Exocytosis involves the release of material from a cell. This happens when secretory cells release their products, for example in the salivary glands and pancreas. Substances within the cell are enclosed by a membrane to form what is called a vesicle. This moves to fuse with the plasma membrane that surrounds the cell, releasing the contained substances into the extracellular matrix, a structural network of molecules outside the cell. Exocytosis is the reverse of what happens in endocytosis, where substances are taken into the cell and part of the cell membrane is used to enclose them.
Through exocytosis, the membrane used to form a vesicle is restored to the surface of the cell. The fused area breaks down to leave the cell membrane complete, with the former contents of the vesicle on the outside of the cell. Cell membranes are negatively charged — when they approach one another they tend to be repelled — so during exocytosis, the presence of what are called fusogenic proteins together with positively charged calcium ions help overcome this.
The Golgi apparatus is a collection of cisterns where proteins synthesized by the cell are sorted and processed for delivery to various sites. There is a secretory pathway from the Golgi apparatus, with lipids and membrane proteins traveling inside vesicles to the plasma membrane of the cell. Generally, when exocytosis occurs, any soluble proteins are released into the extracellular matrix while other proteins and lipids are used in the plasma membrane. This is called the constitutive secretory pathway and it is present in all cells.
Sometimes soluble proteins are stored together with other substances in what are known as secretory vesicles. These are released on demand; examples include hormones, histamine, enzymes of digestion and neurotransmitters. This is known as the regulatory secretory pathway.
In the constitutive pathway, vesicles fuse with the plasma membrane on arrival, but secretory vesicles of the regulatory pathway only do so when the cell receives a signal. Signals can be chemical, such as hormones attaching to surface receptors, or electrical, as in nerve cells. A nerve synapse is able to fire repeatedly because as exocytosis occurs, more vesicles are being primed within the cell.
Secretory vesicles may travel quite a distance to reach the plasma membrane. In nerve cells, vesicles could move across distances of up to around 3 feet (about 1 m) before arriving at the nerve terminal. The process is facilitated by what are called motor proteins, which propel vesicles along tiny tubes known as microtubules.
When vesicles fuse with the cell membrane in the constitutive pathway, their contents become part of the membrane and it increases in size. This is only temporary because the opposite process, endocytosis, is acting simultaneously to remove parts of the membrane. There has to be a balance between addition to and subtraction from the membrane and in growing cells this is weighted toward addition.
