Tissues, Part 4 – Types of Connective Tissues: Crash Course A&P #5


Technically, we’re all just meat. Yeah, you could say that what makes us human
is our emotions, or thoughts, or memories, but physically, we’re not all that different
from this. We’re used to thinking of meat as the muscle
tissue that people cook and eat — but the fact is, you could eat that stuff raw. A chicken
breast or a steak or a pork chop is tender enough that it doesn’t need cooking. What does need cooking is all of the connective
tissue that’s in and around the muscle, and gets in the way of our meal. So one of the main reasons humans began to
roast, and boil, and bake meat in the first place, was to break down those connective tissues
that we can’t chew, so we could get the meat. Now, you might’ve noticed that connective
tissues kind of seem like a grab bag of materials that don’t really fit in other groups. Our muscle, nervous, and epithelial tissue
types are more uniform, more obviously grouped together. Sometimes, our connective tissues just
seem like the leftovers you throw in the stew pot. But appearances are deceiving, and our inability
as a species to tolerate these tissues in our food is just one reminder that no matter
how different tendon, bone, or a hunk of fat may appear, they are indeed very closely related. They all spring from the same embryonic cells,
and their structures are dominated, not by cells, but by an extracellular matrix full
of fibers. And it’s those fibers that have given humans
the most trouble in our meat-eating history. It wasn’t until we figured out how to cook the
stuff the that we were able to denature, or change the natural structure of, those protein
fibers so that they became soft and pliable. Take a chicken for example. You couldn’t
just pull the skin off a living bird — like ethically you couldn’t do that but also
you probably couldn’t. Its living skin is anchored by connective
tissue that’s full of collagen fibers. But if you roast that chicken, and break apart
that collagen, suddenly the skin just peels right off with no effort at all. So connective tissues, as evidenced by this
mess here, are good reminders of how sometimes things are more similar than they appear. They’re also delicious . . . if you’re
into that. So we know what our connective tissues have
in common, and we know that we’ve got four different types — but what do they look like,
what do they do, and where are they found — both in your butcher shop, and on your
body? Perhaps your most diverse type is your classic
connective tissue proper. It comes in both loose and dense subclasses, based on how many
fibers it has in its ground substance. Pull on the back of your hand. See that fleshy
tent there? That’s one example of loose connective tissue. There’s a lot of ground
substance in here, and the elastin fibers help it snap back into place, while the collagen
helps anchor it so you can’t, like, snag your skin on a zipper and watch it just fly
off. But try pulling on your Achilles’ tendon,
or these wing tendons here, and there isn’t a lot of give. That’s because a tendon is
an example of dense connective tissue, with a lot more collagen. You could chew and chew
and chew on a collagen-dense tendon and never get anywhere. That’s why butchers trim off most dense
connective tissue before selling cuts of meat. So loose connective tissues have fewer fibers,
and more cells and more ground substance. Areolar tissue is the most common loose connective
tissue you have, found ALL over your body, just under your epithelial tissue, and wrapped
around your organs. It’s got a loose and random arrangement
of fibers, with just a few fibroblast cells that make those fibers. If you look at it
under a microscope you’ll see that its most obvious feature is that it looks like it has
a lot of open space in it. This makes areolar tissue makes it great at
holding the watery, salty ground substance that surrounding tissues draw from. Your adipose tissue, meanwhile, is your fat
tissue, the loose connective tissue that’s in here. Rather than being mostly ground substance,
this one is mostly cells — adipocytes — which store lipids for later use, insulate the body against
heat loss, and grow pot bellies and love handles. The average person’s weight is about 18
percent adipose tissue, and it’s those fat stores that keep us alive when food is scarce.
