Summary. Mouse oocytes exposed to 1 μg Hoechst 33342 (H-33342)/ml and then fertilized in vitro developed normally into blastocysts and blastocyst outgrowths. After penetration of the zona, the fertilizing spermatozoon showed intense fluorescence upon fusion with the vitelline membrane. Due to fluorochrome leakage from the perivitelline space a faint fluorescence was detected in zona-bound spermatozoa. This fluorescence of zona-bound spermatozoa intensified with increased fluorochrome concentration (10μg/ml), obscuring the fluorescence of the fertilizing spermatozoa. Spermatozoa added to zona-free mouse oocytes (pre-loaded with 1 or 10 μg H-33342/ml) fluoresced within 10 min of insemination, provided the zonae were removed mechanically. Removal by protease digestion induced leakage of fluorochrome, so that all spermatozoa in the vicinity of an oocyte pre-loaded with 10 pg H-33342/ml became labelled. This leakage was not visibly apparent when protease-treated oocytes were exposed to only 1 μg H-33342/ml. The technique could not be applied to zona-free hamster oocytes under our conditions, since the fluorochrome leaked freely from the oocytes whether the zona was removed mechanically or enzymically.
Keywords: gamete fusion; fluorochrome; in-vitro fertilization; mammals