Nanotubular cell–cell protrusions. Schematic diagrams of long, slender cellular extensions that protrude from cells and can form long-distance interconnections with other cells. (A–H) They include (A) filopodia, (B) “long filopodia,” (C) cytonemes, and (D) TNTs—which can be classified as either closed-ended with a terminal gap junction (red) at one end containing connexin 43 (Cx43) or (E) open-ended TNTs, which can in turn be classified as either thin or thick, with thick TNTs also termed (F) tumor microtubes, and (G) contractile TNT-like structures. The latter long, slender, contractile protrusions can contract extracellular collagen inward toward cells while translocating the cell body outward; but (H) they can also become long TNT-like structures (as shown here for cultured MDA-MB-231BO human breast cancer cells stained red for F-actin by ATTO 565 phalloidin and blue for nuclei by DAPI using confocal microscopy imaging by K. Yamada). Consequently, we speculate that these long contractile cell protrusions may be precursors of intercellular connections. The nomenclature for all these nanotubular structures can sometimes appear to differ confusingly when comparing different publications, which apply differing terms to the structures they characterize.