3.15:_Junctions_between_Cells

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<p class="lt-bio-3993" style="background-color: unset;">In many animal tissues (e.g., connective tissue), each cell is separated from the next by an extracellular coating or matrix. However, in some tissues (e.g., epithelia), the plasma membranes of adjacent cells are pressed together. Four kinds of junctions occur in vertebrates:</p>

<ol>
    <li class="lt-bio-3993" style="background-color: unset;">Tight junctions</li>
    <li class="lt-bio-3993" style="background-color: unset;">Adherens junctions</li>
    <li class="lt-bio-3993" style="background-color: unset;">Gap junctions</li>
    <li class="lt-bio-3993" style="background-color: unset;">Desmosomes</li>
</ol>

<p class="lt-bio-3993" style="background-color: unset;">In many plant tissues, it turns out that the plasma membrane of each cell is continuous with that of the adjacent cells. The membranes contact each other through openings in the cell wall called Plasmodesmata.</p>

<span id="Tight_Junctions"></span><span id="Tight_Junctions-2"></span><h2 style="background-color: unset;" class="lt-bio-3993">Tight Junctions</h2>

<p class="lt-bio-3993" style="background-color: unset;">Epithelia are sheets of cells that provide the interface between masses of cells and a cavity or space (a lumen). The portion of the cell exposed to the lumen is called its <strong>apical</strong> surface. The rest of the cell (i.e., its sides and base) make up the <strong>basolateral</strong> surface. Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface. They consist of a network of <strong>claudins</strong> and other proteins. Tight junctions perform two vital functions:</p>

<ol>
    <li class="lt-bio-3993" style="background-color: unset;">They limit the passage of molecules and ions through the space between cells. So most materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides tighter control over what substances are allowed through.</li>
    <li class="lt-bio-3993" style="background-color: unset;">They block the movement of integral membrane proteins (red and green ovals) between the apical and basolateral surfaces of the cell. Thus the special functions of each surface, for example receptor-mediated endocytosis at the apical surface and exocytosis at the basolateral surface can be preserved.</li>
</ol>

<figure><img style="width: 215px; height: 212px;" class="internal" alt="A tight junction separates apical and basolateral membrane regions of two epithelial cells." loading="lazy" width="215" height="196" src="https://bio.libretexts.org/@api/deki/files/5950/TightJunction1.gif?revision=2&amp;size=bestfit&amp;width=215&amp;height=212" aria-describedby="remedy-description-e7764eca221005cc" /><span id="remedy-description-e7764eca221005cc" data-remedy-image-description="e7764eca221005cc" style="display: block; white-space: pre-line;">Lumen is above the cells and ECF below. Red membrane proteins occur along the apical surface facing the lumen; green proteins occur along basolateral surfaces facing ECF. A tight junction near the top of the intercellular gap separates these regions. Each cell contains a dark circular nucleus.</span>
<figcaption>Figure 3.15.1: Tight Junction</figcaption>
</figure>

<div class="box-note"><span id="The_Epithelia_of_the_Human_Lung"></span><span id="The_Epithelia_of_the_Human_Lung-2"></span><h3 class="box-legend"><span class="lt-icon-default">The Epithelia of the Human Lung</span></h3>

<p class="lt-bio-3993" style="background-color: unset;">A report by Vermeer, et al., in the 20 March 2003 issue of Nature provides a striking example of the role of tight junctions. The epithelial cells of the human lung express</p>

<ul>
    <li class="lt-bio-3993" style="background-color: unset;">a growth stimulant, called <strong>heregulin</strong>, on their <strong>apical</strong> surface and</li>
    <li class="lt-bio-3993" style="background-color: unset;">its receptors on the <strong>basolateral</strong> surface. (These receptors also respond to epidermal growth factor (<strong>EGF</strong>), and mutant versions have been implicated in cancer.</li>
</ul>

<p class="lt-bio-3993" style="background-color: unset;">As long as the sheet of cells is intact, there is no stimulation of its receptors by heregulin thanks to the seal provided by tight junctions. However, if the sheet of cells becomes broken, heregulin can reach its receptors. The result is an autocrine stimulation of mitosis leading to healing of the wound. Several disorders of the lung the chronic bronchitis of cigarette smokers, asthma, cystic fibrosis increase the permeability of the airway epithelium. The resulting opportunity for autocrine stimulation may account for the proliferation (piling up) of the epithelial cells characteristic of these disorders.</p>
</div>

<span id="Adherens_Junctions"></span><span id="Adherens_Junctions-2"></span><h2 style="background-color: unset;" class="lt-bio-3993">Adherens Junctions</h2>

