What are endosomes?

Endosomes are membrane-bound organelles that are responsible for regulating the trafficking of proteins, lipids and other subcellular compartments of both the secretory and endocytic pathways. Endosomes can either originate from the trans-Golgi network or from the plasma membrane. Endosomes transpiring from the trans-Golgi network are transported either to the cell membrane to discharge its contents or to lysosomes for degradation. Likewise, endosomes derived from the cell membrane transport to lysosomes for degradation, or its components can be recycled back to the cell membrane via the endosomal recycling pathway.

Endosomes are divided into three different categories, these include:

• Early endosomes – consists of a dynamic tubular-vesicle network. They receive molecules from the extracellular environment, as well as from the Golgi, and sort them into recycling and degradative compartments. Prior to degradation, early endosomes must mature into late endosomes.
• Late endosomes – also referred to as multivesicular bodies (MVBs) consists of cisternal, tubular and multivesicular regions. Late endosomes can either mature into lysosomes or fuse with existing lysosomes to degrade its components.
• Recycling endosomes – major function is to sort and re-export membrane components that have been internalized back to the cell membrane for integration.