Endosomes are membrane-bound organelles found in eukaryotic cells that are part of the endocytic membrane transport pathway originating from the trans Golgi network. They are formed via a complex family of processes collectively known as endocytosis. Endosomes can be classified based on their morphology, distinct protein and lipid composition, and their compartments. There are three major compartments of endosomes:
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Early endosomes: These are the first endosomes to receive newly internalized material from the plasma membrane. They are tubular and vesicular in shape and contain a variety of sorting and recycling proteins.
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Late endosomes: Also known as multivesicular bodies (MVBs), these are mainly spherical, lack tubules, and contain many close-packed intraluminal vesicles. They contain unique lipids named BMP or LBPA, which are not found in any other organelle membrane. Late endosomes/MVBs are sometimes called endocytic carrier vesicles, but it is more likely that transport between these two compartments occurs by a maturation process, rather than vesicle transport. In some circumstances, late endosomes/MVBs fuse with the plasma membrane instead of with lysosomes, releasing the lumenal vesicles, now called exosomes, into the extracellular medium.
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Recycling endosomes: These are responsible for returning membrane proteins and lipids to the plasma membrane and other organelles.
Endosomes generally transport their contents in a series of steps to a lysosome, which subsequently digests the materials. An endosome that is destined to transfer its contents to a lysosome generally goes through several changes along its way. In its initial form, when the structure is often referred to as an early endosome, the specialized vesicle contains a single compartment. Over time, however, chemical changes in the vesicle take place and the membrane surrounding the endosome folds in upon itself in a way that is similar to the invagination of the plasma membrane. In this case, however, the membrane is not pinched off. Consequently, a structure with multiple compartments, termed a multivesicular endosome, is formed. The multivesicular endosome is an intermediate structure in which further chemical changes, including a significant drop in pH level, take place.
