Abstract :
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.
