00:00:00
Check out this amoeba.
00:00:01
Pretty nice. Kind of a rugged, no-frills life
form.
00:00:04
The thing about amoebas is that they do everything
in the same place. They take in and digest
00:00:08
their food, and reject their waste, and get
through everything else they need to do, all
00:00:11
within a single cell.
00:00:12
They don’t need trillions of different cells
working together to keep them alive. They
00:00:16
don’t need a bunch of structures to keep
their stomachs away from their hearts away
00:00:19
from their lungs. They’re content to just
blob around and live the simple life.
00:00:23
But we humans, along with the rest of the
multicellular animal kingdom, are substantially
00:00:27
more complex. We’re all about cell specialization,
and compartmentalizing our bodies.
00:00:32
Every cell in your body has its own specific
job description related to maintaining your
00:00:36
homeostasis, that balance of materials and
energy that keeps you alive.
00:00:40
And those cells are the most basic building
blocks in the hierarchy of increasingly complex
00:00:44
structures that make you what you are.
00:00:47
We covered a lot of cell biology in Crash
Course Bio, so if you haven’t taken
00:00:50
that course with us yet, or if you just want
a refresher, you can go over there now.
00:00:54
I will still be here when you get back.
00:00:56
But with that ground already covered, we’re
going to skip ahead to when groups of similar
00:01:00
cells come together to perform a common function,
in our tissues.
00:01:03
Tissues are like the fabric of your body.
In fact, the term literally means “woven.”
00:01:09
And when two or more tissues combine, they
form our organs. Your kidneys, lungs, and
00:01:12
your liver, and other organs are all made
of different types of tissues.
00:01:15
But what function a certain part of your organ
performs, depends on what kind of tissue it’s
00:01:19
made of. In other words, the type of tissue
defines its function.
00:01:23
And we have four primary tissues, each with
a different job:
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our nervous tissue provides us with control
and communication,
00:01:31
muscle tissues give us movement,
00:01:32
epithelial tissues line our body cavities and organs,
and essentially cover and protect the body,
00:01:37
while connective tissues provide support.
00:01:39
If our cells are like words, then our tissues,
or our groups of cells, are like sentences,
00:01:45
the beginning of a language.
00:01:47
And your journey to becoming fluent in this
language of your body -- your ability to read,
00:01:51
understand, and interpret it -- begins today.
00:02:04
Although physicians and artists have been
exploring human anatomy for centuries, histology
00:02:08
-- the study of our tissues -- is a much younger
discipline.
00:02:11
That’s because, in order to get all up in
a body’s tissues, we needed microscopes,
00:02:15
and they weren’t invented until the 1590’s,
when Hans and Zacharias Jansen, a father-son
00:02:20
pair of Dutch spectacle makers, put some lenses
in a tube and changed science forever.
00:02:24
But as ground-breaking as those first microscopes
were then, they were little better than something
00:02:28
you’d get in a cereal box today -- that is to
say, low in magnification and pretty blurry.
00:02:33
So the heyday of microscopes didn’t really
get crackin’ until the late 1600s, when
00:02:36
another Dutchman -- Anton van Leeuwenhoek
-- became the first to make and use truly
00:02:41
high-power microscopes.
00:02:42
While other scopes at the time were lucky
to get 50-times magnification, Van Leeuwenhoek’s
00:02:46
had up to 270-times magnifying power, identifying
things as small as one thousandth of a millimeter.
00:02:52
Using his new scope, Leeuwenhoek was the first
to observe microorganisms, bacteria, spermatozoa,
00:02:57
and muscle fibers, earning himself the illustrious
title of The Father of Microbiology for his troubles.
00:03:02
But even then, his amazing new optics weren’t
quite enough to launch the study of histology
00:03:06
as we know it, because most individual cells in a
tissue weren’t visible in your average scope.
00:03:11
It took another breakthrough -- the invention
of stains and dyes -- to make that possible.
00:03:15
To actually see a specimen under a microscope,
you have to first preserve, or fix it, then
00:03:20
slice it into super-thin, deli-meat-like sections
that let the light through, and then stain
00:03:25
that material to enhance its contrasts.
00:03:27
Because different stains latch on to different
cellular structures, this process lets us
00:03:32
see what’s going on in any given tissue
sample, down to the specific parts of each
00:03:36
individual cell.
