Transcription
Appeared, and so that's what we say here. It's the tissue remains firm. Um, there's coagulation of cellular proteins; however, the nucleus disappears. The characteristic, uh, circumstance in which coagulative coagulative necrosis occurs is a schemi infarction. Every organ in the body that undergoes schic infarction will actually exhibit coagulative necrosis. The only exception is the brain. So the, again, coagulative necrosis is seen with this esmia of any organ except the brain. Here's an image from your text, Figure 1.5, that just highlights what we mean by coagulative necrosis.
Now let's start with this image here on the right. This is the gulus, what I'm circling, and these are the tubules. And you can clearly see that the Glarus contains nuclei within the cells and that the tubules also contain nuclei within the cells, and that's normal. I'm just—I just put this normal picture here to establish a baseline in your mind. Now let's look here on the left at the necrosis. Here's the glomerulus; you still recognize that it looks sort of like a glomerulus. However, there are no nuclei within the cells. Here are the tubules; still recognize that these are tubules; however, there are no nuclei, no blue nuclei within these cells. And so that, then, this histologic picture is characteristic of coagulative necrosis.
Um, a couple important derivative points regarding coagulative necrosis: usually the area of infarcted tissue is wedge-shaped and pale. All right, usually the area of infarcted tissue is wedge-shaped and pale. Um, this is not so hard to remember. Basically, the—it's—it's this is based on a principle, an anatomic principle, and that is that generally speaking, when vessels enter into an organ and feed an organ, they dichotomously branch as they go through the organ. So let's say that there is a vessel coming this way, and then there's a branch point here, and then another branch point here as the artery branches to feed the organ. If you cut the blood supply here, everything from this vessel and everything from this vessel is going to die, and it's going to create sort of—everything in the distribution of this area will become what you know like a wedge-shaped. Notice that this—this looks like a wedge, a wedge-shaped area of—of um, necrosis. And one of the classic features to be aware of is that the wedge points to the—the area of occlusion, uh, and that's because it's pointing to the area where—where the—where the dichotomus branching was occurring. So I don't think it's hard to remember that these areas are usually wedge-shaped. Why are they pale? Well, same principle. I mean, this is the blood supply coming in, and this is the blood supply branching, and you cut the blood supply. Once you cut the blood supply, this tissue is going to die, and once it dies, blood will obviously not be able to come back into it. So the dead tissue will look pale. No no rocket science, pretty straightforward. Here's a—here's a picture from your text, Figure—um, 1.5, and—and it's basically highlighting that this is the cortex of the kidney, and—um, this area here represents the area of infarction. There was probably an occlusion here, and this tissue within the area of the triangle has died. Now the part—part you can partly recognize by the fact that there's indentation of the tissue here, but also it's a little subtle, but there's disruption of the normal tissue in this region. Um, and so this is a classic example. It's relatively pale, and it's kind of wedge-shaped. You know, gives you an idea of—of what we're referring to.
Uh, another important principle or derivative principle is that we could get red infarction within a tissue, and red infarction requires two things for—in order for it to be seen. First, blood must re-enter the tissue, and second, the tissue must be loosely organized so that it can hold the blood, making—giving it a red color. Um, so two—so these two things are required: blood must re-enter the tissue, and the tissue must be loosely organized. A classic example is shown here in your text, and this is Figure 1.6. This is a testicle, and this testicle has undergone hemorrhagic infarction. Um, basically the idea here is that this testicle is completely red now. Um, in order to understand this, we probably should take a step back, uh, and remind ourselves that—um, coming in this area here—maybe I should create a blank screen to make it more easy to see—when we've got—when we think about the testicle—coming into the testicle, you got this cord which basically the testicle hangs on, um, and then this is the testicle here. Now in this region there is a thick-walled artery and there is a thin-walled vein. The classic mechanism by which testicular infarction occurs is that the testicle twists on this cord, and when it twists on the cord, the artery continues to allow blood to flow in because it's thick-walled; however, the vein collapses. And when the vein collapses, blood comes in, but blood can't go out. And if you recall from our lecture concerning—um, esema, one of the ways by which you can get aeia as blocking of the venous supply because fresh blood would not be then passing across the organ. So the organ will die because the vein—or the tissue will die because the vein is blocked, and blood will continue to re-enter because the artery remains somewhat open relative to the blockage of the vein. As blood piles up within the testicle, the testicle is relatively loose—the tissue of the testicle is relatively loose, and that will then create what we call—um, a red infarction. And it is important for examinations to be familiar with the distinction between a pale infarction, uh, and a red infarction. Okay, so let's continue.
