Characterization of intercellular junctions in the preimplantation mouse embryo by freeze-fracture and thin-section electron microscopy

Intercellular junction formation in preimplantation mouse embryos was investigated with thin-section and freeze-fracture electron microscopy. At the four-cell stage, regions of close membrane apposition with focal points of membrane contact and occasional underlying cytoplasmic densities were observed between blastomeres of thin-sectioned embryos. Corresponding intramembrane specializations were not, however, observed in freeze-fractured embryos. At the 8- to 16-cell stage, small gap and macula occludens junctions and complexes of these junctions were observed at all levels between blastomeres of freeze-fractured embryos. As development progressed from the early to mid 8- to 16-cell stage, the size of the occludens/gap junction complexes increased, forming fascia occludens/gap junction complexes. At the morula stage, gap junctions and occludens/gap junction complexes were observed on both presumptive trophoblast and inner cell-mass cells. Zonula occludens junctions were first observed at the morula stage on presumptive trophoblast cells of freeze-fractured embryos. The number of embryos possessing zonula occludens junctions increased at the mid compared to the early morula stage. At the blastocyst stage, junctional complexes consisting of zonula occludens, macula adherens, and gap junctions were observed between trophoblast cells of freeze-fractured and thin-sectioned embryos. Isolated gap and occludens junctions, adherens junctions, and occludens/gap junction complexes were observed on trophoblast and inner cell-mass cells.