Endosomes form a major sorting compartment within the endomembrane system of human cells. They are classified into different subclasses according to their morphology, composition, acidity and the presence of certain markers. Targeting of molecules to endosomes occurs both through vesicles originating from endocytosis at the plasma membrane and through vesicles originating from the trans-Golgi network. Indeed, vesicles move in both directions between these compartments. Some of the cargo molecules that arrive at endosomes are recycled back to the plasma membrane or to the Golgi apparatus, either directly or via recycling endosomes. Eventually, endosomes fuse with lysosomes, thereby targeting the remaining content for degradation.
Endosomes are usually seen as vesicles located close to the nucleus and the Golgi apparatus. However, differentiation between different types of vesicles in the endomembrane system requires co-staining with marker dyes or of marker proteins. Therefore, vesicle-like stainings are annotated as "vesicles" in the Subcellular Section, while the terms "endosomes", "lysosomes" and "peroxisomes" are only used if co-localization experiments have been carried out.
Read more about the proteome of endosomes as a substructure of vesicles.