The biology of extracellular vesicles: The known unknowns

For many years, double-layer phospholipid membrane vesicles, released by most cells, were not considered to be of biological significance.This stance has dramatically changed with the recognition of extracellular vesicles (EVs) as carriers of biologically active molecules that can traffic to local or distant targets and execute defined biological functions.The dimensionality of the field has expanded with the appreciation of diverse types of EVs and the complexity of vesicle biogenesis, cargo loading, release pathways, targeting mechanisms, and vesicle processing.With the expanded interest in the field and the accelerated rate of publications on EV structure and function in diverse biomedical fields, it has become difficult to distinguish between well-established biological features of EV and the untested hypotheses or speculative assumptions that await experimental proof.With the growing interest despite the limited evidence, we sought in this essay to formulate a set of unsolved mysteries in the field, sort out established data from fascinating hypotheses, and formulate several challenging questions that must be answered for the field to advance. Introduction: Primitive vesicles go complexDuring the course of evolution, the creation of the first cell required sequestration of encoding nucleic acids and their replication machinery from the hazardous external microenvironment.Lipids with polar head groups and hydrophobic tails became a natural solution to this challenge.In the case of phospholipids, for example, a layer of hydrophilic polar head groups and two hydrophobic lipid tails readily form double-layer membranes in the aqueous solution.Moreover, in light of the hydrophobic nature of the tails, the phospholipid membrane required a configuration that minimized edges, resulting in a sphere.The formation of spherical double-layer phospholipid structures in aqueous media occurs spontaneously without enzymatic activity.Phospholipid vesicles are probably the simplest biological structures whose biogenesis can be easily reproduced in a tube.It is therefore not surprising that double-layer phospholipid membranes were used to encapsulate biological molecules.In addition, phospholipid