What are Endosomes?

Endosomes are membrane bound structures within a cell that we call vesicles. They are formed through a complex establishment of processes which is known collectively as endocytosis.   Endosomes are essential for the control of substances in and out of a cell. They act as a temporary vesicles for transportation.

There are three different types of endosomes these are early endosomes, recycling endosomes and late endosomes. 

Early endosomes are composed of morphologically different compartments. Narrow tubular columns and large vesicles with membrane invaginations.  These differences in morphology are essential for separating their individual functions. The main function being the sorting station of the cell. Proteins or lipids may enter the cell by clathrin-mediated endocytosis. This involves uptake of receptors bound to their ligands. They are transported in a vesicle which becomes the early endosome. The early endosomes sort ligands and receptors into different compartments within the endosome.  The receptors are concentrated into a tubular compartment where they are recycled back into the plasma membrane by recycling endosomes. The ligands are concentrated into a sorting compartment before they are released to a late endosome and finally to a lysosome where the final degradation will take place. 

Recycling endosomes are formed by a tubular network and are located close to the micro tubular organising centre.  Recycling endosomes receive internalised material from early endosomes and direct it back to the cell surface or trans golgli network.  The main processes recycling endosomes are engaged in are epithelial cell-cell adhesion, epithelial polarity, cell fate specification and cytokinesis.  

Late endosomes are normally a round or oval shape and the compartments are derived from the vacuolar domains of early endosomes. Due to the multi vesicular appearance late endosomes can also be referred as multi-vesicular bodies. In terms of their function, late endosomes are responsible for managing receptors that are not suitable for recycling, sorting them for lysosomal degradation. Ultimately they fuse with the lysosome resulting in the degradation of the exosomal content. All endosomes go through a maturation process, which causes a hybrid of endosomal compartments accumulating in vesicles. The formation of these vesicles is vital in the development of late endosomes.