<p class="lt-bio-3993" style="background-color: unset;">Adherens junctions provide strong mechanical attachments between adjacent cells. They hold <strong>cardiac muscle</strong> cells tightly together as the heart expands and contracts and hold <strong>epithelial cells</strong> together. Adherens junctions seem to be responsible for contact inhibition and some are present in narrow bands connecting adjacent cells. Others are present in discrete patches holding the cells together . Adherens junctions are built from <strong>cadherins</strong> that are transmembrane proteins (Figure 3.15.2; red labeled) whose extracellular segments bind to each other and whose intracellular segments bind to <strong>catenins</strong> (Figure 3.15.2; yellow labeled), which are connected to actin filaments</p>

<figure><img height="238" width="261" class="internal" alt="Adjacent cells connected by cadherins, connexin channels and filament-anchoring junctions." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5951/CellJunctions.gif?revision=2" aria-describedby="remedy-description-b05597b94dcb5f3f" /><span id="remedy-description-b05597b94dcb5f3f" data-remedy-image-description="b05597b94dcb5f3f" style="display: block; white-space: pre-line;">Two outlined cells share a narrow gap. At the top, red structures labeled Cadherins meet between the cells, with yellow elements on their inner sides. In the middle, green bars labeled Connexin cross the gap. At the bottom, pink plaques anchor brown lines labeled Intermediate filaments, extending into both cells.</span>
<figcaption>Figure 3.15.2: Adheren Junctions</figcaption>
</figure>

<p class="lt-bio-3993" style="background-color: unset;">Human synthesize some 80 different types of cadherins. In most cases, a cell expressing one type of cadherin will only form adherens junctions with another cell expressing the same type. This is because molecules of cadherin tend to form homodimers, not heterodimers. Inherited mutations in a gene encoding a cadherin can cause stomach cancer. Mutations in a gene (<em>APC</em>), whose protein normally interacts with catenins, are a common cause of colon cancer. Loss of functioning adherens junctions may accelerate the edema associated with sepsis and tumor metastasis.</p>

<span id="Gap_Junctions"></span><span id="Gap_Junctions-2"></span><h2 style="background-color: unset;" class="lt-bio-3993">Gap Junctions</h2>

<p class="lt-bio-3993" style="background-color: unset;">Gap junctions are intercellular channels some 1.5&ndash;2 nm in diameter. These permit the free passage between the cells of ions and small molecules (up to a molecular weight of about 1000 daltons). They are cylinders constructed from 6 copies of transmembrane proteins called <strong>connexins</strong>. Because ions can flow through them, gap junctions permit changes in membrane potential to pass from cell to cell.</p>

<figure><img height="387" width="596" class="internal" alt="Gap junction diagram: two six-connexin connexons span adjacent plasma membranes; connexin topology with M1???M4, E1, E2, CL, N- and C-termini shown." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5958/687px-Connexon_and_connexin_structure.svg.png?revision=1" aria-describedby="remedy-description-5bd837d9193cbb22" /><span id="remedy-description-5bd837d9193cbb22" data-remedy-image-description="5bd837d9193cbb22" style="display: block; white-space: pre-line;">This figure illustrates the structure of a gap junction. At top left, two flower-shaped top views of a connexon are labeled 'Closed' and 'Open': the closed view shows six subunits packed in a ring with no central pore; the open view shows the subunits twisted apart, revealing a central channel. The central cross-section shows two gray plasma membranes separated by an 'extracellular' space, with 'Cytoplasm' above and below. Each membrane holds a connexon???'Connexon (cell A)' above and 'Connexon (cell B)' below???each built of six orange cylinder-shaped 'Connexin' proteins; the two connexons align across the extracellular gap, forming a continuous channel between the cytoplasms. At top right, a 3D view labeled 'gap junction' shows two blue membrane sheets joined by several orange connexon channels. A single connexin is drawn as an orange barrel with black dots marking its transmembrane domains. Accompanying text lists the connexin's transmembrane domains (M1???M4), extracellular loops (E1 and E2), cytoplasmic loop (CL), and terminal domains (N and C). The bottom-right 'Connexin structure' diagram shows the protein's topology: four orange transmembrane segments, M1, M2, M3, and M4, span the membrane; E1 and E2 loop out on the extracellular side; CL connects M2 and M3 on the cytoplasmic side; the N-term (NH2) begins in the cytoplasm and the C-term (COOH) extends into the cytoplasm, all connected by a continuous purple polypeptide line.</span>
<figcaption>Figure 3.15.3: The diagram shows a gap junction and its main element. connexon. together with the structure of the connexin. (CC-BY-SA-3.0; Mariana Ruiz)</figcaption>
</figure>

<p class="lt-bio-3993" style="background-color: unset;">Examples of gap junctions include:</p>

<ul>
    <li class="lt-bio-3993" style="background-color: unset;">The action potential in heart (cardiac) muscle flows from cell to cell through the heart providing the rhythmic contraction of the heartbeat.</li>
    <li class="lt-bio-3993" style="background-color: unset;">At some so-called electrical synapses in the brain, gap junctions permit the arrival of an action potential at the synaptic terminals to be transmitted across to the postsynaptic cell without the delay needed for release of a neurotransmitter.</li>
    <li class="lt-bio-3993" style="background-color: unset;">As the time of birth approaches, gap junctions between the smooth muscle cells of the uterus enable coordinated, powerful contractions to begin.</li>
</ul>

