Fig. 2.

The micro- and nano-extrusive activities at the plasma membrane can result in at least three kinds of microextrusive structures such as micro-memtubs (a), membrane blebbing (b) or podosomes (not shown), and membrane ridges or crests (c), based on the shape of extrusion structure and the magnitude of extrusion activity. In the category of micro-memtubs (a), microvilli or nanovilli are probably the most commonly seen micro- or nano-extrusions (i). The adjacent microvilli may become enclosing to engulf extracellular solutes (ii) [10]. Microvilli can release microvesicles to extracellular environments (iii) [13]. The thin and long micro- or nano-extrusions represent the memtubs called nanotubules and nanopodia (iv) [24]