In vitro maturation and ultrastructural observation of cryopreserved minkewhale (Balaenoptera acutorostrata) follicular oocytes

Minke whale (Balaenoptera acutorostrata) follicular oocytes were cryopreser ved by a slow-step freezing procedure using ethylene glycol. The morphologi cally viable proportion of post-thawed minke whale follicular oocytes was 3 9.7%. The maturity of the animals (immature and mature whales) or the prese nce or absence of cumulus cells (CC) did not affect the proportion of morph ologically viable oocytes. Postthawed oocytes were examined for nuclear sta tus after in vitro maturation. The presence of CC (29.1%) significantly enh anced (P < 0.05) the proportion of oocytes at metaphase I/anaphase I/teloph ase I stages compared to results with the absence of CC (13.5%). A total of 4 of 194 postthawed oocytes matured to the second metaphase stage after cu lture for 5.5 days with or without CC. The cryopreserved immature oocytes o btained from immature and mature whales were processed to examine the ultra structure by transmission electron microscopy. Varying ultrastructural dama ge to the cytoplasm was observed as a result of the cryopreservation proced ures. These results show that 20-30% of cryopreserved minke whale follicula r oocytes can resume meiosis in vitro, but damage induced by the freezing a nd thawing procedures was observed.