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  • - [Voiceover] So, let's draw a blood vessel.

  • Here is a blood vessel.

  • In any human, you or me,

  • there's lots of different blood cells

  • travelling around in all the blood vessels of the body.

  • So, you've got your red blood cells,

  • that I'm drawing here.

  • But you've also got your T cells,

  • which are immune cells.

  • You've also got B cells.

  • You've also got something called a macrophage.

  • You've got neutrophils.

  • You've got little platelets,

  • which are actually fragments of cells,

  • they don't have their own nucleoid.

  • And, all in all, you actually have

  • about 10 different kinds of blood cells.

  • And a question you can ask,

  • which is what we're gonna address in this video,

  • is where do all these blood cells come from?

  • Do you know?

  • I'm gonna draw the answer right now

  • and see if you can figure out

  • what exactly I'm drawing.

  • So, this is a bone.

  • Because all these blood cells in the body

  • come from the bone marrow.

  • And here's the bone marrow that I'm drawing here.

  • Now, actually it turns out that they don't come

  • from all the bone marrow of the body.

  • They come from certain places.

  • Some of those places are the head of, for example,

  • the femur, which is the long bone in your thigh.

  • The head of the humerus,

  • which is the long bone in your arm.

  • Those are all long bones.

  • And they also come from something called flat bones.

  • These are very simple names,

  • which I think is always good for us

  • when we're learning a field.

  • Flat bones, such as the one I'm drawing here,

  • which is the sternum.

  • The sternum is the flat bone in your body

  • that connects to all the ribs.

  • Here are some ribs.

  • Of course, it has ribs on both sides.

  • So, the blood cells come from

  • these parts of the long bones

  • and the flat bones of the body.

  • And it turns out,

  • which is interesting and which was not always known,

  • it turns out that all these blood cells in your body

  • have one common precursor,

  • one grandfather, if you will.

  • So, there's one grandfather cell

  • that gives rise to all of these guys.

  • And I'll draw him here. So, here he is. He's purple.

  • His name is complicated.

  • He's called a pluripotent.

  • Pluripotent.

  • If you ever took Latin,

  • you might know that

  • that means sort of able to do anything.

  • Pluripotent hematopoietic stem cell.

  • And the reason that he's called pluripotent

  • is that he is able to give rise

  • to any of the ten blood cells.

  • So, hematopoietic stem cell.

  • You might recall that stem cells

  • are sort of undifferentiated cells that can give rise

  • to many different kinds of cells.

  • So, this grandfather cell gives rise to

  • two different lineages.

  • And those two lineages are

  • the myeloid lineage and the lymphoid lineage.

  • And each of these lineages gives rise

  • to many different cells.

  • The myeloid lineage gives rise to red blood cells,

  • which are biconcave in shape.

  • They are the most common of all blood cells.

  • Now, the myeloid lineage also gives rise to

  • a big cell called a megakaryocyte.

  • Now, you might have never heard of this before,

  • but the megakaryocytes themselves

  • produce platelets, which I think

  • that you've probably have heard of.

  • So, here are platelets.

  • They're little fragments of cells,

  • which actually bud off of the megakaryocytes like this.

  • They kind of squeeze out little pieces of cytoplasm

  • that become platelets.

  • And now I have a challenge for you.

  • Do you think that a macrophage,

  • which is an immune cell

  • that likes to eat up invaders like bacteria,

  • do you think that macrophages come from

  • the myeloid lineage or the lymphoid lineage?

  • So, I was surprised to find out that

  • they actually come from the myeloid lineage.

  • I was surprised because macrophages

  • are immune cells,

  • but they actually come from the same lineage

  • as red blood cells and platelets.

  • So here is a...

  • This is actually not yet a macrophage,

  • this is a monocyte.

  • A lot of crazy words here,

  • but this is a monocyte.

  • Monocytes actually become macrophages

  • once they settle down in the tissues.

  • But, before that, while they're still circulating,

  • they are monocytes.

  • And, in addition to the monocyte,

  • the myeloid lineage gives rise to three guys,

  • one of whom you have heard of probably,

  • two of whom you may not have heard of.

  • I'll just draw them here.

  • So, the one you might have heard of is the,

  • I'm running out of space here,

  • but it's the neutrophil.

  • Neutrophils are the most common

  • immune cell in the blood.

  • The other two are called eosinophils,

  • which are significantly more rare than neutrophils.

  • And, even more rare than eosinophils,

  • are something called the basophils.

  • So, it's the three phils.

  • So, now let's go over to the lymphoid lineage.

  • There's three important cells

  • that come from this one.

  • Two of them you've probably heard of.

  • I'll draw them first.

  • They're both lymphocytes,

  • so it makes sense that

  • they come from the lymphoid lineage.

  • And those are B cells and T cells.

  • And if you recall, B cells are the guys

  • that are going to put out this molecule.

  • Do you know what that is?

  • That's an antibody.

  • B cells make antibodies.

  • And T cells have their own functions

  • that you can learn about

  • in the immune system videos.

  • Now, the lymphoid lineage also gives rise

  • to something called,

  • it actually has a very sort of die morbid name.

  • It's called a natural killer cell.

  • Sometimes, we say NK. Natural killer.

  • So, this is pretty much it.

  • Here we've got our grandfather cell,

  • who gives rise to two lines.

  • You could call these, maybe,

  • the father cells, if you want.

  • And these give rise to our whole array of blood cells.

  • Some of them you've heard of,

  • some of them you haven't.

  • You'll hear more about

  • the ones you haven't heard of in the future.

  • There's one or two more I wanna mention now

  • that are maybe a little more complicated.

  • We have something called a dendritic cell.

  • And the reason I didn't mention it before

  • is because the dendritic cells

  • actually come from both sides,

  • both lineages, which is confusing.

  • They can come from the lymphoid

  • and they can come from the myeloid

  • by way of monocytes.

  • So, monocytes can become dendritic cells.

  • And then we also have another one

  • coming from the myeloid lineage,

  • which is actually fairly important.

  • I could have mentioned it earlier.

  • It's called a mast cell.

  • And mast cells are most notable

  • for causing allergic reactions.

  • They release histamine.

  • You know when you have an allergic reaction