endozomes

Endosomes are membrane-bound organelles found in eukaryotic cells that play a crucial role in endocytosis, a process by which cells absorb molecules from outside by engulfing them with their cell membrane. They are formed when vesicles containing extracellular material fuse with the plasma membrane. Endosomes are transient structures that mature over time, progressively moving from the cell periphery towards the cell interior.

The internal environment of an endosome becomes increasingly acidic as it matures. This change in pH is mediated by proton pumps embedded in the endosomal membrane. This acidification is essential for many endosomal functions, including the dissociation of ligands from their receptors and the degradation of internalized material.

There are different types of endosomes, broadly categorized as early endosomes, late endosomes, and recycling endosomes. Early endosomes are the first endocytic vesicles to form and are primarily involved in sorting internalized molecules. Receptors and their ligands are sorted here, with some destined for degradation and others for recycling back to the plasma membrane. Late endosomes are more acidic than early endosomes and are the precursors to lysosomes. They are the main sites for the degradation of internalized material by lysosomal enzymes. Recycling endosomes are specialized for the rapid return of specific receptors, like transferrin receptors, back to the cell surface.

The endosomal-sorting complex required for transport (ESCRT) machinery is also vital for endosome function, particularly in the formation of intraluminal vesicles within late endosomes, a process leading to the formation of multivesicular bodies (MVBs). MVBs are a distinct stage in endosome maturation, containing numerous smaller vesicles within their lumen. The fate of these MVBs is typically fusion with lysosomes for complete degradation of their contents.