Studying anatomy at the whole body level is straightforward enough but it gets harder
at the microscopic level.
This jumble of cells looks just like this jumble of cells even though they’re totally
different organs.
This study falls within the field of histology, anatomy at the microscopic level.
While it seems intimidating now, if you pursue this field, you’ll find a beautiful, colorfully
stained perspective of human anatomy that looks nothing like the macroscopic version.
You can also gain clinical insight into pathologies and disease that you can more accurately understand
through a microscope.
But before we can appreciate the more ornate histology, we have to get down some basics,
specifically, four types of tissue that make up every organ in our bodies.
[intro]
If you’re new here, welcome, my name is Patrick and right off the bat, I want to tell
you, the goal of this video is to give you somewhere to start.
I’ll have follow up videos so you can get more in depth with the physiology of each
tissue type, but the goal of this video is to get you on your feet and unconfuse those
of you who are feeling lost right now.
Okay, so at the big picture level, the body has all these organs that do different jobs,
and each of your organs has different types of specialized tissue that do jobs within
them.
Each of these tissues are made of specialized cells which work together to get their collective
jobs done.
The four types are:
Muscle tissue in your skeletal muscles, heart, and around blood vessels, nervous tissue in
your brain, spinal cord, and nerves, epithelial tissue in your skin and organs, and connective
tissue pretty much everywhere else which includes stuff like cartilage and tendons but also
your bones and fat cells.
We’ll start with muscle since we only have three types: skeletal, cardiac, and smooth.
All of which have different needs, so they’ll all look and function differently.
Skeletal muscle is the long, spindly stuff that you picture when you hear muscle.
You can usually tell skeletal muscle from its stripey, parallel fibers, or striations,
just like when you’re looking at a piece of steak or chicken breast — that's skeletal
muscle.
It also has multiple nuclei per muscle fiber, these dark dots, which makes it easy to find.
If we took a cross section of it, we’d see distinct bundles of muscle fibers.
Skeletal muscle has quite a bit of variation from sample to sample since muscles have different
shapes and arrangements, but there’s only one of the next muscle.
Cardiac muscle is the other type of striated muscle but is found exclusively in the heart.
This muscle type is built to contract and send signals super quickly, so their structure
reflects that.
Cardiac muscle is more rectangular shaped and has a single nucleus, but it’s dead
giveaway are these things, the intercalated discs.
They’re little passages that open between cardiac cells and let messages pass super
quickly from cell to cell.
The final muscle type is smooth muscle, a muscle type that surrounds organs that need
to expand and constrict like blood vessels, the uterus, and bladder.
They’re the smallest type of muscle and organized into jumbled up sheets, which lets
them contract simultaneously.
That makes them great squeezers, but they’d be terrible biceps.
So when you’re trying to figure out the difference between skeletal, cardiac, and
smooth muscle, look at the shape and layout first.
With a little bit of flashcard time, the patterns become imprinted pretty easily since there’s
only three types of muscle.
But if you’re still not sure, count the nuclei.
Only skeletal muscle will have multiple nuclei, so that leaves cardiac and smooth.
So then, if you find intercalated discs, you know it’s cardiac muscle.
Next up is epithelial tissue, a tissue type that makes up most of our skin and the borders
between different organs.
Now, there are a lot of different types of epithelial tissue that might seem overwhelming,
but there are two things you need to pay attention to.
The shape of the cells and how many layers you see.
Shapes first:
Epithelial cells can be cuboidal, or cube shaped with a round little nucleus, squamous,
or a flat, squashed cell with a flat looking nucleus, or columnar, or column shaped.
They won’t always be super clean cut shapes, but for the most part, they’ll be one of
these three shapes and arranged in a few ways.
When I say arranged, I’m referring to their arrangement on the basement, or basal membrane,
a layer that sticks the epithelial tissue to the tissues around them.
Layers of Simple epithelial cells sit right next to each other in a nice, orderly line.
So for example, you can have simple columnar epithelial cells which means a single layer
of column-shaped cells.
You could also arrange them as stratified epithelial cells, which are cells stacked
on top of each other.
Stratified squamous cells would look like this — layers of squished cells like a big
stack of pancakes.
There are also pseudostratified cells, which is an arrangement unique to columnar cells.
They’re pseudo-stratified since their base is all in one layer, but their tops are uneven.
