- [Voiceover] To help us understand
how cell to cell communication works,
I want you to think back to your
good old elementary school days.
In particular, I want you to think about the things
you'd have to do in order to secretly
talk to your friends during class.
Here you are sitting in your classroom
and you desperately want to tell your friend here something
so you write a little note and you pass it to them.
>From your hand directly to their hand.
They see the note but no one else sees it,
it's just for them.
But let's say you want to tell your friend
across the room something.
I don't know about you but when I was in elementary school
I used to write notes on pieces of paper,
fold it up into a paper airplane,
and I had pretty good aim back then
so I'd throw my paper airplane over to my friend
across some distance
and they'd get my message.
And again, they'd be the only one who got the message,
no one else got my little paper airplane message.
If I wanted to tell a few friends something,
I might call a little huddle and actually say a few things
just to this group of friends here.
My voice would cross this small distance between us.
Now, let's kick things up a notch.
I was, kind of, a rebel in elementary school
and sometimes I'd go to the secretary's desk
and take over the intercom
to say funny things to my friends,
maybe to tell them to meet me at the flagpole
or on the playground at recess or something.
This intercom message that I'd send out,
that would go to everyone,
it would be broadcasted to the whole school.
Those who wanted to come meet me would do that
and those who wouldn't, wouldn't.
We can think of cells as little people
that do really similar things
because you might not always think about it
but it's really, really important
that cells are able to talk to each other.
Evolutionarily, cells being able
to communicate with each other
are a major reason why we're as complex as we are,
as human beings.
I'll give you examples of when cells
might talk to each other as we go along here.
How do cells pass notes to each?
How do they directly communicate with each other?
One way is by, actual, direct contact.
Cells have lots of protein
stuck into their plasma membranes here
that serve a lot of functions.
The most important one is for communication.
Let's look at a macrophage here,
this is a type of white blood cell
that's a part of your immune system.
When these macrophages see a foreign invader,
maybe a little bacteria or a virus,
they can ingest it.
They can ingest it then they break it down
and then they display a little piece of it
which is now called an antigen,
on their surface.
They show it off on their surface with one of these
cell surface proteins here.
This one in particular is called an MHC II protein.
Now, this little antigen here has become
the note that they want to pass on,
this antigen is the message.
Another white blood cell,
maybe a helper T cell might come along
and then grab hold of this antigen here
with one of its cell membrane proteins,
in this case a T cell receptor.
Just by doing this, the macrophage here managed
to pass a message onto the helper T cell here.
Now, based on which antigen this is,
the T cell can decide whether to start
a full-blown immune response.
Maybe it'll go off and ring more alarm bells
by activating other antibody cells which are called B cells
or not, maybe they'll just do nothing.
It just depends on what type of note this is.
When cells directly touch to communicate,
sort of, unsurprisingly this is called
direct cell-cell communication or just direct binding.
Now, what about our other methods of communication?
Cells can also communicate over short distances.
This is our paper airplane here.
For example, let's look at two neurons.
They're in close approximation
but one end of a neuron
doesn't quite touch the start of the next neuron here,
there's a little gap there called the synaptic cleft.
What neurons do is they release
little signals called neurotransmitters
to communicate with each other.
Neurotransmitters get released from the end of this neuron
and they'll diffuse across this little distance here
until they bind onto one of the dendrites
of this next neuron.
That effectively passes the message on
from this neuron to this next neuron.
The paper airplane is thrown from here
to here.
This is called neural communication.
Over here, just like calling a huddle,
one cell can talk to a small group of cells locally as well.
For example, just underneath our skin,
let's say the skin inside our nose,
we have these immune cells called mast cells.
They're really important in mediating allergic reactions,
that's why I purposely picked the nose here.
Let's say you're allergic to certain pollens
and one day in the spring you find yourself
walking through the park.
Inevitably, you'd breathe in some pollen
and then the pollen would go on to attach
to these antibodies stuck to our mast cells.
What happens as a response to this is that
the mast cells release little chemical signals
called histamine.
Histamine acts as our short-range message.
It travels around to cells in the area
then let them know that an allergen has been found
and lets them know to start preparing
for an allergic reaction to take place.
This type of communication is called paracrine signaling,
paracrine, meaning nearby.
Finally, the intercom take over.
How do our cells talk to huge groups of cells at once?
They do that by endocrine signaling.
For example, cells in our Pituitary gland in our brain
make a lot of the important hormones in our bodies.
Let's say that they're making Growth hormone, GH,
to send around to all the cells in the body.
Sending his hormone is their form of communication here.
They'll create the Growth hormone
inside their cell bodies
and they'll release them into the bloodstream.
Now, our Growth hormone can travel through the bloodstream
and get to, literally, any place in the body.
Every cell of your body has the opportunity
to get this Growth hormone message.
It doesn't mean that every cell
will, necessarily, respond to the message.
Some cells just don't have the right proteins
to bind certain hormones,
but either way, you can see that this is a
long distance form of communication.