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a Laboratory of Animal Genetics and Reproduction, Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, Japan
b The Institute of Cetacean Research, Tokyo 104-0055, Japan
Minke whale (Balaenoptera acutorostrata) follicular oocytes were cryopreserved by a slow-step freezing procedure using ethylene glycol. The morphologically viable proportion of postthawed minke whale follicular oocytes was 39.7%. The maturity of the animals (immature and mature whales) or the presence or absence of cumulus cells (CC) did not affect the proportion of morphologically viable oocytes. Postthawed oocytes were examined for nuclear status after in vitro maturation. The presence of CC (29.1%) significantly enhanced (P < 0.05) the proportion of oocytes at metaphase I/anaphase I/telophase 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 culture for 5.5 days with or without CC. The cryopreserved immature oocytes obtained from immature and mature whales were processed to examine the ultrastructure by transmission electron microscopy. Varying ultrastructural damage to the cytoplasm was observed as a result of the cryopreservation procedures. These results show that 2030% of cryopreserved minke whale follicular oocytes can resume meiosis in vitro, but damage induced by the freezing and thawing procedures was observed.
1 Correspondence. FAX: 81 155 49 5462; fukui{at}obihiro.ac.jp
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