Moderate heat treatment increases the penetrability of zonae pellucidae of salt-stored mammalian oocytes by spermatozoa.

The zona pellucida of mammalian oocytes stored in highly concentrated solutions of neutral salts is known to retain its biological and biochemical properties. However, the zona may become resistant to sperm penetration as the storage period is increased. In cattle and hamsters, the penetrability of zonae of salt-stored oocytes was restored or increased by treating the oocytes with moderate heat without altering the gross morphology of the zona. Although this technique did not work for salt-stored human ova, this may have been due to the use of so-called inseminated-unfertilised ova which (1) may have been fertilised but failed to activate, or (2) were not fertilised because of functionally defective zonae.