Extracellular vesicles (EVs) are produced by all known organisms and have important roles in cell communication and physiology. Exosomes are known in the literature to be small round EVs (40 to 100 nm in diameter) and are commonly purified with a serial ultracentrifugation protocol followed by density gradient floatation. Great morphological diversity has been described before regarding EVs found in body fluids such as blood plasma, breast milk and ejaculate. However, a detailed morphological analysis has never been performed on exosomes purified from a single cell type.
Researchers at the University of Gothenburg aimed to analyze and quantify via multiple electron microscopy techniques the morphology of exosomes purified from the human mast cell line HMC-1. The results revealed a novel spectrum of diversity in exosomes, which suggests that subpopulations of exosomes with different and specific functions might also exist. Their findings therefore argue that a new and more efficient way of defining exosome subpopulations is necessary. A system was proposed where exosomes were classified into nine different categories according to their size and shape. Three additional morphological features could also be found in exosomes regardless of their classification. These findings show that morphological diversity is found among exosomes purified from a single cell line, similarly to what was previously observed for EVs in body fluids. This knowledge can help improving the interpretation of experimental results and widening our general understanding of the biological functions of exosomes.
Morphologically diverse exosomes purified from HMC-1.
A) Exosomes visualized after negative staining. At least two different categories of vesicles can be identified in this picture: single vesicles and a long tubule (green arrows and red arrow respectively). B) Montage of 9 cryo-electron micrographs. Green arrows point at single vesicles, yellow arrow points at a double vesicle, small black arrows point at fiducial gold markers. The circular shape is the carbon edge of the holey carbon grid. C) Size distribution of all vesicles included in the analysis (n=1724). In brackets is indicated how many vesicles were larger than 500 nm. D) Percentage of total vesicles that belonged to each morphological category. E) Size distribution for each vesicle category. The top and bottom boundaries of the boxes represent the 75th and 25th percentiles. The top and bottom whiskers represent the 90th and 10th percentiles. The black line in the box represents the median while the white line represents the mean.