the normal extracellular environment of the o ocyte and might therefore modify meiotic regulation and more particula rly progression to metaphase II (MII), The aim of this study was to op timize the treatment using recombinant stimulatory ligands which were known to induce germinal vesicle breakdown (GVBD) and completion of me iosis I, metaphase II (MII), namely recombinant follicle stimulating h ormone (r-FSH), chorionic gonadotrophin (r-HCG) and epidermal growth f actor (EGF), Full-grown intrafollicular oocytes could not resume meios is when the 'ovulatory' stimulus was r-FSH, used at a 100 times higher dose than during culture. r-FSH did not increase progesterone product ion. When 1.5 IU/ml r-HCG was used as meiotic trigger, germinal vesicl e breakdown was obtained in 95% of the oocytes 64% of which extruded a first polar body. r-HCG induced a dramatic increase in progesterone p roduction. When EGF was administered as sole stimulus on day 12 to the attached follicle-enclosed oocytes, only doses greater than or equal to 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 reac hed metaphase II at a rate of 54% (not significant versus r-HCG), EGF did not stimulate progesterone production. Addition of increasing dose s of EGF (0.5; 5; 10; 50 ng/ml) to r-HCG did not increase the GVBD-rat e, but EGF doses greater than or equal to 5 ng/ml improved MI to MII t ransition (P = 0.027), thereby improving the final yield of MII oocyte s 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 ha lf of the cultured follicles. However, in addition to HCG, EGF (greate r than or equal to 5 ng/ml) had a stimulatory effect on completing the first meiotic division. It was concluded that, under the present cult ure conditions, EGF in combination with HCG provided optimal nuclear m aturation.