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Membrane nanotubes (MNTs) are nanotubular cell-to-cell connections enabling cell-tocell signaling and cargo transfer. The presence of local MNT bulges has been reported by several studies. However, a detailed characterization ofMNT bulges concerning their geometrical properties has not been done yet. The aim of our study was to analyze MNT from cone-like photoreceptor cells (661 W) using scanning electron microscopy (SEM) in order to characterize MNT bulges, thereby increasing the knowledge of ultrastructural feature of MNTs. Our SEM analysis of two MNTs with multiple bulges revealed that (i) MNT bulges are characterized by a statistically significant local increase in the MNT diameter (125% and 250% for both MNTs analyzed), (ii) the thickness and length of the MNT bulges correspond to dimensions of mitochondria, peroxisomes and exosomes, and (iii) the MNT bulges seem not to be randomly distributed on the MNTs but exhibit a preferred spacing with a different median value for each MNT. Our findings highlight that the ultrastructure of MNT exhibits interesting properties that need to be further investigated. Introduction Nanotubular cell-to-cell connections termed “tunnelling nanotubes” or “membrane nanotubes” (MNTs) (the term used in this paper) enable diverse possibilities of cell-to-cell signaling and cargo transfer [1] [2] [3]. This includes the exchange of signal carriers (e.g. proteins), organelles (e.g. mitochondria), bacteria, viruses, exosomes, DNA, RNA, or electric long-range coupling [4] [5] [6] [7] [8]. The defining characteristics of MNTs are still being debated and the investigation of MNT properties is ongoing but recently a consensus of leading MNT researchers was published that MNTs can be defined as “tubular membrane connection between non-adjacent cells that allow direct intercellular communication, not necessarily gap junction-mediated”. MNTs also contain F-actin and/or tubulin, have a variable diameter of 50–800 nm and are open-ended [3]. It can be added that MNTs are filled with cytoplasm and have a lipid bilayer [9]. Some MNTs also contain microtubules [10] and have the gap junction protein Cx43 at the end [6]. In some cases, individual MNTs stick together to form a single, thicker, MNT [9]. As recently summarized by Rustom [11], MNTs are “intimately linked to the physiological state and pathological” state of cells and “represent a central joint element of diverse diseases, such as neurodegenerative disorders, diabetes or cancer”. Furthermore, MNTs seem to play an important role in long-range physical cell-to-cell signaling in multicellular organisms possibly enabling novel ways of physical signal transfer [12] and being involved in the functioning of neurosystems [13]. Studies have been published that report the existence of bulges, i.e. local increases of the diameter, of MNTs [14] [15] [16] [17] [6] [18] [19] [20] [21] [22]. This ultrastructural feature of MNTs was attributed to the presence of objects (vesicles or organelles) inside MNTs [6]. Objective Our objective was to document and analyze bulges of MNTs using scanning electron microscopy (SEM) to increase the knowledge of ultrastructural feature of MNTs. To our knowledge, a detailed characterization of MNT bulges concerning their geometrical Ultrastructural features (bulges) of membrane nanotubes between cone-like photoreceptor cells: An investigation employing scanning electron microscopy DOI: 10.19185/matters.201802000011 Matters (ISSN: 2297-8240) | 2 properties has not been done before.