Membrane events involved in fusion of uterine epithelial cells in pseudopregnant rabbits.

During pseudopregnancy in the rabbit some uterine epithelial cells undergo conversion into symplasmata. This event serves as a model for studies of membrane apposition, fusion and fission of the lateral membranes with the use of different ultrastructural techniques. Apposition of lateral membranes occurs by means of proliferation of the tight-junctional belt and macular tight junctions. Membrane fusion is characterized in freeze-fracture replicas by continuously running fracture planes between neighboring membrane leaflets of epithelial cells, in general without reorganization of the particles. It is suggested that the reorganization of particles as well as the blebs or vesicles of smooth membranes, which are occasionally observed, may be artefacts. Membrane fission occurs simultaneously with fusion resulting in irregularly shaped membrane holes on freeze-fracture replicas. These events are rarely seen in thin sections. Staining with tannic acid reveals that only the layers of the plasma membrane are accessible to this agent. The fusion-fission process starts in the lower region of the lateral membranes, whereas the luminal portion with the broad tight-junctional belt remains intact.