Voiceover: All over our
body we have blood vessels

and in those blood
vessels we've talked about

how we have lots of different
kind of blood cells.

We have some red blood
cells that I'm drawing here.

But if you watched our
immune system videos

you'll also know that we have some T-Cells

and some B-Cells and some macrophages

and some platelets.

And all in all actually there's
pretty much 10 different

kinds of blood cells that we
have in our blood at all times.

In this video I'd like to
talk a little bit about where

they're made, which
you might already know.

And also from what precursor
cells they're made,

because cells don't
just come out of nowhere

they come from precursor cells that divide

and produce new ones.

So do you know where all
these cells come from?

Where they're made?

I'm going to draw the
answer cartoonish as always

and not to scale with this blood vessel.

So here's a bone and the answer is that

all these cells come from the bone marrow,

which is inside the bone here.

Voiceover: Now when I
first heard this I thought

to myself, "How is it
possible that blood cells come

"from inside the bone,
get through the surface

"of the bone and get into blood vessels?"

Well you might be
surprised to know that all

these bones are actually
profuse with blood vessels

themselves, they're very
small and difficult to see.

So it's actually quite
easy for cells to hop into

these blood vessels and
go into blood vessels

of the body.

But now let's talk about
those precursor cells.

So it turns out and this is something

that we didn't know for a long time.

But it turns out that there's
one powerful stem cell

in your bone morrow that
can make all 10 kinds

of blood cells and that one
cell, that very powerful cell,

has a bit of a complicated name
which we'll write out here.

It's called a pluripotent ...

Pluripotent if you're good
with your latin you know

that this means, I guess,
"Able to do a lot of things."

That might be a poor
translation but anyway.

Pluripotent hematopoietic
because hematopoiesis

is the process of producing blood cells.

So that's pluripotent
hematopoietic stem cell.

Because Stem Cells as
you may know are cells

that can divide into multiple
different kinds of cells.

And now I'd like to ask
you a question which is,

do you think that a macrophage which,

if you watched the
immunology video as you know,

is a big phagocytic cell
so it's like to swallow

invaders or debris, do you
think that the macrophage

is more closely related to a B-Cell which

as you know produces
anti-bodies or a red-blood cell?

You might be surprised by the answer.

The answer is actually
macrophage is more closely

related to the red blood
cell, which is kind of weird

because macrophages and
B-Cells are both immune cells.

And red blood cells are not.

This pluripotent hematopoietic
stem cell gives rise

to two main lineages
and I'll draw them here.

The first is the myeloid lineage ...

And the other is the lymphoid lineage.

So when this pluripotent
cell here first divides,

It can give rise to one
of these or one of these.

And these are also precursor
cells to the 10 kinds

of red blood cells that we
actually find in our blood.

So these are not the ones
that are going to end up

in our blood, these are guys
who sit in the bone marrow

and make the cells that
end up in our blood.

So now it turns out that
this lymphoid progenitor cell

can make actually three
different kinds of immune cells

in our body, it makes an NK cell ...

It makes a B cell ...

And it can make a T Cell.

And this is three out
of our ten blood cells

that we talked about
and so the other seven

are going to be from the myeloid lineage.

And one of those is, as
you could've guessed ...

The red blood cell.

Another is the megakaryocyte
which you may not

remember what it does so
I'll tell you in a moment

but try to remember.

The megakaryocyte is actually
what makes platelets.

So the megakaryocyte
buds off little pieces

of it's cytoplasm surrounded by membrane

and these are platelets.

But the myeloid lineage also
makes a bunch of immune cells

in addition to the red blood
cells and the platelets.

And you may not have heard of these,

so I'll just list them here.

One of them is the neutrophil, that one

you're most likely to have heard of

and there's two other cells

which are very similar to the neutrophil,

one is called the basophil ...

And one is called the eosinophil.

And don't feel too bad
about not having heard

about these because they're
actually pretty rare

in your blood, they are there,

but there aren't many of them.

And then there's also a
monocyte which is fairly

similar to these three
above and the monocyte

is actually what becomes
a macrophage later on.

You're probably familiar with macrophage,

a monocyte is just a
slightly less differentiated

version of a macrophage.

And finally, there's also mast cells.

And mast cells are the ones that ...

So these are our seven other cells ...

and as you can see what we
said in the very beginning

is in fact true.

The red blood cell is more closely related

to the monocyte or the
macrophage than is a B cell

or a T cell or something like that.

And now I'm adding an
addendum to this video

to mention one other kind
of cell that we've talked

about in the past videos but
that I didn't put on this chart.

And that kind of cell
is the dendritic cell,

it's one of the antigen
presenting cells that we talked

about and I want to ask you, do you think

the dendritic cell comes
from the myeloid lineage

or the lymphoid lineage?

And it's actually a trick question because

the answer is it comes from both.

So we have some dendritic
cells that I'm drawing here,

which come from the myeloid lineage.

And some which I'm
drawing right next to it

which come from the lymphoid lineage.

And it turns out that
the ones that come from

the myeloid lineage are
created from monocytes

whereas the ones that come
from the lymphoid lineage

are not really descendants
of B or T cells,

they come from some lymphoid precursor.

So dendritic cells are
kind of a weird exception

they can be made from
either line and I wanted to

add yet another piece
of information hopefully

not overloading this too
much just to remind you

once again that monocytes in addition to

becoming dendritic cells
can also become macrophages.

And macrophages we are
certainly familiar with,

At least we've talked about
them in the other videos.

So here's a macrophage, and macrophages

if you remember are
kind of like sentinels,

they sit out in the tissues
and watch for invaders

and dendritic cells also behave like that.

So what about this monocyte?

Does he do the same thing?

Well actually no, it
turns out the monocyte

is kind of like a circulating
version of macrophages

or dendritic cells.

So what do I mean by circulating?

I mean that it's actually in the blood.

So monocytes move around in the blood

and then when they go into
the tissues to settle down

and become sentinels,
that's when they are turned

into either macrophages
or dendritic cells.