Trophectoderm differentiation in the bovine embryo: characterization of a polarized epithelium

Blastocyst formation is dependent on the differentiation of a transporting epithelium, the trophectoderm, which is coordinated by the embryonic expres sion and cell adhesive properties of E-cadherin. The trophectoderm shares d ifferentiative characteristics with all epithelial tissues, including E-cad herin-mediated cell adhesion, tight junction formation, and polarized distr ibution of intramembrane proteins, including the Na-K ATPase. The present s tudy was conducted to characterize the mRNA expression and distribution of polypeptides encoding E-cadherin, beta-catenin, and the tight junction asso ciated protein, zonula occludens protein 1, in pre-attachment bovine embryo s, in vitro. Immunocytochemistry and gene specific reverse transcription-po lymerase chain reaction methods were used. Transcripts for E-cadherin and b eta-catenin were detected in embryos of all stages throughout pre-attachmen t development. Immunocytochemistry revealed E-cadherin and beta-catenin pol ypeptides evenly distributed around the cell margins of one-cell zygotes an d cleavage stage embryos. In the morula, detection of these proteins dimini shed in the free apical surface of outer blastomeres. E-cadherin and beta-c atenin became restricted to the basolateral membranes of trophectoderm cell s of the blastocyst, while maintaining apolar distributions in the inner ce ll mass. Zonula occludens protein 1 immunoreactivity was undetectable until the morula stage and first appeared as punctate points between the outer c ells. In the blastocyst, zonula occludens protein 1 was localized as a cont inuous ring at the epical points of trophectoderm cell contact and was unde tectable in the inner cell mass. These results illustrate that the gene pro ducts encoding E-cadherin, beta-catenin and zonula occludens protein 1 are expressed and maintain cellular distribution patterns consistent with their predicted roles in mediating trophectoderm differentiation in in vitro pro duced bovine embryos.