Timing of compaction and inner cell allocation in bovine embryos produced in vivo after superovulation

Preimplantation development in the bovine embryo was examined by relating the occurrence of three morphogenetic processes (compaction, blastulation, and hatching) to the timing of allocation of embryonic cells to the inner cell mass (ICM) or to the trophectoderm (TE). Embryos were collected from 26 cows between Days 4 and 9 postovulation. Compaction started 5 days postovulation at the 32-cell stage. Morulae remained firmly compact until the seventh cell cycle was almost completed. Blastocyst formation started between the 64- and 128-cell stage at Days 6, 7, and 8 postovulation. Hatching was predominant at Day 9 postovulation. ICM and TE cells could successfully be distinguished by differential staining in 107 of 142 embryos (75%). Inner cells could first be detected in 20% of 16-cell embryos. Unexpectedly, it was found that inner cell allocation and compaction were independent processes, since 31% of compacted morulae displayed no ICM. Beyond the 50-cell stage, in vivo compact morulae displayed at least 10 ICM cells, whereas blastocysts with a minimum total cell number of 65 cells displayed at least 23 ICM cells. It can be concluded that the slow in vivo transition from the morula to the blastocyst stage allows sufficient time for allocation of inner cells to the ICM of the embryo.