Exosomes: Revisiting their role as "garbage bags"

In recent years, the term "extracellular vesicle" (EV) has been used to define different types of vesicles released by various cells. It includes plasma membrane-derived vesicles (ectosomes/microvesicles) and endosome-derived vesicles (exosomes). Although it remains difficult to evaluate the compartment of origin of the two kinds of vesicles once released, it is critical to discriminate these vesicles because their mode of biogenesis is probably directly related to their physiologic function and/or to the physio-pathologic state of the producing cell. The purpose of this review is to specifically consider exosome secretion and its consequences in terms of a material loss for producing cells, rather than on the effects of exosomes once they are taken up by recipient cells. I especially describe one putative basic function of exosomes, that is, to convey material out of cells for off-site degradation by recipient cells. As illustrated by some examples, these components could be evacuated from cells for various reasons, for example, to promote "differentiation" or enhance homeostatic responses. This basic function might explain why so many diseases have made use of the exosomal pathway during pathogenesis.