3.25:_Exocytosis

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<p class="lt-bio-4072" style="background-color: unset;">Exocytosis is the reverse of endocytosis and that is just as well. In 30 minutes an active cell like a macrophage can endocytose an amount of plasma membrane equal to its complete plasma membrane. The electron micrograph in Figure 3.25.1 shows a guinea pig phagocyte ingesting polystyrene beads. Several beads are already enclosed in vacuoles while the others are in the process of being engulfed. So the cell must have a mechanism to restore the normal amount of plasma membrane. Exocytosis is that mechanism.</p>

<figure><img class="internal" alt="Microscopic view of bacterial cells, oval-shaped and surrounded by a textured structure, likely cell walls, displayed in black and white." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6132/North.gif?revision=1" /><figcaption>Figure 3.25.1: Guinea pig Phagocyte courtesy Dr. Robert J. North</figcaption></figure>

<span id="The_Secretion_Mechanism"></span><span id="The_Secretion_Mechanism-2"></span><h2 style="background-color: unset;" class="lt-bio-4072">The Secretion Mechanism</h2>

<p class="lt-bio-4072" style="background-color: unset;">Membrane-enclosed vesicles move to the cell surface where they <strong>fuse</strong> with the plasma membrane. This restores the normal amount of plasma membrane and any molecules dissolved in the fluid contents of these vesicles are discharged into the extracellular fluid - this is called <strong>secretion</strong> (e.g., the various components of the extracellular matrix are secreted by exocytosis). Any integral membrane proteins exposed to the <strong>interior</strong> surface of the vesicles will now be displayed at the cell surface because the vesicles turn inside out as they fuse with the plasma membrane. Thus exocytosis does not simply replace plasma membrane, but ensures that the plasma membrane will display its <strong>characteristic cell-surface proteins</strong>.</p>

<figure><img class="internal" alt="Diagram comparing endocytosis and exocytosis: vesicles at the plasma membrane move cargo into or out of the cell via the extracellular fluid." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6134/EndoExocytosis.gif?revision=1" aria-describedby="remedy-description-85aabd3b59544396" /><span id="remedy-description-85aabd3b59544396" data-remedy-image-description="85aabd3b59544396" style="display: block; white-space: pre-line;">A side-by-side diagram contrasting two transport processes at the cell surface. At the top, the label &quot;Plasma membrane&quot; has arrows pointing to the pink membrane outlines in both panels. In the center, the label &quot;Extracellular fluid&quot; identifies the space between the two panels, where small orange dots representing molecules are scattered. Left panel, labeled &quot;ENDOCYTOSIS&quot; at the bottom: a pink circular vesicle sits inside the cell, and the plasma membrane curves inward around it. Black arrows point inward from the extracellular fluid toward the vesicle, indicating material being brought into the cell. Right panel, labeled &quot;EXOCYTOSIS&quot; at the bottom: a pink circular vesicle is positioned at the plasma membrane, fusing with it. Black arrows point outward from the vesicle toward the extracellular fluid, indicating material being released from the cell. Both vesicles are labeled &quot;Vesicle.&quot; The diagram illustrates that endocytosis and exocytosis are reverse processes: one imports extracellular material by engulfing it in vesicles, while the other exports intracellular material by fusing vesicles with the plasma membrane.</span><figcaption>Figure 3.25.2: Exocytosis/ Endocytosis</figcaption></figure>

<p class="lt-bio-4072" style="background-color: unset;">Exocytic vesicles are created from several sources. Some are simply endosomes traversing the cell and others are pinched off from endosomes before they fuse with lysosomes. Others bud off from the endoplasmic reticulum and Golgi apparatus taking their products to the surface of the cell. The exocytosis of lysosomes supplies the membrane needed to repair wounds in the plasma membrane.</p>

<p class="lt-bio-4072" style="background-color: unset;">Some cells specialize in secretion. In cells that secrete large amounts of protein, for example, the protein accumulates in specialized secretory granules formed by the Golgi apparatus. These move to the cell surface and discharge their contents to the outside. For example, exocrine cells in the pancreas synthesize and secrete pancreatic digestive enzymes. The electron micrograph in Figure 3.25.4 shows four cells in the pancreas of a bat. The lumen where their apical surfaces meet leads eventually to the pancreatic duct draining into the small intestine. The spherical bodies (budded off from the Golgi apparatus) contain precursors of digestive enzymes. One is discharging its contents into the lumen by exocytosis (red arrow).</p>

<figure><img class="internal" alt="Electron micrograph of exocytosis at a secretory cell lumen, marked by a red arrow." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6136/Fawcett.gif?revision=1" aria-describedby="remedy-description-d816ea66fe7ef080" /><span id="remedy-description-d816ea66fe7ef080" data-remedy-image-description="d816ea66fe7ef080" style="display: block; white-space: pre-line;">Numerous dark, rounded secretory granules surround a lighter central region labeled Lumen. A red arrow identifies a site at the lumen boundary where secretion occurs. Granular cellular material fills the surrounding area. No scale bar is visible.</span><figcaption>Figure 3.25.4: Pancreas cells of a bat courtesy Fawcett, The Cell: Its Organelles and Inclusions, W. B. Saunders Co., 1966</figcaption></figure>

<p class="lt-bio-4072" style="background-color: unset;">The cells lining our intestine synthesize tiny droplets of fat and discharge them into the lacteals by exocytosis.</p>

<span id="The_Kiss-and-Run_Mechanism"></span><span id="The_Kiss-and-Run_Mechanism-2"></span><h2 style="background-color: unset;" class="lt-bio-4072">The Kiss-and-Run Mechanism</h2>

<p class="lt-bio-4072" style="background-color: unset;">The process described above involves the <strong>fusion</strong> of the exocytotic vesicle with the plasma membrane. In some cells, such as at synapses, a second type of exocytosis also takes place: (1) the vesicles make a brief contact at the plasma membrane, (2) release their contents (neurotransmitters in this case) to the exterior and (3) retreat back into the cytosol. This &quot;kiss-and-run&quot; version of exocytosis does not restore plasma membrane to the cell.</p>

<span id="The_Exosome_Mechanism"></span><span id="The_Exosome_Mechanism-2"></span><h2 style="background-color: unset;" class="lt-bio-4072">The Exosome Mechanism</h2>

<p class="lt-bio-4072" style="background-color: unset;">A third type of exocytosis is found in some cells and involves endosomes themselves invaginating their membrane. As the invaginations break off they produce vesicles within vesicles, called multivesicular bodies. When these fuse with the cell's plasma membrane, these tiny (40&ndash;100 nm) internal vesicles &mdash; called <strong>exosomes</strong> &mdash; are secreted. Exosomes are produced in abundance by dendritic cells and B-cells and enhance their antigen-presenting function.</p>

<figure><img class="internal" alt="An endosome forms a multivesicular body, which releases its internal vesicles as exosomes." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6137/Exosomes.png?revision=1" aria-describedby="remedy-description-1ffc1630d72a75ba" /><span id="remedy-description-1ffc1630d72a75ba" data-remedy-image-description="1ffc1630d72a75ba" style="display: block; white-space: pre-line;">Within the cytosol, an endosome contains small vesicles and shows inward budding at its boundary. An arrow leads to a multivesicular body containing several vesicles. A second arrow leads to fusion with the plasma membrane. Small vesicles then appear in the extracellular fluid, labeled Exosomes. Blue membrane-associated symbols remain on the released vesicles; their identity is not given in a legend.</span><figcaption>Figure 3.25.5: Exosomes</figcaption></figure>

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