Microtubule polarity. Illustration showing the polarity of microtubules (green) in a non-dividing cell. Microtubules are part of a cell's cytoskeleton, providing internal support. The microtubules originate from a production or nucleation site (here a centrosome, red). The ends of the microtubules at this site are negatively charged. The other ends, near the plasma membrane surface of the cell, are positively charged. The cell nucleus (round, blue) is also shown. This illustration is based on a fibroblast cell. For this artwork with labels, see image C023/8762.