The next type of necrosis is liquefactive necrosis, and guess what—in liquefactive necrosis, the tissue becomes liquefied. So we've got necrotic tissue or dead tissue, um, that becomes liquefied. Now, um, the way by which the tissue becomes liquefied is that there is enzymatic lysis of the cells. Um, the cells get acted on by some enzyme, and that is what basically mixes—destroys the tissue, resulting in the liquefaction. It is characteristic—it is seen characteristically in three circumstances. The first is brain infarction. Uh, recall that I told you that when you cut the blood supply to any organ in the body you actually get coagulative necrosis; however, if the blood supply is cut to the brain, that would result in liquefactive necrosis. So that would be one important—important circumstance that results in liquefactive necrosis. One of the questions we can ask ourselves is why is it that within the brain we get liquefactive necrosis, but in all other tissues we get coagulative necrosis? And the answer is because the brain contains microglial cells, which are derivatives—macro—of monocytes. They're sort of the—the macrophages of the brain, and the microglial cells contain hydrolytic enzymes that trash the tissue or destroy the tissue after—after it dies. And so the micro—this is mediated—this brain—this circumstance here is mediated by microglial cells, and that's actually an important principle as well. The second circumstance in which we might see liquefactive necrosis is in an abscess. Now an abscess is a walled-off area of dead tissue, and it becomes liquefied because the neutrophils contain hydrolytic enzymes, and those hydrolytic enzymes—they destroy the tissue, resulting in the liquefication of that tissue. And so if I asked you what mediates liquefactive necrosis in the case of an abscess, the answer is the enzymes from the neutrophils. The third circumstance that we might see liquefactive necrosis is in pancreatitis. What is pancreatitis? Remember that pancreatitis is a disease process in which we activate the enzymes of the pancreas within the pancreas itself. Now normally the enzymes of the pancreas would obviously get activated in the gut and digest food; however, if the enzymes of the pancreas get activated within the pancreatic parenchyma itself, the pancreatic enzymes will digest the pancreas, and it's the digestion of the pancreas that results in a liquefactive necrosis of the pancreas itself. Now I keep emphasizing this pancreas itself because in pancreatitis we also see another type of necrosis of the surrounding fat—the fat that surrounds the pancreas—but that's called fat necrosis, and we'll talk about that in a minute. So actually pancreatitis kind of has two types of necrosis associated with it: both liquefactive of the pancreatic parenchyma and fat necrosis of the peripancreatic fat. And if I asked you what's the mechanism on an examination of—of li—liquefactive necrosis and pancreatitis, the answer is that it's the enzymes from the pancreas that are digesting the—the pancreatic parenchyma.
Another type of necrosis to be familiar with is gangrenous necrosis. This is basically coagulative necrosis, but when you look at the tissue grossly, it resembles mummified tissue. Um, it basically—that is really what we call gangrenous necrosis. Now this is characteristic of esea of the lower limb; that's really the high—the highlight I think here. Um, you can also see it in the GI tract, but it's really—I think the—the classic is esia of the lower limb. Um, it's not uncommonly seen, particularly in diabetics. Now diabetics, they get occlusion or—AOS—not occlusion, but I should say—um, partial occlusion of the poal artery, um, and due to—due to atherosclerosis, and that can result in decreased blood flow to the lower extremity, which then subsequently can cause gangrenous necrosis of that lower extremity. Um, if the dead tissue becomes infected, um, that infected tissue can then undergo liquefactive necrosis, and that can—that also can be a component of gangrenous necrosis, but in that case we call it wet gangrene, the wet being the pus, um, and the inflammatory exudate present due to the superimposed infection. Here's a figure from your text just highlighting this. Um, basically this is the—a foot, um, I think this is of a diabetic patient, and—um, it is just showing you the mummification or the gangrenous necrosis, um, due to gangin—escosis of the extremity. And here it's a little more liquefactive where it looks like there's some superimposed infection, um, which has resulted in—resulted in focal lique—liquefactive necrosis of that tissue. Sorry, tripping on my words.
