Blastocysts derived from bovine zygotes fertilized and matured in vitro and cultured for 7 days in conditioned medium were frozen in 1.36 mol glycerol l−1 and 0.25 mol sucrose l−1. In vitro survival after thawing was unaffected by dilution rate in 0.25 mol sucrose l−1. The proportion of blastocysts that re-expanded after 24 h was 81% (70 of 86) and 47% (33 of 70) hatched. Seven pregnancies beyond 2 months resulted from transfer of 21 blastocysts to 19 recipients. Total embryonic loss was 46.2%, of which 31% occurred between days 21 and 35. In vitro survival after thawing was influenced by culture conditions, the best being culture with oviduct epithelial cells, where 55–82% of blastocysts re-expanded, of which 41–54% hatched. Conditioned medium also supported re-expansion, but low hatching (6%), whereas M199 plus fetal calf serum allowed only limited re-expansion (19–40%). This behaviour was not a consequence of freezing. It is suggested that blastocysts produced in vitro have reduced metabolic activity leading to high embryonic loss before or just at the time of implantation and that oviduct cells create a favourable environment after thawing, allowing hatching in vitro.
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