3.12:_Cilia

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<p class="lt-bio-3989">These whiplike appendages extend from the surface of many types of eukaryotic cells. If there are many of them, they are called <strong>cilia</strong>. If only one, or a few, they are <strong>flagella</strong>. Flagella also tend to be longer than cilia but are otherwise similar in construction.</p>

<span id="Function_of_Cilia_and_Flagella"></span><span id="Function_of_Cilia_and_Flagella"></span><h2 class="lt-bio-3989">Function of Cilia and Flagella</h2>

<p class="lt-bio-3989">Cilia and flagella move liquid past the surface of the cell. For single cells, such as sperm, this enables them to <strong>swim</strong>. For cells anchored in a tissue, like the epithelial cells lining our air passages, this moves liquid over the surface of the cell (e.g., driving particle-laden mucus toward the throat). Both cilia and flagella consist of:</p>

<ul>
    <li class="lt-bio-3989">a cylindrical array of 9 filaments consisting of:
    <ul>
        <li class="lt-bio-3989">a complete microtubule (the A-microtubule) extending into the tip of the cilium. When a cilium is being disassembled, protein complexes move down from the tip of the cilium traveling along A-microtubules.</li>
        <li class="lt-bio-3989">a partial microtubule (the B-microtubule) that doesn't extend as far into the tip. When the cilium is growing, its protein components move up toward the tip of the cilium traveling along B-microtubules.</li>
        <li class="lt-bio-3989">cross-bridges of the motor protein <strong>dynein</strong> that extend from the complete microtubule of one filament to the partial microtubule of the adjacent filament.</li>
    </ul>
    </li>
    <li class="lt-bio-3989">a pair of single microtubules running up through the center of the bundle, producing the &quot;9+2&quot; arrangement.</li>
    <li class="lt-bio-3989">The entire assembly is sheathed in a membrane that is an extension of the plasma membrane.</li>
</ul>

<figure><img class="internal" alt="Cross-section of a cilium showing a 9+2 microtubule arrangement, with nine outer doublet microtubules surrounding two central singlets, often seen in electron microscopy of eukaryotic cells." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5945/Satir.gif?revision=1" />
<figcaption>Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="96" style="font-size: 85%; position: relative;"><svg width="5.783ex" height="1.557ex" role="img" focusable="false" viewbox="0 -666 2556 688" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-97-NCM-N-33" d="M303 353C369 378 431 441 431 526C431 569 410 604 369 631C333 654 292 666 246 666C201 666 162 654 127 631C88 605 68 571 68 528C68 495 90 472 122 472C154 472 176 495 176 527C176 560 157 578 119 580C145 615 186 633 242 633C302 633 332 598 332 527C332 485 324 450 309 421C282 373 245 364 183 364C171 362 165 357 165 348C165 333 172 333 192 333L235 333C310 333 348 280 348 173C348 88 317 14 241 14C176 14 128 36 99 80C134 79 160 105 160 139C160 173 135 198 101 198C62 198 42 178 42 137C42 88 64 49 108 18C147-9 193-22 244-22C301-22 350-3 393 34C436 71 457 117 457 173C457 267 383 332 303 353Z"></path><path id="MJX-97-NCM-N-2E" d="M192 53C192 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data-semantic-="" data-semantic-children="0,1,2,3,4" data-semantic-content="1,3" data-semantic-attributes="latex:\PageIndex{1};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="3" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:3" data-semantic-level-number="1" data-speech-node="true">3</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="12" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:12" data-semantic-level-number="1" data-speech-node="true">12</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true">1</mn></mrow></math></mjx-assistive-mml></mjx-container>: Cilia courtesy of Peter Satir</figcaption>
</figure>

