Ultrastructural features (bulges) of membrane nanotubes between cone-like photoreceptor cells: An investigation employing scanning electron microscopy

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 of MNT 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.