Morphological changes of cultured bovine blastocysts during hatching were observed using time-lapse videomicrography in order to investigate the patterns of the hatching process that occurred in the blastocysts and to determine whether the hatching patterns differed between blastocysts developed from fresh and cryopreserved embryos. Compacted morulae (CMs) were collected from superovulation-treated Japanese Black and Holstein dairy cattle and cultured in a medium in a CO2 culture chamber equipped with an inverted microscope at 38.5 C. Images of resultant blastocysts during the period from blastocoel formation to completion of hatching were taken at 4-sec intervals by a CCD color camera connected to an inverted microscope and recorded by a time-lapse video cassette recorder. In blastocysts developed from fresh CMs, hatching was found to begin with protrusion of trophectoderm cells from zonae pellucidae at the expanded stage. Protrusion of the cells occurred in any site of the trophectoderm. After protrusion, a large or small slit was formed in the zona pellucida in all blastocysts as a result of blastocyst expansion or enlargement of the protrusion. Then, blastocysts completely escaped from the zona pellucida through the slit in the state of expansion. From these findings, the hatching patterns of cattle blastocysts could be classified into 5 types. In blastocysts developed from frozen-thawed CMs, the hatching pattern and length of time needed for hatching are similar to those in blastocysts developed from fresh CMs. In addition, the pregnancy rate of recipients following transfer of frozen-thawed CMs (52.4%) did not differ from that of recipients following transfer with fresh CMs (58.3%). These results suggested that the quality of frozen-thawed cattle embryos is comparable to that of fresh embryos and that there could be a relationship between the hatching pattern of blastocysts and the viability of embryos after transfer.
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