Serum maintains the fertilizability of mouse oocytes matured in vitro by preventing hardening of the zona pellucida

The effects of serum and cumulus cells during oocyte maturation in vitro on subsequent oocyte fertilizability and zona pellucida digestability have been examined. Cumulus cell‐enclosed oocytes were cultured 15–16 hours in medium containing bovine serum albumin plus varying concentrations of fetal bovine serum (FBS). When the serum concentration was decreased incrementally from 5% to 0%, resistance to chymotrypsin digestion increased accordingly in a dose‐dependent manner. The zona pellucida digestion time for oocytes matured in serum‐free medium was increased nearly 500% above that for control oocytes matured in 5% FBS. Also, fertilization was decreased as serum concentration was lowered; the fertilization percentage for oocytes matured in serum‐free serum was reduced by over 90%. This loss of fertilizability was correlated with an absence of sperm within the vitellus following insemination of matured ova. Increasing the serum concentration from 5% to 10% had no effect on fertilization or zona pellucida digestion time. Serum deprivation, even for a short period of time, at the onset of culture significantly reduced the fertilizability of cumulus cell‐enclosed oocytes and increased the zona pellucida digestion time. Removal of serum after oocyte maturation in vitro had little effect. The presence of an intact cumulus oophorus during maturation in vitro was important in the maintenance of fertilizability and zona digestability. These data support the idea that serum deprivation and/or removal of the cumulus cells during oocyte maturation in vitro results in an alteration of the zona pellucida that is manifested as an increased resistance to proteolytic digestion and sperm penetration.