Comparison of embryonic developmental competence of mouse oocytes grown with and without serum

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Abstract
The first objective of this study was to determine whether oocyte growth in serum‐free medium affects the solubility of the zona pellucida to α‐chymotrypsin digestion, which is an index of zona pellucida “hardening” and reflects the potential penetrability of the zona pellucida by sperm. Oocyte‐granulosa cell complexes were isolated from the preantral follicles of 12‐day‐old mice and cultured for 10 days in medium containing 5% fetal bovine serum (FBS) or in serum‐free medium. The zonae pellucidae of oocytes grown in serum‐free medium were four times as hard as freshly isolated germinal vesicle (GV)‐stage oocytes grown in vivo or oocytes grown in vitro in FBS‐containing medium. The hardening of the zonae pellucidae of oocytes grown in serum‐free medium was prevented by addition of fetuin. The second objective was to compare the competence to undergo embryogenesis of oocytes that grew in serum‐free vs. FBS‐containing medium. Approximately 70% of the oocytes underwent maturation regardless of whether the medium was serum‐free or contained FBS. Of the mature ova grown in medium containing FBS, 53% cleaved to the two‐cell stage after insemination compared with only 6% of the ova grown in serum‐free medium. Addition of fetuin to the serum‐free medium used for oocyte growth increased the frequency of cleavage to the two‐cell stage. Of the embryos derived from oocytes that grew in FBS‐containing medium, 70% completed the two‐cell stage to blastocyst transition. In contrast, only about 40% (P < 0.01) of the embryos derived from oocytes that grew in serum‐free medium completed this transition regardless of whether the medium for oocyte growth was supplemented with fetuin. Nevertheless, 12% of the two‐ to four‐cell‐stage embryos derived from oocytes that grew and matured in serum‐free medium containing fetuin developed to live young after transfer to the oviducts of foster mothers. These results suggest that serum‐borne growth factors may function during oocyte growth and maturation to promote events leading to successful fertilization and embryogenesis.

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