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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 you might take an anti-histamine and you do that so that these mast cells can't make you feel crummy.