Fig. 7. Fibronectin dots of 0.1 µm² induce intracellular signalling but do not support cell spreading at distances >5 µm. (A) B16 cells growing on patterned fibronectin substrata with varying dot sizes (as indicated in the lower left corner) and a constant spacing of 5 µm (centre to centre). Cells spread and form actin stress fibres on dots down to 0.25 µm². On 0.1 µm² dots, cells adhere but do not spread. Note missing dots in the vicinity of the cells on 0.25 µm² and 0.1 µm² substrata. Scale bar: 10 µm. (B) A cell sitting on 0.1 µm² dots at a spacing of 2 µm. Phosphotyrosine (PT, green) accumulates in areas of the cell overlying fibronectin dots (red) indicating that an area of 0.1 µm² is sufficient to induce intracellular signalling. (C) Staining for paxilin (Pax, green) of a cell sitting on 0.1 µm² fibronectin dots (red) separated by distances of 4 µm. Like phosphotyrosine, paxilin accumulates over small dots. Scale bars: 5 µm.