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Mikio Furuse investigated the molecular construction of tight junctions. In a 1998 study, introducing claudin-1 or claudin-2 into fibroblasts that normally lack tight junctions produced networks of junction-like strands at contacts between cells. Immunofluorescence located the introduced proteins, while freeze-fracture imaging revealed the strand architecture.^[ref]

The experiment demonstrated that either claudin could build a key structural feature in a simplified cellular setting. Recruitment of occludin further distinguished a strand-forming component from another associated junctional protein.

Occludin is an integral membrane protein concentrated at tight junctions, where neighbouring epithelial or endothelial cells regulate passage between them. It has four membrane-spanning segments and a long cytoplasmic tail. Occludin is a junction component rather than a synonym for the entire sealing structure.

In 1993, Mikio Furuse and colleagues raised monoclonal antibodies against a junction-enriched fraction from chick liver. Immunoelectron microscopy placed the recognized protein directly at membrane-contact points of tight junctions; sequence analysis suggested four membrane crossings. These combined biochemical and spatial observations identified occludin.^[ref]

Later, expression of claudin-1 or claudin-2 in fibroblasts reconstituted extensive junction strands and recruited occludin. This separates the capacity to build a strand from the presence of one particular strand-associated protein.^[ref]