Murine Oocyte Cryopreservation: Comparison Between Fertilization Success Rates Of Fresh and Frozen Metaphase I and II Oocytes

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Abstract
The surplus of oocytes obtained during in vitro and gamete intrafallopian transfer creates the need to either (1) cryopreserve (196 d`C) or (2) fertilize these surplus oocytes and then freeze the resulting embryos. During this pilot study, C57BL6 X CBA female mice (F1 generation) were superovulated by means of hormone stimulation, resulting in adequate numbers of murine oocytes. Oocytes thus obtained were classified according to their nuclear status. The maturation stage of oocytes was evaluated and classified as metaphase I and metaphase II. The success of oocyte cryopreservation in each group was evaluated according to fertilization rate after the insemination procedure. Metaphase II oocytes had a fertilization rate of 33% (frozen) and 84% (fresh), whereas metaphase I oocytes had a 24% (frozen) and a 79% (fresh) fertilization rate. Metaphase I oocytes can mature to the metaphase II stage after having been frozen, metaphase II oocytes may be more vulnerable to chromosome damage, and the nuclear stage prior to freezing has no significant effect on the fertilizability of the frozen-thawed oocytes.