Megakaryoblast

Transcription

Voiceover: I'm gonna start by drawing a blood vessel. Inside the blood vessel I'm gonna draw in some of the cells that you'd expect to see in normal blood. I'm gonna put in a few red blood cells. I'm gonna draw in a couple of platelets which are just fragments of cells and then I'm gonna put in a few white blood cells. Even though all of these cells are found in the blood they're actually not made there. The cells in the blood are made inside the bone. If you were to take a cross section of bone, if you were to take a slice like this and you looked at it, you'd see that on the inside of bone there's a central cavity. This is the outside part of the bone. The hard part and this is the central cavity. Inside that cavity there's this red spongy tissue. That stuff is called bone marrow. Bone marrow. Bone marrow is where all of these blood cells are made. It's the site of hematopoiesis. Hemato means blood and poiesis means to form or to make. Hematopoiesis is just a scary Greek word that means to make blood. Let's take a look at what goes on inside the bone marrow. What's interesting is that as different as all of the blood cells are they all actually originate from the same cell and that cell is called a hematopoietic stem cell. This stem cells gives rise to all of the different cells that you see in the blood. It gives rise first to two different cell lineages or two different cell groups. First there is the myeloid group. Myeloid group. which is different from the lymphoid group. The lymphoid group. All of the cells that you see in the blood belong to either the myeloid group or the lymphoid group. The lymphoid group includes two different types of blood cells. First there is the T cell or actually that's too big because this is a very little cell. That seems about right. There's a T cell and the majority of the cell is taken up by the cell's nucleus, and that's what I'm shading in over here. So much so that this cell has a nickname. It's often referred to as a naked nucleus because it looks like the nucleus isn't surrounded by very much cytoplasm. This is a T cell or a T-lymphocyte. T-lymphocyte. It's very similar in appearance to the other type of lymphoid cell. This cell also has a nucleus that takes up the majority of the cell and this cell is a B cell or a B-lymphocyte. B-lymphocyte, lymphocyte. Those are the two different types of lymphoid cells. What about the different types of myeloid cells. For starters we have a red blood cell. I guess we should draw that in red. This is also very small cell. I'm gonna shade in this cell so that I can show you that the center of this cell is much lighter than the edges or the periphery of the cell. This is not the cell's nucleus, guys because we know that red blood cells don't have nuclei. What I'm trying to show is that the center of the cell is much lighter than the periphery of the cell. I guess I could do a better job of that if I showed you what this cell looks like on its side. This is what a red blood cell looks like when it's laid on its side. It kind of looks like a dumbbell where the edges are much thicker or much wider than the center. The edges would since they're thicker they would be much more densely packed with hemoglobin. Since we know that hemoglobin is what gives red blood cells their red color, the edges would then be darker than the center. Since the center is much thinner it would have a lot less hemoglobin. It would be a lot paler in comparison. We said that this is a red blood cell but since in science we never use a plain and ordinary name of things we call this an erythrocyte. Erythrocyte which is just a fancy name for a plain old red blood cell. What are some of the other types of myeloid cells? We have this one cell that I'm drawing in over here. You might say well, that looks nothing like a cell and you'd be totally right. This is a very odd-looking, very large cell and it's called a megakaryocyte. Even if you never heard of a megakaryocyte before you may have heard of what it gives rise to because a megakaryocyte gives off little blebs of its cytoplasm to form this small cell fragments and these fragments are known as platelets. You may have heard of the platelets before. The rest of the myeloid cells are actually different types of white blood cells. For example you have this one type of white blood cell and the rest of these white blood cells are about twice the size of a red blood cell so that looks like it's twice the size of a red blood cell to me. This cell is called a monocyte. It's known for having this nucleus that's in the shape of a kidney bean. That kind of looks like a kidney bean to me. This is a really cool white blood cell because it protects us from bacteria and viruses and other invading organisms just like any other white blood cell does but it does so in a very interesting way. Let's say that this was a bacterium or actually, no I like the color pink so let's use a color I don't like. Let's say that this was a bacterium. The monocyte would defend us against it. It would attack this bacterium by engulfing itself around the bacterium. When it does that it kind of looks like it's eating the bacteria. It looks like its eating the bacterium. That's a very interesting way of dealing with these invading organisms. Let's clear out that. Mono actually refers to the fact that this cell has a nucleus that's in one piece and I guess a lot of these cells have nuclei that are in one piece. It makes us wonder what types of cells have nuclei that are not in one piece and that would bring us to a neutrophil. This is a neutrophil. A neutrophil. It has a nucleus like I'm drawing in that is broken up into several different pieces or segment. The segments are still held together by little pieces of string. This is called a multi segmented nucleus. Even if you've never heard of a neutrophil before I'm sure you've seen them before and how do I know that? It's because neutrophils are actually the main component of pus. If you like anybody else has ever popped a pimple you've looked at a bunch of neutrophils. What are the other types of myeloid cells? Well, we have this one cell that's called a basophil. Basophil. It looks something like this. You might say, well, that looks an awful lot like a monocyte and you'd be completely correct except this cell has a very unique feature that the monocyte doesn't have, and that is that it has these bright blue granules in its cell's cytoplasm. That make it stick out. The way I like to think of a basophil is like a basophil is like a blueberry muffin and that reminds that it has these bright blue granules in its cell's cytoplasm. For the last type of myeloid cell we talked about. Whoops. For the last type of myeloid cell that we talked about is an eosinophil. It's an eosin-O-P-H, P-H-I-L, eosinophil, okay? It has a nucleus that can sometimes be found in two pieces. This is the first piece and this is the second piece but that's not what makes it so unique. What it's really well known for is the fact that it has these bright, beautiful red granules in its cell cytoplasm. Those are all of the different blood cells with all the lymphoid cells on this side and all the myeloid cells on this side. It turns out that all of these cells don't directly develop from hematopoietic stem cell. Instead they undergo multiple stages of development to mature into their adult form, but don't worry guys we're not gonna hash out each of those different forms and their names and physical characteristics. Instead we're gonna make a very general statement. I guess we could put that up here. We're gonna say that the immature forms. the immature forms of all these cells are called blasts. When the blast mature, they mature they are called cytes. Let me show you what I mean. To spare you guys the agony of having to watch me draw out all of those different forms I just pasted them in here. We can see here that what we said about the immature forms being called blasts and the more mature form being called cytes is true. The T-lymphocyte develops from a T lymphoblast just like an erythrocyte develops from an erythroblast and a megakaryocyte develops from a megakaryoblast. You guys get the pattern. The only wrench I'm gonna throw into this pattern is with the neutrophil, basophil and eosinophil which all actually develop from the same cell and that cell is called a myeloblast. I think that that's a reasonable place to end our discussion on hematopoiesis.