Fig. 1.

The membrane tubular structures (memtubs) of cells. We depict various membrane tubules from non-neuronal cells. In neurons, the membrane tubular structures, including axons, dendrites, and spines, are well elucidated. For non-neuronal cells, tubular or rod-like extrusive membrane structures include “macro”extrusions such as filopodia, invadopodia, cilia, and penetrative pegs (not shown), and “micro”extrusions such as microvilli, foot processes, basal microextrusions, and lateral microextrusions. Lateral microextrusions could form the adhesion zipper when cells contact each other. Micro-memtubs may also be found in intracellular organelles such as the multivesicular body (MVB), a form of late endosomes, and ER