00:03:37
Some stains let us clearly see cells’ nuclei
-- and as you learn to identify different
00:03:41
tissues, the location, shape, size, or even
absence of nuclei will be very important.
00:03:46
Now, Leeuwenhoek was technically the first
person to use a dye -- one he made from saffron
00:03:50
-- to study biological structures under the
scope in 1673, because, the dude was a boss.
00:03:55
But it really wasn’t until nearly 200 years
later, in the 1850s, that the we really got the
00:03:59
first true histological stain.
And for that we can thank German anatomist
00:04:03
Joseph von Gerlach.
00:04:04
Back in his day, a few scientists had been
tinkering with staining tissues, especially
00:04:08
with a compound called carmine -- a red dye
derived from the scales of a crushed-up insects.
00:04:13
Gerlach and others had some luck using carmine
to highlight different kinds of cell structures,
00:04:17
but where Gerlach got stuck was in exploring
the tissues of the brain.
00:04:21
For some reason, he couldn’t get the dye
to stain brain cells, and the more stain he
00:04:25
used, the worse the results were.
00:04:27
So one day, he tried making a diluted version
of the stain -- thinning out the carmine with
00:04:31
ammonia and gelatin -- and wetted a sample
of brain tissue with it.
00:04:35
Alas, still nothing.
00:04:36
So he closed up his lab for the night, and,
as the story goes, in his disappointment,
00:04:40
he forgot to remove the slice of someone’s
cerebellum that he had left sitting in the
00:04:45
He returned the next morning to find the long,
slow soak in diluted carmine had stained all
00:04:50
kinds of structures inside the tissue -- including
the nuclei of individual brain cells and what
00:04:55
he described as “fibers” that seemed to
link the cells together.
00:04:59
It would be another 30 years before we knew
what a neuron really looked like, but Gerlach’s
00:05:03
famous neural stain was a breakthrough
in our understanding of nervous tissue.
00:05:07
AND it showed other anatomists how the combination
of the right microscope and the right stain
00:05:12
could open up our understanding of all of our
body’s tissues and how they make life possible.
00:05:17
Today, we recognize the cells Gerlach studied
as a type of nervous tissue, which forms,
00:05:22
you guessed it, the nervous system -- that
is, the brain and spinal cord of the central
00:05:25
nervous system, and the network of nerves
in your peripheral nervous system. Combined,
00:05:30
they regulate and control all of your body’s
functions.
00:05:33
That basic nervous tissue has two big functions
-- sensing stimuli and sending electrical
00:05:38
impulses throughout the body, often in response
to those stimuli.
00:05:41
And this tissue also is made up of two different
cell types -- neurons and glial cells.
00:05:46
Neurons are the specialized building blocks
of the nervous system. Your brain alone contains
00:05:51
billions of them -- they’re what generate
and conduct the electrochemical nerve impulses
00:05:56
that let you think, and dream, and eat nachos,
or do anything.
00:06:00
But they’re also all over your body. If
you’re petting a fuzzy puppy, or you touch
00:06:04
a cold piece of metal, or rough sandpaper,
it’s the neurons in your skin’s nervous
00:06:07
tissue that sense that stimuli, and send the
message to your brain to say, like, “cuddly!”
00:06:12
or “Cold!” or “why am I petting sandpaper?!”
00:06:15
No matter where they are, though, each neuron
has the same anatomy, consisting of the cell
00:06:20
body, the dendrites, and the axon.
00:06:22
The cell body, or soma, is the neuron’s
life support. It’s got all the necessary
00:06:26
goods like a nucleus, mitochondria, and DNA.
00:06:29
The bushy dendrites look like the trees that they’re
named after, and collect signals from other
00:06:33
cells to send back to the soma. They are the
listening end.
00:06:36
The long, rope-like axon is the transmission
cable -- it carries messages to other neurons,
00:06:41
and muscles, and glands. Together all of these
things combine to form nerves of all different
00:06:45
sizes laced throughout your body.
00:06:47
The other type of nervous cells, the glial
cells, are like the neuron’s pit crew, providing
00:06:52
support, insulation, and protection, and tethering
them to blood vessels.
00:06:56
But sensing the world around you isn't much
use if you can't do anything about it, which
00:06:59
is why we've also got muscle tissues.
00:07:02
Unlike your nervous tissues, your muscle tissues
can contract and move, which is super handy
00:07:06
if you want to walk or chew or breathe.
