Tissues, Part 3 – Connective Tissues: Crash Course A&P #4


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Flo Hyman had always been a tall girl.
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I mean… really tall.
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By her 12th birthday, she was already six feet, and by 17 she’d topped out at just over 6’5’’.
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Initially self-conscious about her stature, she learned to use it to her advantage when
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she started playing volleyball.
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She attended the University of Houston as the school’s first female scholarship athlete,
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and at the age of 21, she was competing in World Championships. Nine years later she
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made it to the 1984 Olympics and helped her team win the silver medal.
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After the Olympics, Hyman moved to Japan where she gained fame playing professional volleyball.
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But all of that ended in 1986 when out of nowhere, she collapsed and died during a game.
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She was 31 years old.
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Hyman’s initial cause of death was thought to be a heart attack, but an autopsy revealed
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that she died from a tear in her aorta, caused by an undiagnosed condition known as Marfan Syndrome.
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Marfan Syndrome is a genetic disorder of the connective tissue. People suffering from it
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have a defect in their connective tissue that substantially weakens it over time.
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And you’ve got connective tissue pretty much everywhere in your body, so it can cause big problems.
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Outwardly, those with Marfan’s tend to to be especially tall and thin, like Flo Hyman,
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with loose, flexible joints and noticeably longer limbs and fingers.
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Those long fingers and bendy joints have actually helped some athletes and musicians do things
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that the rest of us can’t — famous blues guitarist Robert Johnson, piano virtuoso Sergei Rachmaninov,
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and Italian violinist Niccolo Paganini are all believed to have had Marfan Syndrome.
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But these abilities come at a great cost — as people with Marfan’s get older, their weakening
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tissue can cause serious problems in the joints, eyes, lungs, and heart.
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The fact that a single genetic mutation can affect your bones, cartilage, tendons, blood
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vessel walls, and more, shows that all of those structures are closely related, no matter
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how different they may seem.
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We’ve covered the basic properties of nervous, muscle, and epithelial tissue, but we haven’t
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gotten to the most abundant and diverse of the four tissue types — our connective tissue.
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This is the stuff that keeps you looking young, makes up your skeleton, and delivers oxygen
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and nutrients throughout your body. It’s what holds you together, in more ways than one.
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And if something goes wrong with it, you’re in for some havoc.
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And that means we’re gonna be talkin’ about Jello today.
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Uh…we’ll get to that in a minute.
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The springiness here? That’s connective tissue. So is the structure in here, and the
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stuff inside here, and the tendons popping out here
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Connective tissue is pretty much everywhere in your body, although how much of it shows
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up where, varies from organ to organ. For instance, your skin is mostly connective tissue,
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while your brain has very little, since it’s almost all nervous tissue.
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You’ve got four main classes of connective tissue — proper, or the kind you’d find
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in your ligaments and supporting your skin, along with cartilage, bone, and blood.
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Whaaaa?
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Sounds a little weird, but your bones and your blood are just types of connective tissue!
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So, despite the name, your connective tissues do way more than just connect your muscles to your bones.
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Your fat — which is a type of proper connective tissue — provides insulation and fuel storage
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— whether you like it or not — but it also serves structural purposes, like holding your
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kidneys in place, and keeping your eyeballs from popping out of your skull.
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Your bones, tendons, and cartilage bind, support, and protect your organs and give you a skeleton
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so that you can move with a purpose, instead of blobbing around like an amoeba.
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And your blood transports your hormones, nutrients, and other material all over your body. There’s
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no other substance in you that can boast this kind of diversity.
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But if they’re so different, how do we know that anything is a connective tissue? Well,
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all connective tissues have three factors in common that set them apart from other tissue types.
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First, they share a common origin: They all develop from mesenchyme, a loose and fluid
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type of embryonic tissue. Unlike the cells that go on to form, say, your epithelium,
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which are fixed and neatly arranged in sheets, mesenchymal cells can be situated any-which-way,
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and can move from place to place.
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Connective tissues also have different degrees of vascularity, or blood flow. Most cartilage
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is avascular, for example, meaning it has no blood vessels; while other types of connective
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tissue, like the dense irregular tissue in your skin, is brimming with blood vessels.
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Finally — and as strange as it may sound — all connective tissues are mostly composed
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of nonliving material, called the extracellular matrix. While other tissue types are mainly
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made of living cells packed together, the inert matrix between connective-tissue cells
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is actually more important than what’s inside the cells.
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Basically, your connective tissue, when you see it up close, looks and acts a lot like this.
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Yeah. The most abundant and diverse tissue in your body, that makes all of your movements
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and functions possible? Turns out it’s not that different from the dessert that Aunt
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Frances brings to every holiday party.
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The jello that gives this confection its structure is like that extracellular matrix in your
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connective tissue. The actual cells are just intermittent little goodies floating around
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inside the matrix — like the little marshmallows.
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And although it may not look like it in this particular edible model, the extracellular
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matrix is mostly made of two components. The main part is the ground substance — a watery,
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rubbery, unstructured material that fills in the spaces between cells, and — like the
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gelatin in this dessert — protects the delicate, delicious cells from their surroundings.
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The ground substance is flexible, because it’s mostly made of big ol’ starch and
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protein molecules mixed with water.
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The anchors of this framework are proteins called proteoglycans. And from each one sprouts
