Fig. 1: Transition from primary twines to 3D-perpendicular lateral protrusions (3D-PLPs).

a Insets show magnified images of the sections indicated by arrows. The dashed red line shows the original, undeflected position of an extracellular fiber neighboring the cell. Time-lapse images of single cell in elongated shape spreading laterally through force exerting PLPs on neighboring fibers. A single twine contacts a neighboring fiber (upper left; 0′), leading to production of a second twine. The space between them is filled in to form a twine-bridge (upper right; 6′). Twines mature to become capable of exerting inward contractile forces at 12 min and proceed to mature and deflect further by 18 min (bottom images; 12′ and 18′, compare neighboring fiber deflection vs. red dotted line, marking the initial neighboring fiber position). b Twines (pink arrow) were also detected engaging fiber layers suspended above the cell plane (in this example the two-layer separation is 6 µm). After engagement, these twines developed into protrusions that were able to considerably deflect fibers of the top layer, pulling them inwards toward the cell body (decreasing length of yellow arrow). See accompanying movies M1 and M2. Scale bar: a 50 μm, inset 10 μm, b 20 μm.