Endosome definition
Endosome refers to the vesicular compartment/organelle present in animal cells, involved in transport of material ingested by the cell by endocytosis to the lysosomes.
Endocytosis
Every single cell need to ingest material from the extracellular medium, either for their regulation and for their growth. In the case of prokaryotic cells, this process occurs through plasma membrane, and in the case of eukaryotic cells, they resort to a mechanism called endocytosis to internalize/externalize material from/to extracellular medium.
Endocytosis is a mechanism by which the cell absorbs material from extracellular medium through the invagination of the plasma membrane, forming membrane-bound vesicles. These vesicles are then fused with initial endosomes, which fuse with late endosomes and ultimately fuse with lysosomes. It is in lysosomes that endosome charge is destroyed. Through this mechanism the cell is able to ingest macromolecules, small particles or even other cells.
Endosomal Compartments
It is possible to observe endosomal compartments in animal cell by electron microscopy, resorting to an active molecule that is absorbed by the cell. In the path of this molecule, it was possible to perceive that endosomal compartments are distributed from a peripheral zone to a perinuclear region, where they are close to Golgi complex, as can be observed in Figure 1. Endosomes located in peripheral regions just below the membrane are called ‘initial endosomes’. Endosomes located near the nucleus, and consequently near Golgi complex, are called ‘late endosomes’.
The interior of endosomal compartments is maintained acidic by the action of the H + ATPase pump that captures H + from cell’s cytosol and transports it into endosome. Initial endosomes differ from late endosomes because of their protein composition and because the latter are more acidic.
Figure 1 – Electron microscopy image showing different endosomal compartments present in an animal cell. E – initial endosome; M – late endosome; L – lysosome.
Initial endosomes
Initial endosomes are those located near plasma membrane and to which the vesicles originated by endocytosis merge. These initial endosomes have a pH≈6 and a screening function, i.e., henceforth the content of endosomes may follow several pathways: go to late endosomes and thence to lysosomes, be recycled back to plasma membrane, or simply be transferred between the domains of polarized cell.
Late endosomes
The content of initial endosomes is then transported to late endosomes, which are located in a perinuclear region. This transport is done through the movement of vesicles along microtubules in cell’s cytosol. Late endosomes have a pH≈5.5 and evolve to lysosomes, which have a pH≈5.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
Lodish H., Berk A., Zipursky S.L., Matsudaira P., Baltimore D., Darnell J. (2000). Molecular Cell Biology (4th edition). W. H. Freeman, New York.
