3.14:_Adipose_Tissue

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<p class="lt-bio-3991" style="background-color: unset;">Two kinds of adipose tissue are found in mammals: white adipose tissue (WAT) and brown adipose tissue (BAT). White adipose tissue is the most common and is the fat that so many of us complain of acquiring. Brown adipose tissue is present in small mammals (e.g., mice) and in newborn humans. Most of it disappears in adult humans.The cells in both types of fat are called adipocytes although they differ in origin, structure, and function in the two types of tissue.</p>

<table class="mt-responsive-table" align="center">
    <caption>Table 3.14.1: Two classifications of Adipocytes</caption>
    <thead>
        <tr>
            <th class="mt-align-center" scope="col"><strong>WAT Adipocytes</strong></th>
            <th class="mt-align-center" scope="col"><strong>BAT Adipocytes</strong></th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td data-th="WAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">a narrow rim of cytoplasm with its nucleus<br />
            pressed near the margin of the cell<br />
            surrounding</td>
            <td data-th="BAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">Cytoplasm throughout the cell with a central nucleus and</td>
        </tr>
        <tr>
            <td data-th="WAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">a single large membrane-enclosed lipid droplet</td>
            <td data-th="BAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">many small lipid droplets</td>
        </tr>
        <tr>
            <td data-th="WAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">few mitochondria</td>
            <td data-th="BAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">many mitochondria (providing the brown color)</td>
        </tr>
        <tr>
            <td data-th="WAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">modest blood supply</td>
            <td data-th="BAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">rich blood supply</td>
        </tr>
        <tr>
            <td data-th="WAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">serves as a depot of stored energy</td>
            <td data-th="BAT Adipocytes" style="text-align: center; background-color: unset;" class="lt-bio-3991">function is to generate heat</td>
        </tr>
    </tbody>
</table>

<p class="lt-bio-3991" style="background-color: unset;">New adipocytes in <strong>white</strong> adipose tissue are formed throughout life from a pool of precursor cells. These are needed to replace those that die (after an average life span of 10 years). Whether the total <strong>number</strong> of these adipocytes increases in humans becoming fatter as adults is still uncertain. If not, why do so many of us get fatter as we age? Because of the increased <strong>size</strong> of individual adipocytes as they become filled with oil.</p>

<p class="lt-bio-3991" style="background-color: unset;">The adipocytes of white adipose tissue secrete several hormones, including leptin, asprosin (which, during fasting, causes a rapid release of blood sugar (glucose) from the liver) and adiponectin. In addition to serving as a major source of energy reserves, white adipose tissue also provides some mechanical protection and insulation to the body. Obesity is the excessive accumulation of white adipose tissue.</p>

<p class="lt-bio-3991" style="background-color: unset;"><strong>Brown</strong> adipose tissue provides a vital source of heat to maintain body temperature in small mammals (with their high surface to volume ratio) and infants (who usually cannot shiver when they are cold).</p>

<span id="Adipose_tissue_activation_mechanism"></span><span id="Adipose_tissue_activation_mechanism"></span><h2 style="background-color: unset;" class="lt-bio-3991">Adipose tissue activation mechanism</h2>

<p class="lt-bio-3991" style="background-color: unset;">Brown adipose tissue is activated by the following mechanism when the body temperature drops:</p>

<figure><img class="internal" alt="Diagram showing proton flow across the inner mitochondrial membrane. UCP1 generates heat by allowing protons through, while ATP synthase converts ADP to ATP using proton flow in the matrix." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5948/UCP1.png?revision=1" /><figcaption>Figure 3.14.1 Adipose tissue activation process</figcaption></figure>

<ul>
    <li class="lt-bio-3991" style="background-color: unset;">Cold activates the sympathetic nervous system.</li>
    <li class="lt-bio-3991" style="background-color: unset;">Noradrenaline is released by the postganglionic neurons.</li>
    <li class="lt-bio-3991" style="background-color: unset;">The noradrenaline binds to G-protein-coupled receptors on the adipocytes surface.</li>
    <li class="lt-bio-3991" style="background-color: unset;">The second messenger cAMP is generated and moves into the nucleus where</li>
    <li class="lt-bio-3991" style="background-color: unset;">it binds to the promoter of the gene encoding an enzyme that converts thyroxine (T<small><sub><font size="2">4</font></sub></small>) to triiodothyronine (T<small><sub><font size="2">3</font></sub></small>).</li>
    <li class="lt-bio-3991" style="background-color: unset;">T<small><sub><font size="2">3</font></sub></small> enters the nucleus and bind to the promoter of the gene encoding uncoupling protein1 (<strong>UCP1</strong>).</li>
    <li class="lt-bio-3991" style="background-color: unset;">UCP1 inserts into the inner membrane of the mitochondria where</li>
    <li class="lt-bio-3991" style="background-color: unset;">it allows the protons that have been pumped out into the intermembrane space by the electron transport chain</li>
    <li class="lt-bio-3991" style="background-color: unset;">to return to the matrix without having to pass through ATP synthase.</li>
</ul>

<p class="lt-bio-3991" style="background-color: unset;">So instead of cellular respiration (of fatty acids and glucose) generating ATP, it generates heat.</p>

<span id="WAT_can_Acquire_the_Properties_of_BAT"></span><span id="WAT_can_Acquire_the_Properties_of_BAT"></span><h2 style="background-color: unset;" class="lt-bio-3991">WAT can Acquire the Properties of BAT</h2>

<p class="lt-bio-3991" style="background-color: unset;">In mice and perhaps in humans, skeletal muscles that have undergone a period of vigorous exercise secrete a protein hormone called <strong>irisin</strong>. Irisin acts on white adipose tissue to give it the properties of brown adipose tissue:</p>

<ul>
    <li class="lt-bio-3991" style="background-color: unset;">an increase in the number of mitochondria and lipid droplets;</li>
    <li class="lt-bio-3991" style="background-color: unset;">a marked increase in the synthesis of <strong>UCP1</strong>;</li>
    <li class="lt-bio-3991" style="background-color: unset;">an increase in the rate of cellular respiration but with the energy released as heat rather than fueling the synthesis of ATP.</li>
</ul>

<p class="lt-bio-3991" style="background-color: unset;">These brown-like fat cells derived from white fat cells have been called &quot;beige&quot; or &quot;brite&quot; cells. Lean adult humans have deposits of beige cells in the neck and upper chest regions. When they are exposed to cold, their beige cells are activated. Obese people have few or no beige cells. Probably their layers of white adipose tissue provide such good insulation that they are in less danger of heat loss.</p>

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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/3.14:_Adipose_Tissue">3.14: Adipose Tissue</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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