<p class="lt-bio-3989">This electron micrograph (Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="97" style="font-size: 85%; position: relative;"><svg width="5.783ex" height="1.557ex" role="img" focusable="false" viewbox="0 -666 2556 688" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-98-NCM-N-33" d="M303 353C369 378 431 441 431 526C431 569 410 604 369 631C333 654 292 666 246 666C201 666 162 654 127 631C88 605 68 571 68 528C68 495 90 472 122 472C154 472 176 495 176 527C176 560 157 578 119 580C145 615 186 633 242 633C302 633 332 598 332 527C332 485 324 450 309 421C282 373 245 364 183 364C171 362 165 357 165 348C165 333 172 333 192 333L235 333C310 333 348 280 348 173C348 88 317 14 241 14C176 14 128 36 99 80C134 79 160 105 160 139C160 173 135 198 101 198C62 198 42 178 42 137C42 88 64 49 108 18C147-9 193-22 244-22C301-22 350-3 393 34C436 71 457 117 457 173C457 267 383 332 303 353Z"></path><path id="MJX-98-NCM-N-2E" d="M192 53C192 82 168 106 139 106C110 106 86 82 86 53C86 24 110 0 139 0C168 0 192 24 192 53Z"></path><path id="MJX-98-NCM-N-31" d="M269 666C228 624 168 603 89 603L89 564C141 564 184 572 217 588L217 82C217 64 213 52 204 47C195 42 170 39 130 39L95 39L95 0C120 2 174 3 257 3C340 3 394 2 419 0L419 39L384 39C343 39 318 42 310 47C302 52 297 64 297 82L297 636C297 660 295 666 269 666Z"></path><path id="MJX-98-NCM-N-32" d="M237 666C186 666 143 648 106 612C69 576 50 534 50 483C50 449 75 424 106 424C136 424 161 450 161 480C161 513 137 536 105 536C102 536 100 536 98 535C117 584 161 627 224 627C306 627 352 556 352 470C352 403 318 331 250 255L62 43C49 28 50 29 50 0L421 0L450 180L417 180C409 129 402 100 396 91C391 86 361 84 306 84L139 84L236 179C304 243 390 312 419 365C439 400 449 435 449 470C449 588 357 666 237 666Z"></path></defs><g stroke="currentColor" fill="currentColor" stroke-width="0" transform="scale(1,-1)"><g data-mml-node="math" data-latex="\PageIndex{1}" 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data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:12" data-semantic-level-number="1" data-speech-node="true">12</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true">1</mn></mrow></math></mjx-assistive-mml></mjx-container>) shows a cilium in cross section. Each cilium (and flagellum) grows out from, and remains attached to, a <strong>basal body</strong> embedded in the cytoplasm. Basal bodies are identical to centrioles and are, in fact, produced by them. For example, one of the centrioles in developing sperm cells &mdash; after it has completed its role in the distribution of chromosomes during meiosis &mdash; becomes a basal body and produces the flagellum</p>

<span id="The_Sliding-Filament_Model_of_Bending"></span><span id="The_Sliding-Filament_Model_of_Bending"></span><h2 class="lt-bio-3989">The Sliding-Filament Model of Bending</h2>

<p class="lt-bio-3989">Motion of cilia and flagella is created by the microtubules sliding past one another. This requires motor molecules of <strong>dynein</strong>, which link adjacent microtubules together, and the energy of <strong>ATP</strong>. Dynein powers the sliding of the microtubules against one another &mdash; first on one side, then on the other. The bending of cilia (and flagella) has many parallels to the contraction of skeletal muscle fibers.</p>

<span id="Testing_the_Model"></span><span id="Testing_the_Model"></span><h2 class="lt-bio-3989">Testing the Model</h2>

<p class="lt-bio-3989">Remember, the partial microtubules do not extend as far into the tip as the complete microtubules. So if a slice is made a short distance back from the tip:</p>

<ul>
    <li class="lt-bio-3989">A straight cilium should show the complete pattern (center of diagram).</li>
    <li class="lt-bio-3989">In a bent cilium, approximately half the filaments on the upper side should be retracted because of the greater arc on the convex side. So the partial microtubules would disappear being drawn below the plane of the slice. As seen here, bending to the left causes the partial microtubules 4, 5, 6, 7, and 8 to disappear.</li>
    <li class="lt-bio-3989">When the cilium bends the other way, the partial microtubules on the opposite side disappear while they reappear on what is now the lower or concave side.</li>
</ul>

<ul>
    <li class="lt-bio-3989">Electron micrographs (made by Peter Satir) have verified this model precisely.</li>
</ul>

