Cells dividing in 3D fibrin matrices extend protrusions that align with the axis of division. (A) A single well-spread actin-GFP fibroblast embedded in a 3D fibrin gel rounding into a sphere before cytokinesis (t = 84–102 min). The nuclear envelope breaks down at t = 84 min, as shown by the influx of GFP into the nuclear volume (yellow arrowheads). After rounding, cytokinesis occurs within minutes (t = 105–108 min). Long, thin protrusions are apparent throughout the division process. After cytokinesis (t = 117–225 min), the daughter cells respread along the long axis of the protrusions. A movie and 3D renderings are shown in Video 1 and Fig. S1, respectively. Bar, 30 µm. (B) The length, number, and morphology of the long, thin protrusions varied from cell to cell. Shown here are representative dividing cells from multiple experiments just before cytokinesis. (C) 3D renderings were used to calculate the angle θ between the cell axis (black line) and the division axis during anaphase (red line). (D) The cell axis is computed from either the protrusion direction during anaphase (“protrusion,” n = 15), the axis of the cell mass during interphase (“interphase axis,” n = 9), or the direction of the elongated mitotic body (“round body,” n = 14). The clustering of the angle θ near zero indicates a correlation between the protrusion and cell division axes (***, the data do not fit a uniform distribution, K-S test, P < 0.001).