Polymerization of asters to form microtubule templates with varying degrees of percolation. Polymerization was conducted for (a) 10 minutes, (b) 40 minutes, and (c) 130 minutes. As microtubules grew they became interconnected and after 130 minutes a fully percolated system emerged. The frame size of (a), (b) and (c) is the same. The scale bar is 10 μm.