The vesicle lumen is the enclosed interior of a vesicle, separated from the cytosol by its lipid bilayer. It can contain dissolved cargo, ions and portions of membrane proteins facing inward. In a synaptic vesicle, for example, the neurotransmitter is stored in the vesicle lumen before exocytosis.
Membrane topology makes this interior useful to track. When a vesicle buds from the endoplasmic reticulum, its vesicle lumen derives from the interior of the endoplasmic reticulum; fusion with the Golgi apparatus delivers the cargo into the next lumen without exposing it to the cytosol. When a secretory vesicle undergoes exocytosis, the vesicle lumen opens to the extracellular space. Thus an enclosed compartment can transport material through a biological cell while keeping it on the same side of a cell membrane throughout.[ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 13, The Molecular Mechanisms of Membrane Transport and the Maintenance of Compartmental Diversity: conservation of membrane orientation during budding and fusion
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