COMPARISON OF EMBRYONIC DEVELOPMENTAL COMPETENCE OF MOUSE OOCYTES GROWN WITH AND WITHOUT SERUM

The first objective of this study was to determine whether oocyte growth in serum-free medium affects the solubility of the zona pellucida to alpha-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.