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Mouse antral oocytes show two different patterns of chromatin organization, defining oocytes with or without chromatin surrounding the nucleolus (SN: surrounded nucleus; NSN: not surrounded nucleus). We have previously shown that upon injection of eCG, NSN antral oocytes shift towards the SN kind of chromatin organization. We hypothesized that these newly formed SN oocytes were those that would have been ovulated after an ovulatory stimulus. The main objective of this study was to investigate the meiotic and developmental competence of these two types of oocytes. SN and NSN antral oocytes were isolated after i.p. administration of eCG + hCG or eCG-only, in vitro-cultured until completion of metaphase II, and inseminated with capacitated spermatozoa; and their development to the 4-cell stage was examined. This study demonstrates 1) that SN and NSN oocytes isolated after injection of eCG …