If you were to do a quick internet search of the fattest or fattiest organ in the human body,
you'd come across numerous articles that would say it's the brain. Now,
maybe some of you have heard that the brain is made up of primarily fat or fatty tissue
but how can an organ made up of over a hundred billion specialized cells that we call neurons
making trillions of electrical connections be considered fat or fatty. Is it true? Are they
misleading us? Well we're about to answer that question and to do that we're going
to use a real human brain and compare it to actual fatty tissue. So, let's do this.
So, in order for us to discuss what this amazing structure that we call the brain
is made out of or any structure for that matter, we have to mention
histology. Histology translates to the study of tissues or in other words,
what things are made out of. So if I said to you "What's the histology of the achilles tendon?
What's the histology of the epidermis or what's the histology of my t-shirt",
I'm essentially asking what is it made out of. In the case of my t-shirt, we'd say cotton.
Case of the epidermis or the top layer of the skin, we would say epithelial tissue. And I
don't know about you guys - if we said "what's the histology of the brain?" this doesn't really look
like it's made up of fatty tissue or what we refer to as adipose tissue in the lab. But if we take a
look at this cadaver dissection - and let me just orient you - this is the left gluteus maximus
muscle right here and we've left a skin to section over here. So, essentially left butt cheek here.
And on the skin dissection, the top paper thin layer is the epidermis - a little bit thicker but
not quite as thick as the third layer down. Right here is the dermal layer or the dermis, and from
here to here, this yellowy tissue is referred to as the hypodermis or the subcutaneous layer.
Now, if I asked you what is the hypodermis or the subcutaneous layer made out of or its histology,
we would definitely say fatty tissue or in other words adipose tissue. And one of the cool things
about histology or studying what structures are made out of is that eventually you're gonna
have to go on a little bit of a magic school bus journey or in other words, zoom in and zoom in and
zoom into that tissue at the microscopic level. So, if we were to zoom in to this adipose tissue
further and further and further, we would eventually see cells called
adipose sites or simply adipose cells. And in between those cells, we'd see a few protein
fibers such as collagen but the majority of it would be built up by those adipose cells.
Now, those adipose cells are designed to store lipids or oils or in other words fats. And
the more and more lipids and oils they store, they start accumulating these lipid droplets,
they have to increase in size. So, think about collectively all the adipose cells in a certain
area of the body increasing in size - that would mean overall, therefore that the tissue would have
to get thicker and thicker and thicker and vice versa if those cells started to shrink in size.
Now, it's not a secret to most that sometimes in society, adipose or fatty tissue can get
a negative wrap or some negative stereotypes. However, in the land of biology or in the land of
anatomica, like I like to call it, we understand that adipose tissue was absolutely essential
for survival. And what I mean by that is based on some of its functions.
One being that it is an amazing storage of energy. So, think about back in the day when we were
evolving, running around as hunter-gatherers in our little loincloths covering our nakedness or
whatever we were dressed in... We would maybe go gorge ourselves in as much food as we could
whenever it was available to us and we would eat that and if we had any excess, it would get stored
as fat. And that was great because we didn't have refrigerators. We didn't have the same ability to
maintain and store food and just didn't have it at our fingertips all the time so when we needed
to pull from those stores, we could pull it away. Now, another function that's extremely important
is adipose tissue or fatty tissue is an amazing insulator. So, it help insulate us or maintain
body temperature. Now, humans definitely still take advantage of that but think about animals in
like the northern hemisphere or like in the arctic - polar bears or whales swimming in cold water
and that blubber or that adipose tissue would help them maintain their body temperature. So,
extremely important in those colder climates. Now, you would definitely have to argue,
especially looking at this dissection here that if you have this thick covering that it could
also provide some padding and protection for any of the other structures that were deeper or
underneath that adipose tissue. So, now that we've seen what adipose tissue looks and behaves like,
you're probably getting the strong impression that I'm trying to say that the brain is not made
of adipose or fatty tissue, and you'd be correct. And if we just think about the functions - like is
the brain's function to store energy, provide insulation and padding protection for the
body - and we know the answer to that is "no". It's the command center. It processes enormous
amounts of information, sends electrochemical signals - it's an amazing structure, but doesn't
do any of those functions that we mentioned as far as what we saw with adipose tissue.
So, why are these articles or people saying that up to 60% of the brain is made up of fatty tissue.
This thing weighs - the adult human brain weighs about three pounds, so we're gonna say almost two
pounds is made of fatty tissue? No. So, what's going on here? Why are people referring to this
as an adipose or fatty rich organ? The answer comes with again, the histology of the tissue,
but how it's worded in a lot of these articles and how it's explained to a
lot of students initially can be misleading. So, we're going to figure this out by taking a
deep dive in on the magic school bus and seeing what cells make up nervous tissue or the brain.
So, if we were to do what we did with adipose tissue and zoom in and zoom in to the tissue of
the brain, we would see that the brain is made up primarily of two different cellular types;
one type are the neurons and the other are called neuroglia cells.
Many of us have heard of neurons. There's over 100 billion of these things and a neuron
typically has a basic shape or structure if you were to look at it under the microscope.
One, you'd see a central bulbous cell body that would contain the nucleus and other organelles
that you might remember from a biology class like mitochondria, lysosomes, things of that nature
and then it has multiple projections coming off of the cell body called dendrites.
