Organelle rearrangement in the maturing oocyte

Mammalian oocytes acquire a series of competencies during follicular development that play critical roles at fertilization and subsequent stages of preimplantation embryonic development. Before the mammalian oocyte engages in the fertilization process, it must acquire an array of molecular and cellular assets defining its developmental potential. These properties specify competencies to complete meiosis and initiate mitosis. Meiotic maturation requires the acquisition both of nuclear and cytoplasmic competence and this complex mechanism involves most of the organelles, the cytoskeleton and molecules that are relocated from the nucleus to the cytoplasm. Messenger RNAs of maternal origin are accumulated in the oocyte throughout its growth in the ovary. These transcripts are then shuttled to specific sites of the ooplasm, where local translation is promoted. The nucleus as an organelle undergoes significant positional rearrangements during maturation. At the same time, the spindle changes its localization drifting from the cortical region to the centre of the oocyte and then back cortically. The endoplasmic reticulum undergoes significant changes during maturation in its distribution that reflect an enhanced ability to release Ca²⁺. Energy derives from cellular ATP as a result of mitochondrial activity. Mitochondria are themselves subject to redistribution and changes in activity during transition from the germinal vesicle to the metaphase II stage, in a fashion that can profoundly influence the quality of the mature oocyte and the destiny of the ensuing embryo. The dynamics of Golgi membranes during meiosis is still partially unknown but generally, as it occurs in the bovine, involves movements from the centre of the cytoplasm to the cortical area. Afterwards, a second translocation occurs between germinal vesicle breakdown and metaphase I stages, repositioning the Golgi from the periphery to the central cytoplasmic area.