The segregation of inner and outer cells in porcine embryos follows a different pattern compared to the segregation in mouse embryos.

The mammalian blastocyst consists of an inner cell mass (ICM) enclosed by the trophectoderm. The origin of these two cell populations lies in the segregation of inner and outer cells in the early morula. In the present study, the segregation of inner and outer cells has been studied in porcine embryos and is compared with segregation in mouse embryos. For this, nuclei of inner and outer cells were differentially labelled with two fluorochromes after partial complement-mediated lysis of the outer cells. In porcine and mouse embryos compaction and the first appearance of inner cells occur at different stages of development. In porcine embryos compaction was observed as early as the 4-cell stage, while in mouse embryos compaction occurred in the 8-cell stage. The first inner cells segregated in porcine embryos which were in the transition from four to eight cells and inner cells were added during two subsequent cell cycles. In mouse embryos inner cells segregated predominantly during the fourth cleavage division. From the results obtained we conclude that the segregation of inner and outer cells follows a different pattern in mouse and in porcine embryos.