Ultrastructural Comparison between Immature and In Vitro Matured Bovine Oocytes Cryopreserved in Propanediol

Immature and in vitro matured bovine oocytes were cryopreserved in a solution containing propanediol as a cryoprotectant to compare their developmental ability following the treatment. After cryopreservation, the oocytes were inseminated with frozen-thawed bovine spermatozoa and cultured in vitro for a given period of time to determine the developmental capacity of the embryos. The average rates for cleavage, which were measured 2 days after in vitro fertilization (IVF), were 71.2% for freshly collected immature oocytes, 3.4% for frozen-thawed immature oocytes and 6.8% for cryopreserved in vitro matured oocytes, respectively, though none of the cryopreserved oocytes developed to the blastocyst stage. Following cryogenic storage, the oocytes were processed for transmission electron micro-scopy (TEM). Freezing and thawing of the immature oocytes induced remarkable ultrastructural changes in oolemma, microvilli, mitochondria and vesicles in oo-plasm and cumulus cells surrounding the oocytes, whereas the integrity of cell organelles was relatively better preserved in in vitro matured oocytes. The present results suggest that the cryogenic storage of bovine oocytes, both immature and in vitro matured, induced various kinds of ultrastructural damage which was associated with the low developmental capacity of post freeze-thaw oocytes. On the other hand, in vitro matured oocytes were considered to maintain their structural integrity better than immature oocytes following cryopreservation with propanediol.