Endosome

In cell biology, an endosome is a membrane-bound compartment inside eukaryotic cells. It is a compartment of the endocytic membrane transport pathway originating from the trans Golgi membrane. Molecules or ligands internalized from the plasma membrane can follow this pathway all the way to lysosomes for degradation, or they can be recycled back to the plasma membrane. Molecules are also transported to endosomes from the trans-Golgi network and either continue to lysosomes or recycle back to the Golgi. Endosomes can be classified as early, sorting, or late depending on their stage post internalization. Endosomes represent a major sorting compartment of the endomembrane system in cells. In HeLa cells, endosomes are approximately 500 nm in diameter when fully mature.

Endosomes provide an environment for material to be sorted before it reaches the degradative lysosome. For example, LDL is taken into the cell by binding to the LDL receptor at the cell surface. Upon reaching early endosomes, the LDL dissociates from the receptor, and the receptor can be recycled to the cell surface. The LDL remains in the endosome and is delivered to lysosomes for processing. LDL dissociates because of the slightly acidified environment of the early endosome, generated by a vacuolar membrane proton pump V-ATPase. On the other hand, EGF and the EGF receptor have a pH-resistant bond that persists until it is delivered to lysosomes for their degradation. The mannose 6-phosphate receptor carries ligands from the Golgi destined for the lysosome by a similar mechanism.

Endosomes comprise three different compartments: early endosomes, late endosomes, and recycling endosomes. They are distinguished by the time it takes for endocytosed material to reach them, and by markers such as rabs. They also have different morphology. Once endocytic vesicles have uncoated, they fuse with early endosomes. Early endosomes then mature into late endosomes before fusing with lysosomes.

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