00:07:10
Muscle tissue is well-vascularized, meaning
it’s got a lot of blood coming and going,
00:07:15
and it comes in three flavors: skeletal, cardiac,
and smooth.
00:07:18
Your skeletal muscle tissue is what attaches
to all the bones in your skeleton, supporting
00:07:22
you and keeping your posture in line.
00:07:24
Skeletal muscle tissues pull on bones or skin
as they contract to make your body move.
00:07:28
You can see how skeletal muscle tissue has
long, cylindrical cells. It looks kind of
00:07:33
clean and smooth, with obvious striations
that resemble little pin stripes. Many of
00:07:38
the actions made possible in this tissue -- like
your wide range of facial expressions or pantheon
00:07:43
of dance moves -- are voluntary.
00:07:44
Your cardiac muscle tissue, on the other hand,
works involuntarily. Which is great, because
00:07:49
it forms the walls of your heart, and it would
be really distracting to have to remind it
00:07:52
to contract once every second. This tissue
is only found in your heart, and its regular
00:07:58
contractions are what propel blood through
your circulatory system.
00:08:02
Cardiac muscle tissue is also striped, or
striated, but unlike skeletal muscle tissue,
00:08:06
their cells are generally uninucleate, meaning
that they have just one nucleus. You can also
00:08:11
see that this tissue is made of a series of
sort of messy cell shapes that look they divide
00:08:16
and converge, rather than running parallel
to each other.
00:08:19
But where these cells join end-to-end you
can see darker striations, These are the glue
00:08:23
that hold the muscle cells together when they
contract, and they contain pores so that electrical
00:08:27
and chemical signals can pass from one cell
to the next.
00:08:30
And finally, we’ve got the smooth muscle
tissue, which lines the walls of most of your
00:08:33
blood vessels and hollow organs, like those
in your digestive and urinary tracts, and
00:08:38
your uterus, if you have one.
00:08:39
It’s called smooth because, as you can see,
unlike the other two, it lacks striation.
00:08:43
Its cells are sort of short and tapered at the
ends, and are arranged to form tight-knit sheets.
00:08:48
This tissue is also involuntary, because like
the heart, these organs squeeze substances
00:08:52
through by alternately contracting and relaxing,
without you having to think about it.
00:08:56
Now, one thing that every A&P student has
to be able to do is identify different types
00:09:01
of muscle tissue from a stained specimen.
00:09:02
So Pop Quiz, hot shot!
00:09:04
See if you can match the following tissue
stains with their corresponding muscle tissue
00:09:07
types. Don’t forget to pay attention to
striations and cell-shape!
00:09:11
Let’s begin with this. Which type of tissue
is it?
00:09:13
The cells are striated. Each cell only has
one nucleus. But the giveaway here is probably
00:09:17
the cells’ branching structure; where their
offshoots meet with other nearby cells where
00:09:21
they form those intercalated discs. It's cardiac
muscle.
00:09:25
Or these -- they’re uninucleate cells, too,
and they also are packed together pretty closely
00:09:30
together. But…no striations. They’re smooth,
so this is smooth muscle.
00:09:34
Leaving us with an easy one -- long, and straight
cells with obvious striations AND multiple
00:09:38
nuclei. This could only be skeletal muscle
tissue.
00:09:40
If you got all of them right, congratulations
and give yourself a pat on your superior posterior
00:09:44
medial skeletal muscles -- you’re well on
your to understanding histology.
00:09:48
Today you learned that cells combine to form
our nervous, muscle, epithelial, and connective
00:09:53
tissues. We looked into how the history of
histology started with microscopes and stains,
00:09:58
and how our nervous tissue forms our nervous
system. You also learned how your skeletal,
00:10:03
smooth, and cardiac muscle tissue facilitates
all your movements, both voluntary and involuntary,
00:10:08
and how to identify each in a sample.
00:10:12
Thanks for watching, especially to all of
our Subbable subscribers, who make Crash Course
00:10:16
possible to themselves and also to everyone
else in the world. To find out how you can
00:10:20
become a supporter, just go to subbable dot
com.
00:10:22
This episode was written by Kathleen Yale,
edited by Blake de Pastino, and our consultant
00:10:26
is Dr. Brandon Jackson. Our director and editor
is Nicholas Jenkins, the script supervisor
00:10:30
and sound designer is Michael Aranda, and
the graphics team is Thought Café.