So at first glance, it looks like they’re in different layers but they’re not.
Since I’m a teacher, I imagine pseudostratified columnar tissue like a bunch of little kids
lining up after recess.
Some are tall, some are short, and when they’re standing in line it seems like there’s a
layer of tall kids and another hidden layer of short kids, but their feet are all on the
same ground, so they’re actually in one layer.
Right, but back to histology.
When we name epithelial cells, we say the layer style first then the shape.
Simple squamous for example starts with the layer style, simple, then the shape, squamous.
Or stratified cuboidal, multiple layers of cube shaped cells.
Finally, and this one kind of doesn’t fit nicely into the naming conventions, there’s
transitional epithelium, which is a jumble of epithelial cells around organs like the
bladder or urethra that lets them stretch.
So when you have a slide of epithelial cells in front of you, remember to identify shape
and layers.
That epithelial tissue is often spattered with nervous tissue, which makes up some of
our nervous system organs like the brain, spinal cord, and peripheral nerves.
This is the kind of tissue that has the job of sensing stimuli from the outside environment
and then transmitting messages from your brain elsewhere.
So your senses, your brain, your reflexes, all of that uses this tissue type.
In general, we can split nervous tissue into two cell types: neurons and glial cells.
Neurons are without a doubt the superstars of the nervous system.
Partly because they have this beautiful branching shape, but also because these are the cells
that actually transmit nervous impulses around the body, whether in the brain or along nerves.
But we actually have more of the other type of nervous tissue — glia.
The name given to any of the supporting cells in the nervous system which includes cells
like astrocytes, which as the name implies are star shaped, satellite cells which as
the name implies look like they orbit the cell body, or Schwann cells, which don’t
offer us any fun mnemonics, boo, but do look like prosciutto-wrapped cheese sticks, yay.
If you’re a student trying to learn this stuff for the first time, you’ll probably
spend more time identifying the parts of the neuron cell as opposed to identifying different
glial cells.
And if you’re a layperson, you can use this to start looking at science journalism with
a little more skepticism.
Does that news story about Alzheimers show you an actual picture of neurons or did they
take some artistic liberties with the stock photos?
Pictures: brain, spinal cord, peripheral nerves, neurons x 4, astrocytes, satellite cells,
schwann cells x 2
Finally we have connective tissue, which is everything else.
Tendons and ligaments are some of the clear cut ones, but things like fat, bone, and blood
count as connective tissue too.
Because there are so many types of connective tissue, each type is going to look a little
different.
Again, I’ll go more in depth in a future video, but for now, we can divide them into
three categories: loose, dense, or specialized connective tissue, and the names are self
explanatory once we see the components they’re built from.
This includes some kind of extracellular matrix, or ECM.
This is the non-living material — non-specialized fibers like collagen and elastin fibers that
give the connective tissue some tensile strength, as well as ground substance, which is this
liquid, intermediary substance that particles can be dissolved in.
So loose connective tissue will be pretty heavy on the ground substance and a little
lighter on the fibers, while the dense connective tissue looks more like thick bundles of fibers.
Regardless of its density, it also has living cells.
The most common of which are fibroblasts, cells that can support the extracellular matrix,
but you’ll also have immune cells scattered around the ground substance too.
And if you include some of the specialized connective tissue, you get some more exciting
cell types.
For example, cells called chondrocytes maintain cartilage.
Bone has specialized cells called osteoblasts that help build bone and osteoclasts that
resorb bone in a constant remodeling process.
And fat, which counts as connective tissue, has living cells in it called adipocytes that
store energy among other things.
There are so many diverse types of tissues out there, and like I said at the beginning
this is supposed to be an introduction.
And this makes me happy, of you noticed some of those illustrations throughout the video,
those came from my friends over at KenHub, so I want to give them a little shoutout.
They’re an anatomy learning platform that has all kinds of anatomy and histology quizzes,
over 700 fun videos, detailed articles and an atlas of human anatomy.
They’re not paying me to plug them, I just like their service.
I’ve got a link to them down in the description if you want to try them out.
Otherwise, you can find the more detailed, follow up videos for each tissue type in this
playlist here, and another playlist for basics of anatomy right here.
And since it’s probably the beginning of your semester, you should subscribe to my
channel and hit the bell icon so you get notifications when I upload.
Your grades will thank you.
Have fun, be good.
Thanks for watching.