Check out this amoeba.
Pretty nice. Kind of a rugged, no-frills life
form.
The thing about amoebas is that they do everything
in the same place. They take in and digest
their food, and reject their waste, and get
through everything else they need to do, all
within a single cell.
They don’t need trillions of different cells
working together to keep them alive. They
don’t need a bunch of structures to keep
their stomachs away from their hearts away
from their lungs. They’re content to just
blob around and live the simple life.
But we humans, along with the rest of the
multicellular animal kingdom, are substantially
more complex. We’re all about cell specialization,
and compartmentalizing our bodies.
Every cell in your body has its own specific
job description related to maintaining your
homeostasis, that balance of materials and
energy that keeps you alive.
And those cells are the most basic building
blocks in the hierarchy of increasingly complex
structures that make you what you are.
We covered a lot of cell biology in Crash
Course Bio, so if you haven’t taken
that course with us yet, or if you just want
a refresher, you can go over there now.
I will still be here when you get back.
But with that ground already covered, we’re
going to skip ahead to when groups of similar
cells come together to perform a common function,
in our tissues.
Tissues are like the fabric of your body.
In fact, the term literally means “woven.”
And when two or more tissues combine, they
form our organs. Your kidneys, lungs, and
your liver, and other organs are all made
of different types of tissues.
But what function a certain part of your organ
performs, depends on what kind of tissue it’s
made of. In other words, the type of tissue
defines its function.
And we have four primary tissues, each with
a different job:
our nervous tissue provides us with control
and communication,
muscle tissues give us movement,
epithelial tissues line our body cavities and organs,
and essentially cover and protect the body,
while connective tissues provide support.
If our cells are like words, then our tissues,
or our groups of cells, are like sentences,
the beginning of a language.
And your journey to becoming fluent in this
language of your body -- your ability to read,
understand, and interpret it -- begins today.
Although physicians and artists have been
exploring human anatomy for centuries, histology
-- the study of our tissues -- is a much younger
discipline.
That’s because, in order to get all up in
a body’s tissues, we needed microscopes,
and they weren’t invented until the 1590’s,
when Hans and Zacharias Jansen, a father-son
pair of Dutch spectacle makers, put some lenses
in a tube and changed science forever.
But as ground-breaking as those first microscopes
were then, they were little better than something
you’d get in a cereal box today -- that is to
say, low in magnification and pretty blurry.
So the heyday of microscopes didn’t really
get crackin’ until the late 1600s, when
another Dutchman -- Anton van Leeuwenhoek
-- became the first to make and use truly
high-power microscopes.
While other scopes at the time were lucky
to get 50-times magnification, Van Leeuwenhoek’s
had up to 270-times magnifying power, identifying
things as small as one thousandth of a millimeter.
Using his new scope, Leeuwenhoek was the first
to observe microorganisms, bacteria, spermatozoa,
and muscle fibers, earning himself the illustrious
title of The Father of Microbiology for his troubles.
But even then, his amazing new optics weren’t
quite enough to launch the study of histology
as we know it, because most individual cells in a
tissue weren’t visible in your average scope.
It took another breakthrough -- the invention
of stains and dyes -- to make that possible.
To actually see a specimen under a microscope,
you have to first preserve, or fix it, then
slice it into super-thin, deli-meat-like sections
that let the light through, and then stain
that material to enhance its contrasts.
Because different stains latch on to different
cellular structures, this process lets us
see what’s going on in any given tissue
sample, down to the specific parts of each
individual cell.
Some stains let us clearly see cells’ nuclei
-- and as you learn to identify different
tissues, the location, shape, size, or even
absence of nuclei will be very important.
Now, Leeuwenhoek was technically the first
person to use a dye -- one he made from saffron
-- to study biological structures under the
scope in 1673, because, the dude was a boss.
But it really wasn’t until nearly 200 years
later, in the 1850s, that the we really got the
first true histological stain.
And for that we can thank German anatomist
Joseph von Gerlach.
Back in his day, a few scientists had been
tinkering with staining tissues, especially
with a compound called carmine -- a red dye
derived from the scales of a crushed-up insects.
Gerlach and others had some luck using carmine
to highlight different kinds of cell structures,
but where Gerlach got stuck was in exploring
the tissues of the brain.
For some reason, he couldn’t get the dye
to stain brain cells, and the more stain he
used, the worse the results were.
So one day, he tried making a diluted version
of the stain -- thinning out the carmine with
ammonia and gelatin -- and wetted a sample
of brain tissue with it.
Alas, still nothing.
So he closed up his lab for the night, and,
as the story goes, in his disappointment,
he forgot to remove the slice of someone’s
cerebellum that he had left sitting in the
He returned the next morning to find the long,
slow soak in diluted carmine had stained all
kinds of structures inside the tissue -- including
the nuclei of individual brain cells and what
he described as “fibers” that seemed to
link the cells together.
It would be another 30 years before we knew
what a neuron really looked like, but Gerlach’s
famous neural stain was a breakthrough
in our understanding of nervous tissue.
AND it showed other anatomists how the combination
of the right microscope and the right stain
could open up our understanding of all of our
body’s tissues and how they make life possible.
Today, we recognize the cells Gerlach studied
as a type of nervous tissue, which forms,
you guessed it, the nervous system -- that
is, the brain and spinal cord of the central
nervous system, and the network of nerves
in your peripheral nervous system. Combined,
they regulate and control all of your body’s
functions.
