overview of the types of connective tissue in the body and learn why those
types of tissues show up in the anatomy that they do.
If you're new here, welcome! My name is Patrick and on this channel
we learn anatomy.
On the chance that you've already
watched my histology basics video, the first minute or two of this video is
going to sound familiar. Obviously I'd recommend watching this
one anyway for a refresher but if you want to skip to the new information you
can click this time on the screen (2:20). So for the rest of you let's get into
this lesson.
Connective tissue is one of the four types of tissue we have in our
bodies. The others: muscle, nervous, and epithelial
all serve unique purposes and have pretty clear-cut roles in the body.
Like, muscles produce force. Nervous tissue transmits signals and
epithelial tissues line organs and secrete mucus.
Connective tissue is literally everything else! Ligaments and fascia are
some of the obvious types of connective tissue but
so is bone and blood and lymph. Like yes we have
liquid connective tissue that is a real thing. And in general,
we can split connective tissue into three categories: loose proper,
dense proper, and specialized. We're going to build on this flow chart over the
course of the video and if you want it as a study guide there's
a copy link down in the doobly-doo. So as expected
the tissue gets its name depending on what it's built from and each proper
connective tissue has some degree of the following
three components: ground substance, structural fibers,
and living cells. Ground substance is kind of a liquidy, intermediary substance
for dissolved particles to float around in. Stuff like electrolytes or enzymes
can float from a capillary through ground substance to make its way across
tissues to a cell. The fibers in connective tissue give the
tissue some kind of structure and mechanical properties. Again, think of
them like building materials. You could choose the dense,
thick collagen fibers if you want resistance to tensile force,
or thin reticular fibers which usually get laid down in a mesh work style,
creating more spaces between fibers. Then there's elastin which is stretchier and
more pliable. There's even variation within those
fibers for specific purposes. Our bodies make 28 types of collagen,
again, all with different purposes. Like type 1
collagen is more appropriate for ligaments while type
2 collagen is more common between our joints. And while a long strand of
collagen protein isn't alive, there is a living part of connective
tissue: cells. The most common of which are fibroblasts --
cells that live to supply the tissue around it with those structural fibers
hence /fibro/blast/. That's not all though.
There are also wandering immune cells like the occasional macrophage
that goes looking around for bad guys and as you could probably infer,
loose connective tissue is going to be heavier on the ground substance and
lighter on the fibers while dense connective tissue is the
opposite it has a higher density of fibers
and less ground substance. We'll come back to specialized connective tissue
but for now let's address the elephant in the room.
What's the point of loose connective tissue?
I mean, "if the point of connective tissue is to connect,
then shouldn't a material with a high fiber content and higher tensile
strength just be a better option, like always?" asked an 18 year old me in front
of the entire class proving that i didn't do the assigned reading when i
was a freshman in college. So just like in our house metaphor,
there's a time and place to use beefier materials.
I want you to think the same thing about connective tissue -- there's a time and place
for each material. You'll use dense connective tissue proper for anatomy
that needs to resist a straightforward strong tensile force. Best example is
probably ligaments: the regular dense connective tissue that
attaches bone to bone. They exist to prevent any type of
excessive motion in one particular direction.
Like your anterior talofibular ligament keeps you from too much ankle
inversion, so a strong bundle of dense connective tissue ties down your
talus to your fibula to try to restrict that motion. And it's
not just the density of fibers that gives ligaments their strength,
but also their arrangement ligaments fibers run in parallel directions
unlike irregular dense connective tissue which might not have the same single
direction tensile strength, but it's good at resisting
multi-directional force. That's what makes it perfect for the capsule
around your shoulder. The glenohumeral joint needs a large range of motion,
so having a bunch of spindly dense ligaments around it wouldn't be a great
idea (it would look like a big fishing net) but
you still need to keep the ball of the humeral head
in the shallow glenoid fossa, or socket, so you do need
dense tissue, which is perfect! Irregular for the multi-directionality,
and dense for the strength. This type of connective tissue also makes up eighty
percent of connective tissue in our dermis:
the middle layer of skin. This will be a separate video but
the skin as an organ does so many complicated and intricate tasks.
It's got hair follicles and capillaries and nerves and oil glands all around it
to do stuff like thermoregulation and sensation.
So to give it a little extra structural integrity, we have
irregular dense connective tissue right in the middle of the dermis. But you'll
notice above that is a less dense layer called the papillary layer, and below
that in the hypodermis is adipose tissue (which is still connective tissue) again
this is a perfect demonstration of form matching function. Loose connective
tissue is soft and squishy, so it isn't as strong when it's pulled apart
but it offers more of like a mattress quality. The most dominant kind
is areolar tissue, referencing all the space, the open air of loose tissue. Our
bodies use this tissue in the papillary layer of the skin for
exactly that reason. We can fill all kinds of stuff in that
layer. And that's what makes loose connective tissue so unique, is that this
shift towards more ground substance and less structural fibers.
This allows more dissolved stuff or immune cells like macrophages and mast cells
Just like areolar tissue, reticular tissue is pretty spaced out
and has predominantly reticular fibers to give it support. I know! Anatomists are
terrible at naming things. To make things worse, that layer of dense
irregular tissue in the dermis is called the reticular layer, despite not being
reticular tissue. Finally, that adipose tissue in the
hypodermis is loose connective tissue. I know it
doesn't seem like it's connecting anything to anything,
but when you imagine loose connective tissue more as support than connection
it becomes more clear. And I hesitated to call adipose tissue loose connective
tissue because during my fact check for the script
I found sources that said fat was loose connective tissue while some said it was
firmly a specialized connective tissue in the
same category as blood, lymph, cartilage, and bone. Our final
category in the flow chart. All of these tissue types technically count as
connective tissue, and like cartilage, which you'd think
should be dense connective tissue is actually specialized. But that's a
good question to jump off on though: what's so special about it? Well allow me
to introduce to chondrocytes. Cells that are unique to cartilage.
Like the fibroblasts and other connective tissue,
chondrocytes make extracellular matrix, but they make it differently.
And when we're looking at these specialized connective tissues,
It's those unique cells like chondrocytes that make them special.
Like our bones are mostly collagen that have been hardened with calcium.
The structure of bone really isn't that special.
It's still the same stuff as in other connective tissue,
but osteocytes, or mature bone cells, are unique to bone. Erythrocytes, or red
blood cells, (or red blood corpuscles, if you want to be
that guy) are unique to blood and so on.
But all of those cells started
from the same layer as an embryo and get replenished from the same stem
cells as an adult. So really these seemingly unrelated tissues
have a lot more in common than we first thought,
and they all fall under the connective tissue umbrella.
One other thing that I
find fascinating is how blood, a connective tissue, is made inside of
bones, another connective tissue. I have a video
that you can check out right here if you're interested in the physiology of
bone. Not memorizing a skeleton but like some
good physiology. Otherwise check out these playlists to
keep on learning, and subscribe right here hit the bell so