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Video transcript

- [Voiceover] So in the next couple of videos, we're going to talk about myeloproliferative disorders. Myeloproliferative disorders. This group of diseases involves the bone marrow because that's what myelo means. It's just a term for the bone marrow. And in this group of disorders, you are proliferating, you are proliferating or you are making too much bone marrow. Now to be clear, there are a lot of things that we make in the bone marrow, so I've drawn a bone right here. This is your standard femur and the marrow is usually located in the shaft of long bones like the femur or maybe the humerus or even in your vertebral bodies in the spinal column. But we'll use this as an example right here. The bone marrow makes all of the blood cells in our blood stream, and it all starts from a single precursor that's known as a pluripotent. Pluri meaning almost the same thing as plural, so many potency in making hematopoietic, hematopoietic, which you might recognize as the same sound as the term hematology, which is the study of blood, stem cells. So pluripotent hematopoietic stem cells are cells within the bone marrow that have the ability to make all different types of blood cells that we find in the blood stream. So I'll draw one of them right here, and let's go through and make a quick chart of all the different types of blood cells that we can have. There's one key lineage up there and I'll draw another one down here that's gonna be less important for these disorders we're about to talk about. Both of these are also stem cells, so I'll code them both in yellow. Up here we're talking about the myeloid stem cells. This is the myeloid stem cell which is why it's most relevant for the type of disorders we're about to talk about. Whereas down here we are talking about the lymphoid stem cell. The lymphoid stem cell that you might recall sounds a lot like lymphocytes and that's sure enough what it produces. The lymphoid stem cell will go on to make three main types of cells over time, so I'm sort of abbreviating it here. There are many steps in between. And I'll draw them out in this color right here just to distinguish them 'cause they're not stem cells but they have a different property that I'll mention in a minute. But the lymphocytes or the lymphoid cells that you end up with are your B cells, you get some T cells as well, and in addition you also get these natural killer cells. And that's all I'll say about that lineage for now. Up here for the myeloid stem cells, there are four specific types of pathways that we go through, so I'll draw them out right here. The first type of cell that can branch off a myeloid stem cell is called a proerythroblast. A proerythroblast. And this guy will go through a whole bunch of divisions to eventually make what's referred to as a, or rather, as an erythrocyte. So this is an erythrocyte that you may more commonly refer to as a red blood corpuscle, or a red blood cell. Next down here, what we can make off the myeloid stem cell is something that's called a myeloblast. A myeloblast. And the myeloblast itself can divide into three different types of cells as well. And I'll draw them each right here, but the sizes are not going to be quite representative. I'm just trying to show them here schematically. We'll get some basophils, which you may have heard of before, eosinophils. So that's eosinophils. And finally you can also get some neutrophils. Neutrophils. Another lineage that you can have dividing off of the myeloid stem cell is through the monoblast, so a monoblast. And this guy, which you may be able to guess, will divide into this cell right here, which is known as a monocyte. And for the final lineage, I'm going to draw somewhat of a bigger cell right here because it's supposed to accommodate a bigger nucleus. This is a megakaryoblast. A megakaryoblast. Mega meaning big, karyo meaning nucleus. The megakaryoblast will divide into big megakaryocytes, so I'll write that here, megakaryocyte, and then these then will fragment. It's very unusual. It's not like the other cells we talk about above, but these cells will break apart into pieces. So there are six pieces I drew here. So there should be six pieces that come out on the other side right here. And these aren't perfect but they show that there are six pieces coming off the megakaryocyte, and each of these pieces are known as a platelet. So these are your platelets right here. And that's the gist of how you get all the different blood cells off of this single pluripotent hematopoietic stem cell in the bone marrow. Myeloproliferative disorders occur when there's a defect, so I'll write that up here. There's a defect in a receptor tyrosine kinase, which may be something you've heard of before if you're familiar with enzymes. A defect in a receptor tyrosine kinase that is responsible or involved in cell division. The activation of this receptor tyrosine kinase regulates whether cells are allowed to divide along these various lineages or if not, if they're supposed to pause until it's more appropriate for them to divide. The mutation that occurs allows this receptor tyrosine kinase to just always be on, and there are different points along the way where we can have this receptor tyrosine kinase turned on. Say if we have the mutation occurring along this path right here. That defect is associated with a myeloproliferative disorder known as polycythemia, polycythemia vera, where you have poly, meaning a lot of different cells that are produced, and so you'll get a bunch of erythrocytes, basophils, neutrophils, all of these guys, in the blood stream. Or let's say you have the defect right here, where the myeloblast branches off. In that case, you would have a disorder known as chronic myelogenous, chronic myelogenous leukemia. Chronic myelogenous leukemia. And you might recall leuk meaning white, which refers to leukocytes, which I've colored in all of these little beige colors here. All of these refer to leukocytes. This defect will produce a whole bunch of leukocytes and it'll mainly be these that we see up here through the myeloblast lineage. Or let's say we have a defect in the platelet lineage around here. We can have a disorder that's known as essential, essential thrombocythemia. Thrombocythemia. And thrombocyte, I should've mentioned this earlier when I was writing out platelets, but thrombocyte means the same thing as platelets. Or finally you could even have a defect so early on right here, before you even get to the pluripotent hematopoietic stem cell. And you have what's referred to as primary myelofibrosis. Primary myelofibrosis, which you might recall as meaning fibrosis or connective tissue being put in the bone marrow, which is not where it should be. So that would be a big problem for us. And when this occurs, you'll get an additional defect known as myeloid metaplasia, which means various other organs in your body, like what I've drawn here is your liver or your spleen, will start making these hematopoietic stem cells, causing them to become really big. So your liver might end up being that big and bulge out, or your spleen could also become pretty large as well. And in fact, when you have enlargement of your liver, or what's known as hepatomegaly, or enlargement of your spleen, which is known as splenomegaly, these were things that you can also see with these other disorders that I've mentioned up here, because now they're kicked into overdrive in producing these types of cells. And not just in these organs but in other bones of the body like the skull, for instance. You can start having these hematopoietic stem cells put out these cells at a rate that they're not supposed to be, which can cause the bones to grow and produce an abnormal appearance that's very recognizable. Many of these disorders can also go on to produce what are called acute leukemias. I think we know what leukemias are already, but acute just refers to what stage of development these leukocytes will be put out into the blood stream. And acute leukemias will happen when you have a defect that occurs before the leukocytes are put out. So can you guess which of these four right here would not have an acute leukemia associated with it? I think you reasoned correctly. If you think about polycythemia vera, we have the defect here and so sure enough we can produce abnormal amounts of leukocytes. Chronic myelogenous leukemia, that's before leukocytes are produced. Primary myelofibrosis, the defect is here. That's definitely before leukocytes are produced down here. But with essential thrombocythemia, the defect happens after your leukocytes branch off. So you will not see acute leukemias occur with essential thrombocythemia. And there are small nuances sort of like that that we can talk about to describe the different types of myeloproliferative disorders and how you can differentiate when one occurs versus the other, which we'll talk about in the next couple of videos.