Okay, uh, caseous necrosis. This is another type of necrosis. Um, it's characterized as a soft, friable necrotic tissue with a quote-unquote "cottage cheese-like" appearance. Um, I'm sure you've heard that term before. Um, the way I think about this—it's a little strange—but there's the way I think about it: it's basically liquefactive necrosis with something mixed in. Um, and the reason I think about it that way—it kind of goes back to cooking. If you've ever made a gravy, uh, you know that you take some—you take something very liquidy, you add some flour. When you mix in the flour, that sort of thickens up the soup. So in the same way, in caseous necrosis, you've got an infection; it's creating liquefactive necrosis, uh; however, you throw in there something that thickens up the soup, making it a little more cottage cheese-like in appearance. And so what thickens the soup? The wall of fungus, I.E., fungal infection or mycobacterium like TB. Uh, so this is really what results in caseous necrosis. It's characteristic of TB and fungal infection, in particular—the granulomatous inflammation that's seen with fungus and TB. This is a classic, uh, example of caseous necrosis, um, highlighted here—as outlining here in red. This is the white cottage cheese appearance of caseous necrosis.
Fat necrosis refers to necrosis of adipose tissue. Um, it has a classic chalky white appearance. The reason it picks up this chalky—chalky white appearance is because when fat dies, fatty acids are released, and the fatty acids then have the ability to bind with calcium, which is a—in which is a process called saponification. Important term to be familiar with: saponification. Um, basically what happens here is that when calcium deposits on that dead fat, it gives a white, chalky appearance to the tissue. Um, here's a classic example. Uh, this is a pancreas—this long, extended organ is a pancreas—Figure 1.9 from your text, and here we can see sort of these little white, chalky deposits at the outer edge of this tissue, which is the fat surrounding the pancreas, and this would be a classic example of fat necrosis. Now let me take a step back and remind you because sometimes students can get confused. In—in pancreatitis, where you get—chewing up of the pancreas by pancreatic enzymes, you get liquefactive necrosis of the pancreas, and you get fat necrosis of the peripancreatic fat again. Um, so—so that's fat necrosis. Now fat necrosis is classically seen in two regions or with—with two—two processes. The first is trauma to fat, and the highest yield here would be breast trauma—trauma to the breast. The breast is a relatively fatty organ; it contains a lot of adipose tissue. Um, trauma to the bre—breast, for example—if a woman is in a car accident and the breast is crushed against the steering column or steering wheel, that can result in fat necrosis, which would release fat. Now that could result in destruction of the fat cells, release of the fatty acids, and calcium binding those fa—fatty acids, resulting in the classic appearance of fat necrosis. Um, so trauma can actually re—trauma to fat can release those fatty acids, resulting in fat necrosis. Um—um, probably it's—it's important to also mention here that this really comes into play particularly in the breast because it can clinically present in two ways. One way by which fat necrosis can present, especially on exams, is as a mass. And—uh, basically when the fat—when the fat necrosis occurs, there is often a giant cell reaction because—as because of the inflammatory response to the fat. And so—um, there can be a breast mass. A woman comes in, a biopsy is performed, and they might tell you on the exam that—um, the patient—the biopsy shows giant cells, fat, and calcification. And so you would think about fat necrosis then. Um, it's also important to be aware of fat necrosis in the breast because—um, the way by which—u—mam—the—the—the sort of the—uh—one of the ideas behind mammography is that—uh, we look for—one of the findings in mammography, I should say, is calcification. U—that's one of the abnormal findings. Um, and the presence of calcification is a—sometimes an indicator of ductal carcinoma in situ; however, it can also be seen in benign circumstances like fat necrosis. A woman does not necessarily have to give a history of trauma for fat necrosis. It could have happened—in a circumstance where maybe the woman would not even have been aware of trauma; for example, she's a softball player or plays a sport and happens to get hit with the ball on the chest, and that could result in fat necrosis of the breast. Another circumstance in which fat necrosis would be characteristic is the pancreatitis-mediated damage of peripancreatic fat, but I think I've really—uh—harped on that enough. Saponification—high-yield term and—and concept—I've already mentioned it. When fat is damaged, fatty acids are released, whether that be by trauma or by lipase from pancreatitis, um, and that allows the fat to join calcium, and that's called saponification. And if you are interested in sort of a side point, um, this is the way by which soap is made as well. Uh, so the soap—the consumer soap that we purchase, um, is actually made by the similar process of saponification.