<p class="lt-bio-3993" style="background-color: unset;">Several inherited disorders of humans such as certain congenital heart defects and certain cases of congenital deafness have been found to be caused by mutant genes encoding connexins.</p>

<span id="Desmosomes"></span><span id="Desmosomes-2"></span><h2 style="background-color: unset;" class="lt-bio-3993">Desmosomes</h2>

<p class="lt-bio-3993" style="background-color: unset;">Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia (e.g., the skin). Desmosomes are attached to intermediate filaments of keratin in the cytoplasm. <strong>Pemphigus</strong> is an autoimmune disease in which the patient has developed antibodies against proteins (cadherins) in desmosomes. The loosening of the adhesion between adjacent epithelial cells causes blistering. Carcinomas are cancers of epithelia. However, the cells of carcinomas no longer have desmosomes. This may partially account for their ability to metastasize.</p>

<span id="Hemidesmosomes"></span><span id="Hemidesmosomes-2"></span><h3 style="background-color: unset;" class="lt-bio-3993">Hemidesmosomes</h3>

<p class="lt-bio-3993" style="background-color: unset;">These are similar to desmosomes but attach epithelial cells to the basal lamina (&quot;basement membrane&quot;) instead of to each other. <strong>Pemphigoid</strong> is an autoimmune disease in which the patient develops antibodies against proteins (integrins) in hemidesmosomes. This, too, causes severe blistering of epithelia.</p>

<span id="Plasmodesmata"></span><span id="Plasmodesmata-2"></span><h3 style="background-color: unset;" class="lt-bio-3993">Plasmodesmata</h3>

<p class="lt-bio-3993" style="background-color: unset;">Although each plant cell is encased in a boxlike cell wall, it turns out that communication between cells is just as easy, if not easier, than between animal cells (Figure 3.15.4). Fine strands of cytoplasm, called <strong>plasmodesmata</strong>, extend through pores in the cell wall connecting the cytoplasm of each cell with that of its neighbors.</p>

<figure><img height="291" width="387" class="internal" alt="Diagram of two plant cells showing plasmodesma connecting cytoplasm, with apoplastic (cell wall) and symplastic (cytoplasm) transport pathways." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5954/1052px-Apoplast_and_symplast_pathways.svg.png?revision=1" aria-describedby="remedy-description-368f8a99ba98eafd" /><span id="remedy-description-368f8a99ba98eafd" data-remedy-image-description="368f8a99ba98eafd" style="display: block; white-space: pre-line;">Cross-section diagram of two adjacent plant cells, left and right, each containing a large white vacuole surrounded by yellow cytoplasm and enclosed by a green cell wall. A narrow channel, labeled 'Plasmodesma,' passes through the point where the two cell walls meet, connecting the cytoplasm of both cells. Two transport routes are shown with directional arrows moving from left to right: orange arrows represent the apoplastic pathway, traveling through the cell wall; blue arrows represent the symplastic pathway, traveling through the cytoplasm. The orange apoplastic arrows run along the outer cell wall and are diverted around the plasmodesma, since the cell wall is interrupted there. The blue symplastic arrows curve through the cytoplasm of the left cell, converge, and pass directly through the plasmodesma into the cytoplasm of the right cell, demonstrating that plasmodesmata allow molecules to move between plant cells without crossing membranes. Legend: orange line = 'Apoplastic pathway (through cell wall)'; blue line = 'Symplastic pathway (through cytoplasm)'. Labels with leader lines identify Cell wall, Cytoplasm, Vacuole, and Plasmodesma.</span>
<figcaption>Figure 3.15.4: A diagram showing the different routes through which water can travel through a plant. Plasmodesmata allow molecules to travel between plant cells through the symplastic pathway. Image used wtih permission from Wikipedia (public domain; Jackacon).</figcaption>
</figure>

<p class="lt-bio-3993" style="background-color: unset;">Plasmodesmata provide an easy route for the movement of ions, small molecules like sugars and amino acids, and even macromolecules like RNA and proteins, between cells. The larger molecules pass through with the aid of actin filaments. Plasmodesmata are sheathed by a plasma membrane that is simply an extension of the plasma membrane of the adjoining cells. This raises the intriguing question of whether a plant tissue is really made up of separate cells or is, instead, a syncytium: a single, multinucleated cell distributed throughout hundreds of tiny compartments.</p>

<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/03:_The_Cellular_Basis_of_Life/3.15:_Junctions_between_Cells">3.15: Junctions between Cells</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0 (creativecommons.org)</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball (biology-pages.info)</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content (biology-pages.info)</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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