That basic nervous tissue has two big functions
-- sensing stimuli and sending electrical
impulses throughout the body, often in response
to those stimuli.
And this tissue also is made up of two different
cell types -- neurons and glial cells.
Neurons are the specialized building blocks
of the nervous system. Your brain alone contains
billions of them -- they’re what generate
and conduct the electrochemical nerve impulses
that let you think, and dream, and eat nachos,
or do anything.
But they’re also all over your body. If
you’re petting a fuzzy puppy, or you touch
a cold piece of metal, or rough sandpaper,
it’s the neurons in your skin’s nervous
tissue that sense that stimuli, and send the
message to your brain to say, like, “cuddly!”
or “Cold!” or “why am I petting sandpaper?!”
No matter where they are, though, each neuron
has the same anatomy, consisting of the cell
body, the dendrites, and the axon.
The cell body, or soma, is the neuron’s
life support. It’s got all the necessary
goods like a nucleus, mitochondria, and DNA.
The bushy dendrites look like the trees that they’re
named after, and collect signals from other
cells to send back to the soma. They are the
listening end.
The long, rope-like axon is the transmission
cable -- it carries messages to other neurons,
and muscles, and glands. Together all of these
things combine to form nerves of all different
sizes laced throughout your body.
The other type of nervous cells, the glial
cells, are like the neuron’s pit crew, providing
support, insulation, and protection, and tethering
them to blood vessels.
But sensing the world around you isn't much
use if you can't do anything about it, which
is why we've also got muscle tissues.
Unlike your nervous tissues, your muscle tissues
can contract and move, which is super handy
if you want to walk or chew or breathe.
Muscle tissue is well-vascularized, meaning
it’s got a lot of blood coming and going,
and it comes in three flavors: skeletal, cardiac,
and smooth.
Your skeletal muscle tissue is what attaches
to all the bones in your skeleton, supporting
you and keeping your posture in line.
Skeletal muscle tissues pull on bones or skin
as they contract to make your body move.
You can see how skeletal muscle tissue has
long, cylindrical cells. It looks kind of
clean and smooth, with obvious striations
that resemble little pin stripes. Many of
the actions made possible in this tissue -- like
your wide range of facial expressions or pantheon
of dance moves -- are voluntary.
Your cardiac muscle tissue, on the other hand,
works involuntarily. Which is great, because
it forms the walls of your heart, and it would
be really distracting to have to remind it
to contract once every second. This tissue
is only found in your heart, and its regular
contractions are what propel blood through
your circulatory system.
Cardiac muscle tissue is also striped, or
striated, but unlike skeletal muscle tissue,
their cells are generally uninucleate, meaning
that they have just one nucleus. You can also
see that this tissue is made of a series of
sort of messy cell shapes that look they divide
and converge, rather than running parallel
to each other.
But where these cells join end-to-end you
can see darker striations, These are the glue
that hold the muscle cells together when they
contract, and they contain pores so that electrical
and chemical signals can pass from one cell
to the next.
And finally, we’ve got the smooth muscle
tissue, which lines the walls of most of your
blood vessels and hollow organs, like those
in your digestive and urinary tracts, and
your uterus, if you have one.
It’s called smooth because, as you can see,
unlike the other two, it lacks striation.
Its cells are sort of short and tapered at the
ends, and are arranged to form tight-knit sheets.
This tissue is also involuntary, because like
the heart, these organs squeeze substances
through by alternately contracting and relaxing,
without you having to think about it.
Now, one thing that every A&P student has
to be able to do is identify different types
of muscle tissue from a stained specimen.
So Pop Quiz, hot shot!
See if you can match the following tissue
stains with their corresponding muscle tissue
types. Don’t forget to pay attention to
striations and cell-shape!
Let’s begin with this. Which type of tissue
is it?
The cells are striated. Each cell only has
one nucleus. But the giveaway here is probably
the cells’ branching structure; where their
offshoots meet with other nearby cells where
they form those intercalated discs. It's cardiac
muscle.
Or these -- they’re uninucleate cells, too,
and they also are packed together pretty closely
together. But…no striations. They’re smooth,
so this is smooth muscle.
Leaving us with an easy one -- long, and straight
cells with obvious striations AND multiple
nuclei. This could only be skeletal muscle
tissue.
If you got all of them right, congratulations
and give yourself a pat on your superior posterior
medial skeletal muscles -- you’re well on
your to understanding histology.
Today you learned that cells combine to form
our nervous, muscle, epithelial, and connective
tissues. We looked into how the history of
histology started with microscopes and stains,
and how our nervous tissue forms our nervous
system. You also learned how your skeletal,
smooth, and cardiac muscle tissue facilitates
all your movements, both voluntary and involuntary,
and how to identify each in a sample.
Thanks for watching, especially to all of
our Subbable subscribers, who make Crash Course
possible to themselves and also to everyone
else in the world. To find out how you can
become a supporter, just go to subbable dot
com.
This episode was written by Kathleen Yale,
edited by Blake de Pastino, and our consultant
is Dr. Brandon Jackson. Our director and editor
is Nicholas Jenkins, the script supervisor
and sound designer is Michael Aranda, and
the graphics team is Thought Café.
This page was last edited on 26 November 2020, at 00:13