Now what is—sort of a derivative point, but it gives us an opportunity to—to touch on a—on a high-yield—idea—is that dis—saponification is—is an example of something called dystrophic calcification. So why don't I take a step back and just remind you that generally speaking, calcium does not deposit in tissues of the human body. Um, when it deposits in tissues in the human body, there's really two underlying mechanisms by which that occurs. The first is dystrophic calcification, and the idea here is that there is a dead or dying tissue, and that dead or dying tissue becomes a nidus for the calcium to deposit. The serum calcium is normal; the serum phosphate is normal. Um, however, calcium can deposit on the tissue because the tissue is dying. So one example of dystrophic calcification would be saponification, where you have dead fat which then picks up the calcium. Um, another example of dystrophic calcification, as long as we're on the topic, would be psammoma bodies. So the psammoma bodies that we're so used to hearing about in papillary carcinoma of the thyroid and in meningiomas and in papillary serous carcinoma of the ovary, um, all of those are examples of dystrophic calcification. The tumor cells outgrow their blood supply; they're growing rapidly; they die, and it's on top of those dying cells that calcification occurs. Again, in dystrophic calcification, the serum calcium is normal, and the serum phosphate is also normal. Dystrophic calcification can be contrasted against—or can—can be contrasted with—um, another process which is called metastatic calcification. In metastatic calcification, the serum calcium or the serum phosphate—either one—is elevated, and the elevated serum calcium or elevated serum phosphate has the ability to force calcium into tissues, which then, based on its high concentration, can precipitate out as calcifications. That's called metastatic calcification, and it's always associated with a high serum calcium or a high serum phosphate. Now let—let me just remind you that the term metastatic calcification doesn't mean that the patient has metastatic cancer. That's unfortunately sometimes confused by students. Metastatic calcification just means that the serum calcium is high throughout the blood, and therefore the calcium can deposit throughout all the tissues of the body, and so it kind of is metastatic. It so happens that one of the causes of a high serum calcium is metastatic cancer to the bone, which could then result in metastatic calcification due to the high serum calcium. However, I really want to make it clear that when we say metastatic calcification, uh, what we mean is calcification that occurs in multiple tissues due to a high serum—or a high phosphate. All right, so let's keep moving forward.
Fibrinoid necrosis. This is another type of necrosis. It refers to necrotic damage of the blood vessel wall—necrotic damage to the blood vessel wall. This damage results in leaking of proteins from the blood vessel into the wall, which then results in a bright pink staining on histology. So let me show you a picture. This is Figure 1.10 on your text, and this is a blood vessel highlighted here—I'm outlining it for you—and you can clearly see that there is this bright pink staining to the wall of the blood vessel. This is an example of fibrinoid necrosis. Let me take you a step back and remind you that this is characteristic of two pathologic processes. The first is malignant hypertension. When—why don't—why don't we go back? What is malignant hypertension? Well, hypertension or high blood pressure can be divided into benign and malignant. Benign, um, hypertension is a sort of low-grade hypertension that creates long-term, progressive, chronic damage to organs. Now malignant hypertension is a very high blood pressure that presents with clinical findings such as headache, renal failure, papilledema, um, and is a medical emergency. One of the consequences of malignant hypertension is that the blood vessel wall can actually necrose due to the high pressure, um, and that's called fibrinoid ne—and that's an example, I should say, of fibrinoid necrosis. So very important to remember. Another example of fibrinoid necrosis is vasculitis. So in—in—in a subset of vasculitis, uh, the characteristic finding would be fibrinoid necrosis, and we'll have a chance to discuss that when we—just—when we do the blood vessel chapter. Now I'd like to say one more thing, um, and trying to give you a classic, uh, question that might show up on an exam. Um, let's just ask you: what circumstance—in what circumstance would a 30-year-old woman present with fibrinoid necrosis? Now the reason I asked this question is just sort of jog your memory and to stick a little point into your brain, and that is that you know a 30-year-old woman would not normally have malignant hypertension, and the vast majority of 30-year-old women would not have vasculitis. But there is one circumstance in which a—a woman could get very high blood pressure, and that's preeclampsia. Now what happens in preeclampsia? In preeclampsia, usually in the third—usually presents in the third trimester with an elevated blood pressure, uh, often with proteinuria, and—uh—the consequence of preeclampsia is—is—is—um, fibrinoid necrosis of—of what organ?—of the placenta. So fibrinoid necrosis of the placenta—of the placental blood vessels—is a consequence, um, of preeclampsia. And believe me, that's a very commonly tested principle. Okay, so that really ends our discussion of necrosis, and now we're actually going to move on to apoptosis. Remember that I told you the way I sort of think about apoptosis is it's cellular suicide. It is energy-dependent. Uh, the cell decides that it wants to purposely use energy to kill itself, and it's genetically programmed. It runs through a genetic program in order to—to—to destroy itself. Usually invol—it involves single cells or small groups of cells as opposed to necrosis, which involves a large group of cells. Um, a few classic examples to be familiar with: the endometrium, when it sheds, actually sheds by apoptosis during the menstrual cycle. Um, during embryogenesis, cells are often removed, and that removal of cells occurs by apoptosis. Now, for example, the spaces that are present between your fingers and toes—that occurs—um, by removal of tissue via apoptosis. And sometimes you'll see patients with—uh, something called syndactyly, where there are two toes stuck together, um, and that is because apoptosis did not occur during embryogenesis for that—in that particular tissue. And a—and a third classic example of apoptosis would be—um, the killing of virally infected cells by CD8-positive T cells. Now recall that when a virus infects a cell, the proteins of that virus are actually produced by the genetic machinery of the cell, and therefore, because these are endogenous proteins, they'll be expressed on MHC class I. MHC class I is present on all nucleated cells and platelets, um, and it expresses endogenous protein. MHC class I, once it expresses this viral proteins, um, will be recognized by CD8-positive T cells, and the CD8-positive T cells—the means by which they kill virally infected cells is actually apoptosis. So that becomes relatively high-yield.
