Exocytosis: The final step in the secretion pathway

Exocytosis is a process by which cells expel something, generally large particles that cannot pass through the cell membrane. The basic process is an intracellular vesicle merging with the membrane and opening up so its contents face the extracellular milieu (see an animation). It is also the mechanism by which plasma membrane and cell surface receptors are restored from the necessary endocytotic processes.

Here is a description of the process of exocytosis, the last step in the secretory pathway.

Secretory vesicle

The process starts with a vesicle. A vesicle is a “bubble” made of plasma membrane. If it contains material that will be expelled from the cell, it is called “secretory” (for secretion). Many cell components are secreted, including proteins and enzymes, neurotransmitters and other hormones and antibodies. These vesicles also carry receptors meant for the cell surface or antigens to be presented to immune cells that go from being intravesicular to extracellular.

The vesicles may be endosomes that traverse the cell, a part of an endosome that pinched off or a lysosome. Some secretory vesicles are formed by budding off from the Golgi apparatus or endoplasmic reticulum. Some cells specialize in this process and are constantly turning over Golgi and organellar membranes with the cell surface. If the material is not already present in a vesicle, a portion of the organellar membrane engulfs it to create one. Some need the help of a temporary protein coat (i.e., coated vesicles).

Vesicular transport

Vesicles move through the cell via signals so they know where to go. These are now understood to include cytoskeletal proteins, such as kinesin and dynamin (i.e., motor proteins) attached to microtubules. The motor proteins walk the vesicles along the microtubule to their destination. Actin filament polymerization is another such motor.

Membrane fusion

When the vesicle meets the intracellular side of the plasma membrane, they fuse. At neural synapses, this fusion is considered “kiss and run” because the contents are expelled and then the vesicle retreats back into the cell, intracellularly pinching off where it had fused. In other exocytotic processes, the vesicle fully opens up and becomes part of the membrane.

Vesicle contents

The contents of the vesicle become part of the extracellular fluid. Whatever was embedded in the vesicle membrane becomes part of the cell’s membrane. Sometimes secretion of the contents of a vesicle is only needed in certain circumstances. This is called regulated secretion. A signal from the cell surface triggers the vesicle to travel to and merge with the membrane, releasing its contents. Calcium signaling is one cascade involved in such processes. When secretion occurs without a triggering signal, it is called constitutive secretion. See an image depicting the difference at HistologyWorld.