Cumulus cells steroidogenesis is influenced by the degree of oocyte maturation

BACKGROUND: The possibility to predict the ability of a germ cell to properlysustain embryo development in vitro or in vivo as early as possible isundoubtedly the main problem of reproductive technologies. To date, only theachievement of nuclear maturation and cumulus expansion is feasible, as all thestudies on cytoplasmic maturation are too invasive and have been complicated bythe death of the cells analyzed. The authors studied the possibility to test the cytoplasmic quality of pig oocytes by evaluating their ability to producesteroidogenesis enabling factor(s). To this aim, oocytes matured under different culture conditions that allowed to obtain gradable level of cytoplasmicmaturation, were used to produce conditioned media (OCM). The secretion of thefactor(s) in conditioned media was then recorded by evaluating the ability of thespent media to direct granulosa cells (GC) steroidogenesis.METHODS: In order to obtain germ cells characterized by a different degree ofdevelopmental competence, selected pig oocytes from prepubertal gilts ovarieswere cultured under different IVM protocols; part of the matured oocytes wereused to produce OCM, while those remaining were submitted to in vitrofertilization trials to confirm their ability to sustain male pronucleardecondensation. The OCM collected were finally used on cumulus cells grown asmonolayers for 5 days. The demonstration that oocytes secreted factor(s) caninfluence GC steroidogenesis in the pig was confirmed in our lab by studying E2and P4 production by cumulus cells monolayers using a radioimmunoassay technique.RESULTS: Monolayers obtained by growing GC surrounding the oocytes for five days represent a tool, which is practical, stable and available in most laboratories; by using this bioassay, we detected the antiluteal effect of immature oocytes,and for the first time, demonstrated that properly matured germ cells are able todirect cumulus cells steroidogenesis by inhibiting E2 production (P < 0.01).Nevertheless, only fully competent oocytes were able to suppress estrogensproduction, while those cultured under unfavourable conditions were unable toexert any inhibitory effect on the functions of cumulus cells (P < 0.01).CONCLUSION: These results demonstrated that good quality oocytes can be easilyselected on the basis of their ability to affect granulosa cell steroidogenesisthus reducing failures in reproductive technologies due to the transfer offertilized oocytes with a scarce ability to sustain embryo development.[...]