<figure><img class="internal" alt="Diagram of a mechanical system with petals and circular insets showing detailed components, connected by lines and arrows indicating movement and function." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5946/Cilium.gif?revision=1" />
<figcaption>Figure <mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="98" style="font-size: 85%; position: relative;"><svg width="5.783ex" height="1.557ex" role="img" focusable="false" viewbox="0 -666 2556 688" aria-hidden="true" style="vertical-align: -0.05ex;"><defs><path id="MJX-99-NCM-N-33" d="M303 353C369 378 431 441 431 526C431 569 410 604 369 631C333 654 292 666 246 666C201 666 162 654 127 631C88 605 68 571 68 528C68 495 90 472 122 472C154 472 176 495 176 527C176 560 157 578 119 580C145 615 186 633 242 633C302 633 332 598 332 527C332 485 324 450 309 421C282 373 245 364 183 364C171 362 165 357 165 348C165 333 172 333 192 333L235 333C310 333 348 280 348 173C348 88 317 14 241 14C176 14 128 36 99 80C134 79 160 105 160 139C160 173 135 198 101 198C62 198 42 178 42 137C42 88 64 49 108 18C147-9 193-22 244-22C301-22 350-3 393 34C436 71 457 117 457 173C457 267 383 332 303 353Z"></path><path id="MJX-99-NCM-N-2E" d="M192 53C192 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data-semantic-annotation="nemeth:number;depth:2" data-semantic-id="3" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true"><use data-c="2E" href="#MJX-99-NCM-N-2E"></use></g></g><g data-mml-node="mn" data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-id="4" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true" transform="translate(2056,0)"><use data-c="31" href="#MJX-99-NCM-N-31"></use></g></g></g></g></svg><mjx-assistive-mml unselectable="on" display="inline"><math data-latex="\PageIndex{1}" data-semantic-structure="(5 0 1 2 3 4)"><mrow data-mjx-texclass="ORD" data-latex="1}" data-semantic-type="punctuated" data-semantic-role="sequence" data-semantic-annotation="depth:1" data-semantic-="" data-semantic-children="0,1,2,3,4" data-semantic-content="1,3" data-semantic-attributes="latex:\PageIndex{1};texclass:ORD" data-semantic-owns="0 1 2 3 4" data-semantic-level-number="0" data-speech-node="true"><mn data-latex="3" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:3" data-semantic-level-number="1" data-speech-node="true">3</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="12" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:12" data-semantic-level-number="1" data-speech-node="true">12</mn><mrow data-mjx-texclass="ORD" data-latex="{.}"><mo data-latex="." data-semantic-type="punctuation" data-semantic-role="fullstop" data-semantic-annotation="nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:{.};texclass:ORD" data-semantic-operator="punctuated" data-semantic-level-number="1" data-speech-node="true">.</mo></mrow><mn data-latex="1" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:1" data-semantic-level-number="1" data-speech-node="true">1</mn></mrow></math></mjx-assistive-mml></mjx-container>: Cilia Model courtesy Peter Satir</figcaption>
</figure>

<span id="Other_Parallels"></span><span id="Other_Parallels"></span><h2 class="lt-bio-3989">Other Parallels</h2>

<p class="lt-bio-3989">There are other parallels between the sliding filaments of skeletal muscle and the sliding microtubules of cilia. Both are powered by ATP. Both motors &mdash;dynein in cilia, myosin in skeletal muscle &mdash; are ATPases and both are regulated by calcium ions.</p>

<span id="The_Primary_Cilium"></span><span id="The_Primary_Cilium"></span><h2 class="lt-bio-3989">The Primary Cilium</h2>

<p class="lt-bio-3989">Motile, &quot;9+2&quot;, cilia are found only on certain cells in the vertebrate body, e.g., the epithelia lining the airways. But almost every cell in mammals has &mdash; or had &mdash; a single <strong>primary cilium</strong>. The primary cilium grows out of the older of the two centrioles that the cell inherited following mitosis. The primary cilium does not beat because it lacks the central pair of microtubules; that is, it is &quot;9+0&quot;. Where functions have been identified, they all involve sensory reception. Some examples are as follows:</p>

<ul>
    <li class="lt-bio-3989"><strong>Mechanoreceptors</strong>: A primary cilium extends from the apical surface of the epithelial cells lining the kidney tubules and monitors the flow of fluid through the tubules. Inherited defects in the formation of these cilia cause polycystic kidney disease.</li>
    <li class="lt-bio-3989"><strong>Chemoreceptors</strong>: We detect odors by receptors on the primary cilium of olfactory neurons. Many types of cells detect extracellular signaling molecules, e.g., nutrients, growth factors, hormones, with receptors localized on their primary cilium. These signals may be transduced into the nucleus where they alter gene expression.</li>
    <li class="lt-bio-3989"><strong>Photoreceptors</strong>: The outer segment of the rods in the vertebrate retina is also derived from a primary cilium.</li>
</ul>

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<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/3.12:_Cilia">3.12: Cilia</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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