Oocyte maturation involves important changes required for activation competence

The Authors studied the influence of the culture conditions on the ability of oocytes matured in vitro to activate in response to parthenogenetic activation (7% ethanol, etOH, for 5 min). Sheep oocytes were matured as cumulus enclosed oocytes (COCs) or as COCs cocultured with suspended granulosa cells in the presence or absence of gonadotropins. The reactivity of the eggs was evaluated at three different levels. First, intracellular calcium (Ca) was evaluated with the fluorescent dye Indol at the time of exposure of the eggs to the activating stimulus, parthenogenetic development was then ascertained 5h after etOH by recording the formation of pronuclei (PN), and 4 days later by recording early embryo development. Intracellular Ca levels after etOH were significantly higher in oocytes matured in the presence of granulosa cells than in COCs cultured alone. The presence of gonadotropins throughout maturation further increased the amount of Ca released after etOH and FSH was significantly more effective than LH. Culture conditions similarly influenced the formation of IN (Fig 4) and the subsequent development of parthenogenetic embryos (Table 2) These results demonstrate that: a) culture conditions throughout maturation affect the ability of the egg to activate, b) follicle somatic cells in coculture are required for the acquisition of activation competence, c) gonadotropins and FSH in particular improves the reactivity of the oocytes, d) the different reactivity of the eggs are due to the different ability to release Ca following parthenogenetic stimulation.[...]