Bovine embryos produced in vitro differ considerably in quality from embryos developed in vivo. The in vitro production system profoundly affects the competence to form blastocysts, the number of cells of the total embryo and of the inner cell mass (ICM), and the incidence of apoptosis. To our knowledge, the effects of different postfertilization regimens before and after completion of the fourth embryonic cell cycle on these aspects have not yet been investigated. In the present study, we assessed the blastulation rate by stereomicroscopy and the cell number of the total embryo, of the ICM, and of the cells with apoptotic changes by confocal laser-scanning microscopy after staining with propidium iodide and TUNEL. Two groups of embryos were developed in heifers, after superovulation, until 45 or 100 h postovulation (po) and, after collection on slaughter, were further cultured in vitro until Day 7 po. A third and fourth group comprised embryos that were produced entirely in vitro or in vivo. The results indicate that passage in vivo of the fourth cell cycle does not prevent acceleration of the formation of the blastocoele in vitro but may be the critical factor contributing to a higher cell number in the total blastocyst and its ICM. The lower quality of in vitro-produced embryos can be attributed to the ICM having less viable cells because of a lower number of cells and a higher incidence of apoptosis that appears to be determined before completion of the fourth cell cycle.
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