EZRIN BECOMES RESTRICTED TO OUTER CELLS FOLLOWING ASYMMETRICAL DIVISION IN THE PREIMPLANTATION MOUSE EMBRYO

During preimplantation development in the mouse, two phenotypically di stinct cell populations appear at the 16-cell stage: nonpolarized inne r cells that give rise to the inner cell mass and polarized outer cell s that give rise mainly to the trophectoderm. The divergence of these two cell lineages is due to asymmetrical cell divisions during the tra nsition from the 8- to the 16 cell stage which can occur following bla stomere polarization. During compaction, at the 8-cell stage, cytoplas mic organelles accumulate in the apical domain, a surface pole of micr ovilli forms, and blastomeres flatten onto one another. During the div ision from the 8- to the 16-cell stage, the only asymmetrical structur e maintained is the pole of microvilli, At the 16-cell stage, only bla stomeres inheriting a large part of this apical structure can reestabl ish a polarized organization. The mechanisms involved in the formation and stabilization of the apical pole of microvilli are still unknown. Ezrin is an actin-associated protein that has been proposed to play a role in the formation of microvillous structures. This led us to stud y the expression of ezrin during early development of the mouse embryo . We observed that ezrin mRNA and protein are present in the mouse ooc yte and throughout preimplantation embryo development, although the am ount of protein present decreases continuously during early developmen t, particularly after the 8-cell stage, at the time of compaction. Two isoforms of ezrin phosphorylated on tyrosine residues are present dur ing all of preimplantation development while a third non-tyrosine-phos phorylated isoform appears at the 8-cell stage and its relative amount increases from the 8-cell stage to the blastocyst stage. Before compa ction, ezrin is distributed around the cell cortex. However ezrin beco mes restricted to the microvilli of the apical pole after compaction. At later stages, ezrin is found in the microvilli of the apical surfac e of outer cells. finally, ezrin remains associated with the microvill ous pole during the transition from the 8- to 16-cell stage and is fou nd only in the outer cells after division. Thus, ezrin is the first cy tocortical protein described that is totally segregated in outer cells at the 16-cell stage after an asymmetrical division. (C) 1996 Academi c Press, Inc.