Ultrastructural characteristics of sheep oocytes during in vitro maturation (IVM)

The aim of this study was to describe the ultrastructure of sheep oocytes during 24 h of in vitro maturation (IVM). Ovaries were collected from a slaughterhouse and the follicles aspirated. The selected cumulus oocyte complexes (COC's) were divided into groups and transferred to maturation medium, where they were maintained for 6, 12, 18 or 24 h. Control immature oocytes were also collected (0 h). After different maturation intervals the structures were removed and processed for transmission electron microscopy. Immature oocytes (0 h) demonstrated pleomorphic mitochondria and presumptive lipid droplets as normally expected, and a moderate number of vesicles scattered throughout the cytoplasm. After 6 h of IVM, there was a migration of the mitochondria and presumptive lipid droplets to the cytoplasm periphery and the onset of cumulus cell expansion. Between 12 and 18 h of IVM, cytoplasmic rearrangements of the organelles were also observed e.g. aligning and increasing of cortical granules under the plasma membrane. Hooded mitochondria were observed during oocyte maturation. After 24 h of IVM, the oocytes showed signs of degeneration, which can be related to cellular aging - because of the excessive time in the maturation medium, after acquisition of developmental competence. This work described the ultrastructural aspects of sheep oocytes during IVM and provided insight into oocyte maturation in the ovine species.