With no fat stores, you’d die within a few days of your last sandwich. Reticular tissue is like areolar tissue, but
with a woven mess of reticular fibers — rather than collagen and elastin fibers — hence
the name. This tissue provides the soft internal framework,
or stroma, of the spleen, lymph nodes, and bone marrow, and it supports lots of developing
blood cells. Just as your areolar tissue is a kind of sponge for watery ground substance,
your reticular tissue is what holds your blood in place in many of your organs. Really, all of these loose connective tissue
proper types share an airy dispersal of fibers … which is why they’re easier to eat after cooking —
and why you can pull cooked chicken skin apart. On the other hand, you can’t easily rip
a tendon or ligament in two, or even chew it, because it’s made of that dense regular
tissue, full of tight bundles of collagen fibers all running parallel. You can see how neat and smooth a slide sample
looks under a scope, the fibers being those white, flexible structures. They provide great
resistance to tension when that tension is exerted in one direction. That’s why you find this tissue in your
tendons, which connect muscle to bone or other muscle, and your ligaments, which bind bones
together anywhere you’ve got a joint. But what if those collagen fibers aren’t
all stacked regularly in rows? Then it forms dense irregular tissue, whose
fibers are thicker and arranged erratically — it’s found wherever tension might be
exerted in lots of different directions, like the leathery dermis underlying your skin. And finally, your body has places that require
more elasticity than rigidity, like say, around your joints. That’s where you’ll find
dense elastic tissue — for example, connecting your vertebrae so that your spine can curve
and twist. Some of our largest artery walls are made
of this stretchy elastic tissue, too, which provides both support and flexibility. From fat to tendons, connective tissue proper
is the most diverse group in this tissue family. But for the last few minutes you’ve also
been watching a different type of connective tissue bob up and down as I talk — cartilage Cartilage doesn’t have any blood or nerves,
and it stands up against both tension and compression pretty well — it’s that whitish
gristle you see at the end of pork ribs or chicken wings in your grocer’s freezer — and
it’s another thing that you’re not going to have much luck chewing. Hyaline cartilage is your most common type
— it’s kind of glassy looking and provides pliable support. It connects your ribs to your
sternum and keeps the tip of your nose all perky. Its ground substance is rich with those sticky,
starchy proteoglycans, and although it has collagen fibers, when you look at it under
a scope, you can’t really see them — instead the tissue looks glassy, hence the name “hyal”
meaning glassy, or transparent. Elastic cartilage is very similar to hyaline,
but with more elastic fibers that are easier to see, and it’s found in places where strength
and stretchability are needed. Like, it’s why I can pull on my ear without it ripping
off. And just as your body needs firm parts and
stretchy parts, it also needs shock absorbers. That’s where your fibrocartilage comes in.
It’s dominated by thick fibers of collagen and is good at withstanding lots of pressure,
so it makes up the discs between your vertebrae, and your knee joints where it keeps your bones
from grinding together. And speaking of bones! Although you might not think of something
so hard and durable as living tissue, bones definitely are. The word “bone” can refer to an entire
organ — like your femur or scapula — or just bone tissue. And that bone, or osseous
tissue, is just calcified connective tissue, perfect for supporting and protecting your
body’s various structures. Spongy bone tissue is typically found in the
heads of long bones and in the inner layer of flat bones like the sternum. This spongy
tissue is strong, but porous, even to the naked eye, and it uses this extra room to
make and store bone marrow. Compact bone tissue on the other hand is quite
dense, with no visible spaces. It forms the external layer of your bones and stores calcium
for bone cells to use to make more tissue. Now, if bone doesn’t conjure up your traditional
image of a tissue, blood probably doesn’t either — I mean, it doesn’t seem to connect
things, or lend support, and yet, blood is our fourth type of connective tissue. And when you think about it, it clearly does
connect distant parts of your body and provides some rigidity to other parts. Just like other connective tissues, blood
develops from mesenchyme and is made up of cells surrounded by an extracellular nonliving
matrix: In this case, the ground substance is your blood plasma, which has protein fibers
floating around in it. Your blood’s main job, of course, is delivering
goods — it transports cells, and nutrients, and hormones, and wastes, and all kinds of
other stuff, keeping all the parts of your body connected in the process. Most of your blood cells are erythrocytes,
or your famous red blood cells that zoom around, carrying oxygen and carbon dioxide through
your body. You’ve also got larger, infection-fighting white blood cells, or leukocytes in the mix,
and your platelets, the small cell fragments needed for blood clotting so that a paper
cut doesn’t bleed you dry. Unlike other connective tissues, it doesn’t
really have what you would call fibers, but instead has a bunch of protein dissolved in
the plasma, and those protein molecules will form fiber-like structures when your blood
needs to clot. So. You take your four primary types of tissues,
and all of the subtypes of each that’s taken us weeks to explore, and you can probably
see how they can come together in many, many combinations to form all of your body’s
organs. And just like that chaotic cafeteria packed
with middle-schoolers — we’d be hopeless messes without our hierarchy of organization,
our inner and outer boundaries, and the ability of our systems to communicate with each other. THAT is what our tissues do for us, and we
wouldn’t exist without them. Today we wrapped up our four-episode series
on tissues, with a survey of the sub-groups of loose and dense connective tissue, including
areolar, adipose, reticular, tendons, ligaments, dense irregular, and dense elastic tissue.
We also talked about the three cartilage types, spongy and compact bone connective tissue, and how
and why our blood is also a connective tissue type. Thanks to all of you for watching, especially to our
Subbable subscribers, who make Crash Course possible to themselves and also to the rest of the world. To find out how you can become a supporter, you can go to Subbable.com. And don’t forget to
go to Youtube.com/CrashCourse and subscribe. This episode was written by Kathleen Yale,
the script was edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson.
It was directed by Nicholas Jenkins & Michael Aranda and our graphics team is Thought Cafe.

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