Ultrastructural events during oocyte maturation in bovine. A: Oocyte from tertiary follicle with intact germinal vesicle (GV), showing mitochondria (arrows) in peripheral position. Note the great amount at vesicles throughout the ooplasm. B: Oocyte after 12 hours of in vitro maturation (IVM) presenting mitochondria clustered (arrows) mostly at cortical areas. Microvilli loosen from the zona pellucida. Observe the general organization of organelles in small groups. C: Oocyte after 12 hours of IVM. Cortical granules clusters are located at periphery of the ooplasma, close to the plasma membrane. Note a group of hooded/pleomorphic mitochondria. D: Oocyte after 18 hours of IVM showing cortical granules (arrows) aligned to the plasma membrane. E: oocyte after 18 hours of IVM. Observe the peculiar arrangement of organelles, with endoplasmic reticulum in close association with mitochondria and vesicles (arrows). F: mature oocyte after 24 hours of IVM that have extruded the first polar body (PB). Note the expanded cumulus cells. GV: germinal vesicle, PS: perivitelline space, ZP: zona pelucida, Mv: microvilli, Mt: mitochondria, CG: cortical granules, ER: endoplasmic reticulum, CC: cumulus cells, PB: polar body.