The process of oocyte maturation is underlined by a redistribution of cellular organelles, among which mitochondria play a functional role for the acquisition of fertilization and developmental competence. In this paper, we applied electron and confocal microscopy by using DIOC6 and JC-1 stain to evaluate mitochondria distribution pattern and activity during different stages of oocyte growth in the ascidian Styela plicata. Three categories of oocytes at the germinal vesicle stage underlying the vitellogenic process were characterized on the basis of size, pigmentation and accessory cells. Mitochondria were spread throughout the cytoplasm at the smallest oocyte stage and gradually migrated to the periphery of the subcortical cytoplasm at the intermediate stage. At the fully grown oocyte stage, mitochondria were aggregated in the subcortical cytoplasm. This pattern of polarized mitochondria distribution correlates significantly with an increase in mitochondria potential and activity. In this paper we discuss the relationship of mitochondria to the acquisition of oocyte developmental competence.

Medium that contains 17β-estradiol has been reported to support in vitro growth of bovine oocytes, isolated from early antral follicles, until the final stage. The aim of this study was to determine the effects of androstenedione in medium on such growing bovine oocytes. Oocyte–granulosa cell complexes were collected from early antral follicles and cultured for 14 days in medium supplemented with 17β-estradiol (0, 10 and 100 ng/ml) or androstenedione (0, 10 and 100 ng/ml). The mean diameter of oocytes measured after seeding on the culture substrate was 96.9 μm (n = 191). Either steroid was necessary for maintainance of the organization of oocyte–granulosa cell complexes over the 14-day culture period. In the 17β-estradiol- or the androstenedione-supplemented medium about 80% or 65%, respectively, of viable oocytes were recovered. In both groups the increase in oocyte size was significant after 14 days. The in vitro grown oocytes were cultured for a further 22–24 h for oocyte maturation; 13% and 30% of oocytes grown in the 10 and 100 ng/ml 17β-estradiol-supplemented medium reached metaphase II, respectively; more than 64% of oocytes grown in the androstenedione-supplemented medium matured to metaphase II. These results show that androstenedione, as 17β-estradiol, can maintain the viability of bovine oocyte–granulosa cell complexes and support the growth of oocytes, and that androstenedione promotes the acquisition of oocyte meiotic competence efficiently at a low dose.

Après avoir décrit succinctement les quatre grandes étapes de l'ovogenèse - stade protoplasmique, vitellogenèse endogène, vitellogenèse exogène et maturation finale -, cette courte revue compare la dynamique de croissance ovocytaire et ovarienne chez les poissons à pontes unique, multiples et continues. Elle présente ensuite, pour les différents modes de reproduction, l'essentiel des connaissances actuelles sur la régulation, par les stéroïdes sexuels, des différentes étapes du cycle reproducteur.