Occluding junctions of mammary epithelial cells in nonproliferating primary culture occasionally display an atypical pattern of intramembrane strands oriented predominantly perpendicular, instead of roughly parallel, to the apical border of the junction. To test whether the orienting influence was a centripetal cytoskeletal tension often observed in epithelial sheets on fixed substrates, we seeded cells at low density; this allows them to spread maximally while forming a barely confluent pavement. The result was a fourfold increase in the percentage of junctions with the strongly aligned, atypical pattern. Closely similar configurations were observed as the earliest detectable effect of chelation of extracellular Ca++, which induced pronounced centripetal contraction of the cell body. Externally imposed tension, applied so as to stretch cells in one direction only, affected the positions of strands in stretched junctions as might be predicted, by flattening their undulations, increasing their alignment parallel to the apical border. Thus mechanical tension alone, whether inherent in the cytoskeleton or imposed on the cell surface by exogenous force, can cause coordinate lateral displacement of macromolecular assemblies within the membranes of both joined cells.

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