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lots and lots of long, starchy strands called glycosaminoglycans, or GAGs, radiating out
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from those proteins like brush bristles.
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These molecules then clump together to form big tangles that trap water, and if you’ve
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ever made glue out of flour, you know that starch, protein and water can make a strong
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and gooey glue.
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But running throughout the ground substance is another important component: fibers, which
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provide support and structure to the otherwise shapeless ground substance. And here, too,
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are lots of different types.
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Collagen is by far the strongest and most abundant type of fiber. Tough and flexible,
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it’s essentially a strand of protein, and stress tests show that it’s actually stronger
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than a steel fiber of the same size. It’s part of what makes your skin look young and
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plump, which is why sometimes we inject it into our faces.
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In addition, you’ve also got elastic fibers — which are longer and thinner, and form
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a branching framework within the matrix. They’re made out of the protein elastin which allows
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them to stretch and recoil like rubber bands; they’re found in places like your skin,
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lungs, and blood vessel walls.
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Finally, there are reticular fibers — short, finer collagen fibers with an extra coating
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of glycoprotein. These fibers form delicate, sponge-like networks that cradle and support
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your organs like fuzzy nets.
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So, there’s ground substance and fibers in all connective tissue, but let’s not
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forget about the cells themselves.
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With a tissue as diverse as this, naturally there are all kinds of connective tissue cells,
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each with its unique and vital task — from building bone to storing energy to keeping
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you from bleeding to death every time you get a paper cut.
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But each of these signature cell types manifests itself in two different phases: immature and
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mature. You can recognize the immature cells by the suffix they all share in their names: -blast.
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“Blast” sounds kinda destructive, but literally it means “forming” — these
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are the stem cells that are still in the process of dividing to replicate themselves. But each
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kind of blast cell has a specialized function: namely, to secrete the ground substances and
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fiber that form its unique matrix.
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So chondroblasts, for example, are the blast cells of cartilage. When they build their
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matrix around them, they’re making the spongy tissue that forms your nose and ears and cushions your joints.
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Likewise, osteoblasts are the blast cells of bone tissue, and the matrix they lay down
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is the nexus of calcium carbonate that forms your bone. Once they’re done forming their
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matrix, these blast cells transition into a less active, mature phase. At that point,
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they trade in -blast for the suffix -cyte. So an osteoblast in your bone becomes an osteocyte
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— ditto for chondroblasts becoming chondrocytes.
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These cyte cells maintain the health of the matrix built by the blasts, but they can sometimes
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revert back to their blast state if they need to repair or generate a new matrix.
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So, the matrices that these cells create are pretty much what build you — they assemble
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your bone and your cartilage and your tendons and everything that holds the rest together.
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Not bad for a bunch of marshmallows floating in jello.
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BUT! There is another class of connective tissue cells that are responsible for an equally
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important role. And that is: protecting you, from pretty much everything.
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These are cells that carry out many of your body’s immune functions.
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I’m talking about macrophages, the big, hungry guard cells that patrol your connective
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tissues and eat bacteria, foreign materials, and even your own dead cells.
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And your white blood cells, or leukocytes that scour your circulatory system fighting
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off infection, they’re connective tissue cells, too.
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You can see how pervasive and important connective tissue is in your body. So a condition that
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affects this tissue, like Marfan Syndrome, can really wreak havoc.
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One of the best ways of understanding your body’s structures, after all, is studying
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what happens when something goes wrong with them. In the case of your connective tissue,
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Marfan Syndrome affects those fibers we talked about, that lend structure and support to
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the extracellular matrix.
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Most often, it targets the elastic fibers, causing weakness in the matrix that’s the
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root of many of the condition’s most serious symptoms.
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About 90 percent of the people with the disease experience problems with the heart and the
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aorta — the biggest and most important artery in the body. When the elastic fibers around
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the aorta weaken, they can’t provide the artery with enough support. So, over time,
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the aorta begins to enlarge — so much so that it can rupture.
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This is probably what happened to Flo Hyman. She was physically exerting herself, and her
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artery — without the support of its connective tissue — couldn’t take the stress, and it tore.
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There’s SO MUCH going on with your connective tissue — so many variations within their
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weird diversity — that we’re going to spend one last lesson on them next week, exploring
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the subtypes that come together to make you possible.
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But you did learn a lot today! You learned that there are four types of connective tissue
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— proper, cartilage, bone, and blood — and that they all develop from mesenchyme, have
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different degrees of blood flow, and are mostly made of extracellular matrix full of ground
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substance and fibers. We touched on different blasts, and cyte, and immune cell types, and
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discussed how Marfan Syndrome can affect connective tissue.
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Thanks for watching, especially to our Subbable subscribers, who make Crash Course possible
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for themselves and also for the rest of the world. To find out how you can become a supporter,
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just go to subbable.com.
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This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant,
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is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor
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and sound designer is Michael Aranda, and the graphics team is Thought Café.


This post was previously published on YouTube.

Photo credit: Screenshot from video.