WEBVTT

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Voiceover: All over our
body we have blood vessels

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and in those blood
vessels we've talked about

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how we have lots of different
kind of blood cells.

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

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But if you watched our
immune system videos

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

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and some B-Cells and some macrophages

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and some platelets.

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

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

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

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they're made, which
you might already know.

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And also from what precursor
cells they're made,

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because cells don't
just come out of nowhere

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they come from precursor cells that divide

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and produce new ones.

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So do you know where all
these cells come from?

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Where they're made?

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I'm going to draw the
answer cartoonish as always

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and not to scale with this blood vessel.

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So here's a bone and the answer is that

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all these cells come from the bone marrow,

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which is inside the bone here.

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Voiceover: Now when I
first heard this I thought

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

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"from inside the bone,
get through the surface

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"of the bone and get into blood vessels?"

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Well you might be
surprised to know that all

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these bones are actually
profuse with blood vessels

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

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

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these blood vessels and
go into blood vessels

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of the body.

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But now let's talk about
those precursor cells.

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So it turns out and this is something

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that we didn't know for a long time.

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

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in your bone morrow that
can make all 10 kinds

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

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

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It's called a pluripotent ...

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Pluripotent if you're good
with your latin you know

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

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That might be a poor
translation but anyway.

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Pluripotent hematopoietic
because hematopoiesis

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is the process of producing blood cells.

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So that's pluripotent
hematopoietic stem cell.

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Because Stem Cells as
you may know are cells

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that can divide into multiple
different kinds of cells.

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

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do you think that a macrophage which,

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if you watched the
immunology video as you know,

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

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invaders or debris, do you
think that the macrophage

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is more closely related to a B-Cell which

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

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You might be surprised by the answer.

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The answer is actually
macrophage is more closely

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

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because macrophages and
B-Cells are both immune cells.

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And red blood cells are not.

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This pluripotent hematopoietic
stem cell gives rise

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

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The first is the myeloid lineage ...

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And the other is the lymphoid lineage.

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So when this pluripotent
cell here first divides,

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

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And these are also precursor
cells to the 10 kinds

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

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

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

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and make the cells that
end up in our blood.

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So now it turns out that
this lymphoid progenitor cell

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can make actually three
different kinds of immune cells

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

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It makes a B cell ...

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And it can make a T Cell.

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And this is three out
of our ten blood cells

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that we talked about
and so the other seven

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are going to be from the myeloid lineage.

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

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The red blood cell.

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Another is the megakaryocyte
which you may not

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

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but try to remember.

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The megakaryocyte is actually
what makes platelets.

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So the megakaryocyte
buds off little pieces

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of it's cytoplasm surrounded by membrane

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and these are platelets.

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But the myeloid lineage also
makes a bunch of immune cells

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in addition to the red blood
cells and the platelets.

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And you may not have heard of these,

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so I'll just list them here.

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One of them is the neutrophil, that one

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you're most likely to have heard of

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and there's two other cells

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which are very similar to the neutrophil,

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one is called the basophil ...

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And one is called the eosinophil.

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And don't feel too bad
about not having heard

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about these because they're
actually pretty rare

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in your blood, they are there,

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but there aren't many of them.

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And then there's also a
monocyte which is fairly

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similar to these three
above and the monocyte

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is actually what becomes
a macrophage later on.

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You're probably familiar with macrophage,

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a monocyte is just a
slightly less differentiated

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version of a macrophage.

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And finally, there's also mast cells.

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And mast cells are the ones that ...

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So these are our seven other cells ...

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

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is in fact true.

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The red blood cell is more closely related

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to the monocyte or the
macrophage than is a B cell

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or a T cell or something like that.

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And now I'm adding an
addendum to this video

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

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

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And that kind of cell
is the dendritic cell,

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

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about and I want to ask you, do you think

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the dendritic cell comes
from the myeloid lineage

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or the lymphoid lineage?

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And it's actually a trick question because

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the answer is it comes from both.

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

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which come from the myeloid lineage.

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And some which I'm
drawing right next to it

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which come from the lymphoid lineage.

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And it turns out that
the ones that come from

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the myeloid lineage are
created from monocytes

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whereas the ones that come
from the lymphoid lineage

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are not really descendants
of B or T cells,

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they come from some lymphoid precursor.

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So dendritic cells are
kind of a weird exception

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they can be made from
either line and I wanted to

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add yet another piece
of information hopefully

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not overloading this too
much just to remind you

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once again that monocytes in addition to

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becoming dendritic cells
can also become macrophages.

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And macrophages we are
certainly familiar with,

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

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So here's a macrophage, and macrophages

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if you remember are
kind of like sentinels,

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they sit out in the tissues
and watch for invaders

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and dendritic cells also behave like that.

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So what about this monocyte?

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Does he do the same thing?

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Well actually no, it
turns out the monocyte

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is kind of like a circulating
version of macrophages

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or dendritic cells.

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So what do I mean by circulating?

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I mean that it's actually in the blood.

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So monocytes move around in the blood

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and then when they go into
the tissues to settle down

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and become sentinels,
that's when they are turned

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into either macrophages
or dendritic cells.