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The timing and mode of hamster blastocyst escape from the zona pellucida in vitro ("hatching") and in vivo were compared on the basis of dynamic and static images captured on videotape and computer, respectively. In vivo, most embryos (&gt; or = 80%) escaped between 0030 h and 0600 h on Day 4 of pregnancy. The zona gradually thinned globally and greatly increased in diameter, eventually disappearing. Attachment of blastocysts to the uterine epithelium followed within 4 h. Retarded embryos also showed global zona lysis. In contrast, zona escape in vitro was delayed &gt; or = 29 h both in blastocysts cultured from 1-cell embryos and in blastocysts flushed from uteri of mated, naturally cycling hamsters at 2330 h on Day 3. Zonae were focally lysed; then blastocysts egressed through the hole, leaving most of the zona intact. Global zona lysis never occurred, and retarded embryos did not escape. In conclusion, the striking morphological and temporal differences in zona escape indicate that the hatching behavior routinely observed in cultured hamster blastocysts is not representative of normal events, and implicate a uterine contribution to normal zona escape that is lacking in culture.

Zona Pellucida Escape by Hamster Blastocysts in Vitro is Delayed and Morphologically Different Compared with Zona Escape in Vivo1