**1. Extracellular vesicles**

Cells are basic structural, functional, and biological units delimited by a plasma membrane that contains all the molecules necessaries for living. Cells could be a complete organism, such as yeast or bacterium, or form part of a multicellular organism which is specialized, such as neurons or adipocytes. The communication between cells is very important to react to the environment and to initiate signaling cascades in all the organisms, from bacteria to eukaryotes. This communication involves the secretion of proteins to the extracellular environment through direct secretion (classical secretion pathways) or can be mediated by the secretion of extracellular vesicles (EVs). This communication allows cells sending and receiving messages about the inside and the outside environment. Different strategies are applied for cellular crosstalk. Current studies have emerged as EVs as an important mechanism of cell communication [1, 2]. The secretion of EVs is a well conserved process through all the organisms, from bacteria to mammals [3].

The term extracellular vesicle includes a heterogeneous group of membrane vesicles with diverse origins, sizes, and shapes. EVs are being identified in almost all the cells, from prokaryotic to eukaryotic, and not only in healthy conditions but also associated with many diseases [4]. The composition, origin, and functions are the focus of attention of researchers and clinicians. The number of papers published yearly in Pubmed related to EVs has increased exponentially during the last 10 years (**Figure 1**). Despite the enormous amount of data published and because of that, there are many new questions unanswered. We still cannot claim the complete understanding of the function of these vesicles in the cell.

The release of EVs to the media allows the cell sending and receiving messages without direct interaction. The content of EVs could participate in controlling important processes, such as growth and differentiation, pathogenesis or metabolic processes. The diversity and complexity of EVs are enormous. Each cell can produce different types of EVs, varying the biogenesis process and the content. The term of EVs is a generic term for all types of vesicles. There are different types of EVs attending to their function, size, and content [5]. Three EV types are mainly accepted: exosomes (30–90 nm), microvesicles (100 up to 1 μm), and apoptotic bodies. The EVs are formed by lipid bilayer with integrated proteins. This capsule protects the inside content from the proteases and nucleases that can affect their content. EVs transport lipids, proteins, growth factors, and RNA. In summary, the EVs are able to transfer genetic material and proteins that can contribute to change the receiving cell. Although, the main components are the same, different EVs from different cells or different EVs from the same cell can possess different contents and characteristics.

**Figure 1.** *Evolution of the number of papers published in PubMed over the past 21 years (1997–2018).*
