Ultrastructural Changes during Maturation and Cryopreservation of Follicular Oocytes of Antarctic Minke Whales (<i>Balaenoptera bonaerensis</i>)

Important information for cryopreservation and in vitro culture could be obtained through ultrastructural observation of minke whale oocytes. In the present study, transmission electron microscopy was used to examine the ultrastructure of immature and in vitro mature Antarctic minke whale oocytes before and after freezing and thawing. Close communication existed between ooplasm and cumulus cell projections, and between mitochondria and membranous organelles, including the smooth endoplasmic reticulum in the ooplasm of minke whale immature oocytes. Cytoplasmic organelles, such as mitochondria and lipid droplets, were clustered in the ooplasm cortex region, while vesicles were located centrally in the ooplasm. After in vitro maturation, cytoplasmic organelles were observed to have distributed throughout the ooplasm, and the morphology of the mitochondria was changed. Cytoplasmic degeneration was observed in the minke whale oocytes after freezing and thawing. The slow freezing procedures employed in this study appeared to generate extensive and severe ultrastructural damages in minke whale ooplasms.