TIMING OF COMPACTION AND INNER CELL ALLOCATION IN BOVINE EMBRYOS PRODUCED IN-VIVO AFTER SUPEROVULATION

Preimplantation development in the bovine embryo was examined by relat ing the occurrence of three morphogenetic processes (compaction, blast ulation, and hatching) to the timing of allocation of embryonic cells to the inner cell mass (ICM) or to the trophectoderm (TE). Embryos wer e collected from 26 cows between Days 4 and 9 postovulation. Compactio n started 5 days postovulation at the 32-cell stage. Morulae remained firmly compact until the seventh cell cycle was almost completed. Blas tocyst formation started between the 64- and 128-cell stage at Days 6, 7, and 8 postovulation. Hatching was predominant at Day 9 postovulati on. ICM and TE cells could successfully be distinguished by differenti al staining in 107 of 142 embryos (75%). Inner cells could first be de tected in 20% of 16-cell embryos. Unexpectedly, it was found that inne r cell allocation and compaction were independent processes, since 31% of compacted morulae displayed no ICM. Beyond the 50-cell stage, in v ivo compact morulae displayed at least 10 ICM cells, whereas blastocys ts 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 t he morula to the blastocyst stage allows sufficient time for allocatio n of inner cells to the ICM of the embryo.