title: Megapinocytosis: a novel endocytic pathway.
abstract: M2 macrophages showed large endocytotic structures, very different from classical macropinosomes that we named megapinosomes. As observed in the scanning electron microscope, megapinosome formation started with a large (diameter of several micrometers) invagination of the plasma membrane. When the invagination was almost completed, the remaining opening was closed by an actinomorphous centripetal arrangement of many (about 50-100) microvilli-like structures. In transmission electron microscopy using high-pressure freezing, we observed that the megapinosome was filled with a trabecular meshwork that originated from the highly structured plasma membrane. The trabecular meshwork was topologically part of the cytosol and separated from the extracellular fluid by a lipid bilayer. According to ultrastructural features, we could define different phases of megapinosome formation and decay. Megapinosomes became more frequent when M2 macrophages were inoculated with human cytomegalovirus. We did not find megapinosome formation in M1 macrophages.
authors.affiliation: Institute of Virology, University Medical Center Ulm, Albert-Einstein-Allee, 89081, Ulm, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Bauer
authors.affiliation: Institute of Virology, University Medical Center Ulm, Albert-Einstein-Allee, 89081, Ulm, Germany.|Central Facility for Electron Microscopy, Ulm University, Albert-Einstein-Allee 11, 89081, Ulm, Germany.
authors.forename: Narmadha
authors.initials: N
authors.lastname: Subramanian
authors.affiliation: Institute of Virology, University Medical Center Ulm, Albert-Einstein-Allee, 89081, Ulm, Germany.|Central Facility for Electron Microscopy, Ulm University, Albert-Einstein-Allee 11, 89081, Ulm, Germany.
authors.forename: Clarissa
authors.initials: C
authors.lastname: Villinger
authors.affiliation: Institute of Virology, University Medical Center Ulm, Albert-Einstein-Allee, 89081, Ulm, Germany.
authors.forename: Giada
authors.initials: G
authors.lastname: Frascaroli
authors.affiliation: Institute of Virology, University Medical Center Ulm, Albert-Einstein-Allee, 89081, Ulm, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Mertens
authors.affiliation: Central Facility for Electron Microscopy, Ulm University, Albert-Einstein-Allee 11, 89081, Ulm, Germany. paul.walther@uni-ulm.de.
authors.forename: Paul
authors.initials: P
authors.lastname: Walther
mesh_terms: D002478:Cells, Cultured; D006801:Humans; D008264:Macrophages / Q000166:cytology* / Q000378:metabolism* / Q000648:ultrastructure; D046529:Microscopy, Electron, Transmission; D010873:Pinocytosis
keywords: Electron microscopy; Endocytosis; High-pressure freezing; Macrophages; Macropinocytosis; SEM; TEM