CHARACTERISTICS OF ACTIN FIBERS AND ULTRASTRUCTURE OF THE CONTACT REGIONS INVOLVED IN THE SEPARATION OF BLASTOMERES OF 2-CELL MOUSE EMBRYOS, FROZEN-THAWED WITHOUT THE ZONA-PELLUCIDA

Freezing of embryos deprived of the zona pellucida (ZP) decreases thei r survival rate immediately after thawing, and gives rise to the separ ation of their blastomeres in a high percentage of cases. We have stud ied the ultrastructure and the characteristics of actin fibers in the cell-to-cell contact region in mouse embryos frozen-thawed without the ZP at the two-cell stage. Our results indicate that most of the embry os that retain their blastomeres united after freezing and thawing sho w either the presence of a midbody, or a contact region with a close a pposition of the plasma membranes but without an organized actin corte x in their contact region. Only a small percentage of embryos that ret ain their blastomeres united after freezing and thawing show a contact region with widely separated plasma membranes and an organized actin cytocortex. (C) 1997 Academic Press.