Objective: UPlanSApo 100x oil/1.40	Exposure: 250 ms
Microscope: Olympus DSU/IX81	Gain: 3
Camera: Hamamatsu ImagEM	Interval: 500 ms

By adding or removing globular tubulin proteins, the length of polymeric microtubules can be increased or decreased. Because the two ends of a microtubule are not the same, however, the rate at which growth or depolymerization occurs at each pole is different. The end of a polarized filament that grows and shrinks the fastest is known as the plus end and the opposing end is called the minus end. For all microtubules, the minus end is the one with exposed alpha-tubulins. In an animal cell, it is this end that is located at the centriole-containing centrosome found near the nucleus, while the plus end, comprised of exposed beta-units, is projected out toward the cell's surface. Microtubules are continuously being assembled and disassembled so that tubulin monomers can be transported elsewhere to build microtubules when needed. The digital video above illustrates the tracking of microtubule +TIPs (plus end tracking proteins) in Gray fox lung fibroblast cells labeled with a chimera of EB3 fused to mEGFP. Each of the three cells in the video are expressing the EB3 fusion at different levels (thus are imaged at different intensities).