Let’s say you want to get a piercing. Under your own free will, you’re choosing for
someone to take a needle, quickly poke a hole in your body, which you can then adorn with
all kinds of cool decorations. People get piercings for all kinds of cultural and aesthetic
reasons, but a few spots on the body get pierced more than others including different spots
on the ear, nose, and belly button. Here in the US, the most common visible piercing is
through the earlobe, but it’s fairly common to see a piercing a few centimeters higher
on the ear. This is a structure called the helix, and it’s what people are talking
about if they want to pierce their cartilage. And while the lobe and helix are near each
other, they behave differently than each other. Like, there’s a reason you see the larger
gauges in earlobes and not in cartilage piercings. That has to do with the entire nature of cartilage
and the role it plays in our bodies. This type of tissue is part of the bigger type
of connective tissue, one of our four tissue types, the others being nervous, muscle, and
epithelial tissues. And as we’ll see, connective tissue encompasses a lot of different cell
types, from the cartilage in your ears and nose to the liquid blood in your veins. In
today’s episode, we’ll talk about the role of connective tissue in our body and
see how researchers are tackling different injuries and surgeries with their deeper understanding
of this particular anatomy.
When you think of connective tissue, you probably don’t
think of anything exciting. It’s just kinda there, holding stuff together, right? I mean,
it’s not contractile like muscle and it doesn’t send electrical signals like nervous
tissue, so what is it actually doing? Well, aside from making sure your body doesn’t
fall apart, a lot. Connective tissue is probably the hardest tissue type to define in a concise
way because it includes so many different types of cells including the ones in bones,
the cushioning between your joints, even your red blood cells are considered connective
tissue. It can be liquid like blood and lymph, or proper connective tissue like ligaments.
But there are a few features that most connective tissue will have in common, so let’s break
them down. All connective tissue is going to start with the extracellular matrix, a
collection of fluid and structural fibers. How those things are arranged and organized
then tells us more about the tissue. This matrix can have a lot of structure to it and
make a firm, dense tissue, or very few fibers and be looser and more cushioning. This lets
us sort connective tissue into two big groups. Like we have loose connective tissue that
make up the more pliable structures that hold organs in place. And we also have dense connective
tissue that makes up some of the extra thick structures in our body like tendons and ligaments. That
makes sense, ligaments connect bones to bone and tendons connect bones to muscles. Between
those two slides for instance, you can probably guess which one is dense and which one is
loose based on the pictures alone. This one is dense while this one is loose. If you got
it right, good job, ten Seeker bucks for you. By the way, those Seeker bucks have no monetary
value and can buy you absolutely nothing. As the name implies,
that dense connective tissue just has more fibers in it compared to the loose connective tissue.
But those fibers are where some of the interesting properties come in. A lot of those fibers
are made from collagen, a strong protein that gives structure and support to tissues. Collagen
is the most common protein in our body and we have twenty eight versions of it, each
with slightly different properties. Some of those collagen proteins are so durable that
they can resist breaking down for tens of millions of years. In fact, back in the early
2000’s, scientists found preserved collagen in a T rex fossil from almost seventy million
years ago. Collagen might also be the casing for your sausage or hot dogs, but honestly,
the sausage manufacturing process is a nebulous black box of mystery, so who knows what you’re
actually getting. Other than those tough collagen fibers, the matrix can be built with thinner,
crosslinking fibers called reticular fibers, as well as elastic fibers that can give some
stretchiness to your tissues. That elasticity is especially useful in places like the lungs,
bladder, and major blood vessels like the aorta. Now, if you look back at the loose
connective tissue picture, you’ll notice what looks like a lot of empty space. That’s
ground substance, a gelatinous material that’s mostly water with a few dissolved proteins. It
acts kind of like a glue for the fibers, and gives the cells and capillaries a way to exchange
nutrients and chemicals. All your connective tissue will have those three things: fibers,
ground substance, and the actual living part, cells. Like adipose tissue, or fat, is a
connective tissue. It has plenty of structural fibers, but its cells, or adipocytes, store
lipids for energy. Still counts as a specialized connective tissue. Now, I’ve been spending
so much time at the cellular level because once we see how all those cells, fibers and
ground substance come together, the tissues themselves are less uniform. Like bone for
example. It may be more rigid than what you typically picture as connective tissue, but
it has structural fibers like collagen, some ground substance, and all kinds of different
cells like osteocytes. Even our fluid connective tissue like lymph and blood have a liquid
matrix and cells, just like typical connective tissue. They just don’t have supporting
fibers. We have episodes dedicated to both lymph and blood by the way, so make sure to
check those out if you’d like a refresher. Now, we opened the episode talking about cartilage,
which is only one of those connective tissue types. More specifically, it’s a type of
supportive connective tissue. And that type of cartilage in the ear that we mentioned
isn’t necessarily representative of the rest of the body’s cartilage. Again, it
seems like this stuff is simple but there are so many variations that all do unique
things. For instance, you might not notice your hyaline cartilage now, but you will if
you develop arthritis. This is the most common type of cartilage in our body. It’s thin,
glassy looking, and structurally weaker than the other types of cartilage. You have a
version of this coating the ends of certain bones called articular cartilage, creating
a smooth surface so your joints can bend around with less friction. When that smooth surface
breaks down over time, that joint is more likely to develop osteoarthritis. It might
seem weird that hyaline cartilage is so common, considering that the cartilage we’re used
to seeing superficially is very different. We’re more used to elastic cartilage which
includes tissue in the ear and tip of the nose, but you also have some around your trachea. And
you guessed it, this stuff is more elastic than the other types. No matter how much you
try to deform or bend your nose cartilage, it always springs back to center. When people
break their nose, they’re breaking the nasal bone which only goes about a third of the
way down your whole nose. The nasal cartilage just goes along for the ride. There’s also
a less elastic, but more supportive fibrous cartilage. If you were to dissect a human
spine, you’d find thick fibrous discs between each vertebrae. There is some gelatinous goop
within those disc too but the outer layer is fibrous. And if you looked inside your
knees you’d find a curved meniscus sitting on top of the tibia. Both of these pieces
of cartilage provide cushioning to bony structures and have lots of collagen fibers in them.
Now, that meniscus can present some problems. A meniscus tear is a fairly common orthopedic
injury. Unfortunately, the meniscus can’t repair itself very well, so doctors will usually
elect for surgery if the tear is bad enough. Depending on the exact location and severity
of the tear, surgeons might stitch it back together or just take the whole meniscus out. But
those surgeries can have all kinds of complications like an increased risk of arthritis or a build
up of thick, fibrous tissue inside the knee. In the past few years, researchers have tried
using stem cell injections as a new treatment option. Unfortunately, they’ve run into
some challenges with delivering the right kind of stem cells to the injured spot. So
back in 2017, researchers at the University of Pennsylvania came up with a creative workaround.
They made a microscopic scaffold — a patch of matrix without cells in it, and loaded
it with two special chemicals. One was an enzyme to loosen the dense matrix, and the
other was a growth factor that attracted stem cells to the location. They tried out their
treatment on cow meniscus, and it worked as expected. The enzyme and growth factor combo
allowed the meniscus to start its repair process. Now, they still have to test this treatment
in large animals before beginning human trials, but it’s an interesting start.
Studying connective tissue is a great reminder for me that even when anatomy seems straightforward,
it has the ability to surprise you with its depth and complexity. Man! I love this stuff.
Thanks for watching this episode of Seeker Human. I’m Patrick Kelly.