Those dendrites take signals in from other neurons that are talking to it and they take that signal
in towards the cell body. And then typically they have a really long process called the
axon and that takes the signal away from the cell body and that axon is
going to be important in this discussion of fatty and adipose tissue in the brain.
Now, axons, you guys, can be multiple feet long. For example ,you can have a neuron that
originates in the lumbar or lower spinal cord and goes all the way down to engage a muscle
in the toe. I mean, think of like Shaquille O’Neal - that's a pretty long axon of a neuron.
So, these are pretty tremendous - or we could say amazing cells that send electrochemical signals in
milliseconds. Now obviously, the neurons that just stay in the brain aren't going to be feet long but
you have neurons that leave the central nervous system to go to tissues throughout the body.
But again, we're focusing in on the brain here and remember I mentioned the other cell type,
the neuroglia cells. Glia means glue because they originally thought that these neuroglia cells were
kind of like the scaffolding that kind of helped and support and hold the neurons together. We
have found they do so many other functions besides just helping support the neurons.
And something that's astounding about the neuroglia cells - remember I mentioned 100 billion
neurons... However, there are anywhere from 10 to 50 trillion neuroglia cells. So they dominate
as far as numbers compared to the neurons and remember, I also mentioned they do more than just
provide support and scaffolding for the neurons. Depending on the type of neuroglia cell,
some of them will engulf pathogens or damaged tissues, kind of like white blood cells will,
other neuroglia cells will help contribute to the blood-brain barrier
and another specific type will help form this structure, and this is the type that we're really
concerned about in our discussion here, will help form a structure called the myelin sheath.
So, many of you may have heard of the myelin sheath. This is the part that gets damaged
in a disease called multiple sclerosis and it's also responsible for this idea about the brain
being made up of fatty or adipose tissue, and it also creates this stuff called white matter
and maybe you've heard of that but I can show you on the brain here - white matter you can
actually see with the naked eye because it gives off a glistening color or those myelin
sheaths give off a glistening color. If you see the tip of the probe,
this lighter portion of the area - of the brain would be considered white matter and the gray
outer portions would be called gray matter. Now, simply put in the gray matter, that's where you're
talking about cells like forming - or neurons forming connections like synapses and talking to
each other but that white matter contains those myelinated axons and again, this idea about how
the brain is potentially made up of fatty tissue and there's tons of white matter in the brain.
So let's explain how that works. So, going back to our wonderful cell biology class that maybe we
took in high school or junior high or college, remember, every cell in the human body has an
outer covering called the cell membrane or the plasma membrane. But what was that cell membrane
made out of? It was made out of phospholipids. The key word there being "lipids" and remember
lipids are another name for fats or oils. And so, in the case of the nervous system, there's
all these neurons going through the brain and the spinal cord and all of those axons traveling to
send signals to and from different neurons and communicate through other regions of the brain,
and then there's all those neuroglia cells. And there's a specific type of neuroglia
cell called an oligodendrocyte. Yes, it is very fun to say so you can say yes, oligodendrocyte.
Now, an oligodendrocyte, imagine it positioning itself in between multiple neurons that are
passing by, and then it sends out extensions of its cell membrane, which we know again,
is made of phospholipids and then what it does is imagine it grabbing a hold of multiple axons
of neurons surrounding it and then wrapping it around multiple times with its cell membrane
material again, made of phospholipids, kind of like a toilet paper roll.
So if you were to kind of compare this - hopefully it focuses in right there, the actual
cardboard would be the neuron going through and then the actual sheets of
toilet paper would be multiple layers - multiple layers of phospholipids
and that gives us this idea of multiple multiple lipid layers
in certain areas of the brain, specifically in the white matter, contributing to our nervous system.
Now, what's the whole point of this? Well, we know that lipids are great at insulating things. Now,
in the skin we were insulating from temperature, we're not doing that in the brain or the spinal
cord or in the nervous system, we're insulating something else. Remember again, it was that axon.
We're insulating the electrical signal or the impulse. And we could go into a whole
other video on action potentials and how the electrical signal is transmitted down an axon,
but in general, what we know is when an axon is myelinated, because there's some axons throughout
the body that are not myelinated, but those that are myelinated and insulating that electrical
impulse can send the signal up to 50 times faster. So, wrapping all this up to understand why some
people refer to the brain as being made up of fatty and adipose tissue, again,
it's due to the formation of all of this white matter that was created by those oligodendrocytes
going and wrapping the axons - like again, the toilet paper roll. But it can wrap the axons
up to a hundred layers of phospholipids from the cell membrane. That's pretty crazy to think about.
But is it the same? Is the true adipose tissue that we found in the hypodermis or
the subcutaneous layer, no. But it is unique to the nervous system and you don't find this in
any other tissues of the human body where you have multiple layers of phospholipids.
So that insulation, not for warmth but to speed up those electrical signals so you can contract
your muscles, feel sensations and have this awesome amazing functioning nervous system.
Thanks for watching everyone, if you feel like supporting the channel, you can take
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of the video or any future videos you'd like to see. Also, protect those brains - those myelinated
axons need to fire on all cylinders up to 50 times faster and we'll see in the next video.