E-cadherin is a cell adhesion molecule critical for the compaction of preimplantation mouse embryos and the maintenance of epithelial cell layers. This study investigates the role of E-cadherin in epithelial biogenesis by examining embryos homozygous for E-cadherin deficiency. These embryos, derived from heterozygous transgenic mice, died around the time of implantation, despite undergoing compaction due to residual maternal E-cadherin. At the blastocyst stage, E-cadherin-negative embryos failed to form a trophectodermal epithelium or a blastocyst cavity, indicating the essential role of E-cadherin in epithelial biogenesis. Mouse preimplantation embryos serve as a model for studying epithelial formation from a nonpolarized cell. During preimplantation development, blastomeres maximize cell-cell contacts to form a compact morula, a prerequisite for the formation of two cell types: the trophectoderm epithelium and the inner cell mass (ICM). The first morphological changes occur at the 8-cell stage, leading to cellular asymmetry, including apical membrane microvilli and actin cytoskeleton polarization. This asymmetry is maintained in the outer cell layer at the 16-cell morula stage. Compaction is mediated by E-cadherin, which is expressed on the cell surface of oocytes and early embryos. De novo synthesis of E-cadherin begins at the late 2-cell stage and is developmentally regulated, occurring only in cells destined for epithelial differentiation. To study E-cadherin's role, the gene was inactivated in embryonic stem (ES) cells using homologous recombination, generating heterozygous mice. E-cadherin-negative embryos failed to form a trophectodermal epithelium or blastocyst cavity, demonstrating E-cadherin's pivotal role in epithelial biogenesis. These embryos also exhibited defects in hatching and cell-cell adhesion, likely due to reduced adhesive strength. Despite these defects, some E-cadherin-negative embryos showed epithelial differentiation, indicating that part of the epithelial differentiation program occurs in their absence. However, the ICM of E-cadherin-negative embryos was composed of loosely attached cells, and ES cell lines derived from these embryos exhibited reduced cell-cell adhesiveness. These findings highlight the central importance of E-cadherin in the first morphogenetic events of mammalian development, particularly in providing cell polarity and the integrity of the trophectoderm epithelium. The results suggest that E-cadherin molecules necessary for terminal epithelial differentiation accumulate during early cleavage stages, with some maternally contributed and additional zygotic E-cadherin required for proper epithelial structure organization. Post-translational modifications and the assembly of theE-cadherin is a cell adhesion molecule critical for the compaction of preimplantation mouse embryos and the maintenance of epithelial cell layers. This study investigates the role of E-cadherin in epithelial biogenesis by examining embryos homozygous for E-cadherin deficiency. These embryos, derived from heterozygous transgenic mice, died around the time of implantation, despite undergoing compaction due to residual maternal E-cadherin. At the blastocyst stage, E-cadherin-negative embryos failed to form a trophectodermal epithelium or a blastocyst cavity, indicating the essential role of E-cadherin in epithelial biogenesis. Mouse preimplantation embryos serve as a model for studying epithelial formation from a nonpolarized cell. During preimplantation development, blastomeres maximize cell-cell contacts to form a compact morula, a prerequisite for the formation of two cell types: the trophectoderm epithelium and the inner cell mass (ICM). The first morphological changes occur at the 8-cell stage, leading to cellular asymmetry, including apical membrane microvilli and actin cytoskeleton polarization. This asymmetry is maintained in the outer cell layer at the 16-cell morula stage. Compaction is mediated by E-cadherin, which is expressed on the cell surface of oocytes and early embryos. De novo synthesis of E-cadherin begins at the late 2-cell stage and is developmentally regulated, occurring only in cells destined for epithelial differentiation. To study E-cadherin's role, the gene was inactivated in embryonic stem (ES) cells using homologous recombination, generating heterozygous mice. E-cadherin-negative embryos failed to form a trophectodermal epithelium or blastocyst cavity, demonstrating E-cadherin's pivotal role in epithelial biogenesis. These embryos also exhibited defects in hatching and cell-cell adhesion, likely due to reduced adhesive strength. Despite these defects, some E-cadherin-negative embryos showed epithelial differentiation, indicating that part of the epithelial differentiation program occurs in their absence. However, the ICM of E-cadherin-negative embryos was composed of loosely attached cells, and ES cell lines derived from these embryos exhibited reduced cell-cell adhesiveness. These findings highlight the central importance of E-cadherin in the first morphogenetic events of mammalian development, particularly in providing cell polarity and the integrity of the trophectoderm epithelium. The results suggest that E-cadherin molecules necessary for terminal epithelial differentiation accumulate during early cleavage stages, with some maternally contributed and additional zygotic E-cadherin required for proper epithelial structure organization. Post-translational modifications and the assembly of the