The morphology of apoptosis—let's go back and remind ourselves of the meaning of the word apoptosis. Apoptosis means the falling of leaves. It's a Greek term that means the falling of leaves. And so the idea is that—let's pretend that this is the cell, and the cell is going to die. Well, the way by which the cell dies is that this cell eventually shrinks. The nucleus in the cell shrinks, and then eventually little pieces of the cell sort of bud off and fall off the cell, um, as if leaves are falling from a tree. And so the word apoptosis means the falling of leaves. And when the cell eventually dies and breaks up, you see the falling of pieces—falling off of—pieces of the cell. So that—that's the name apoptosis. Now from this image that I just drew, um, you get the idea that a couple things have to happen. First of all, the cell has to shrink, and when the cell shrinks, it becomes very pink because the cytoplasm gets concentrated within that cell. Um, the second is that the nucleus has to condense or shrink, and then it has to fragment. And—um—the third is that the apoptotic bodies will fall off from the cell. And it's important to remember that those apoptotic bodies are removed by macrophages. Apoptotic bodies are removed by macrophages. There is no inflammation—no acute inflammation that follows apoptosis, and this is important because it's a contrasting point against necrosis. Remember that necrosis is followed by acute inflammation, whereas apoptosis—which—which is not followed by inflammation. Here's a classic image, Figure 1.11 in your text, um, and you can see here that this—I'm boxing out here—the apoptotic cell. So notice, if you compare it to its neighbors, its cytoplasm—which is this stuff here—its cytoplasm is pink because it's getting concentrated as the cell shrinks, and the nucleus here is a little smaller than the rest of the neighboring cells because the cell—the nucleus is also shrinking. Once you understand the morphology, you can then understand how apoptosis is mediated. Now the key mediator of apoptosis is something called a caspase, and that's high-yield. So caspases are the enzymes that mediate apoptosis, and what do they do? They do a couple things. Number one, they activate proteases. And why do they activate proteases? Because a cytoskeleton needs to be broken down. Let's go back and remind ourselves that a cell maintains its normal size and shape by the presence of a skeleton which is called a cytoskeleton. That cytoskeleton is predominantly composed of protein. If we want to take a cell from this size and we want to bring it down to this size, I.E., if we want to shrink it, we've got to trash that cytoskeleton. And who's going to do that? The proteases. All right, in the same way, if this—
Is the cell, and the cell contains this nucleus, and the cell is shrinking, and the nucleus is going to shrink? Then you also need to be able to crunch down that nucleus and break it down. And so endonucleases are the mechanism by which the the the DNA of the cell gets broken down. So apoptosis, the key key mediator is a caspase, or the caspases. And the caspases eventually activate the proteases, which allow for shrinking and breaking down of the cytoskeleton and endonucleases, which allow for breaking down and shrinking of the DNA. Okay. Now, very high-yield caspases, which are the key enzymes that sort of mediate apoptosis, are activated by three major pathways, and each of these pathways is relatively high-yield.
Now, the first is called the intrinsic mitochondrial pathway. And the idea here is that if there is something that um is going to result—let's say that there's some force that is going to cause apoptosis—so what are some things that can cause apoptosis? Cellular injury. Do you guys remember from cellular injury what I told you um was the key sign of um irreversible damage? It's uh irreversible injury; the key sign was membrane damage. And one of those membranes that was going to be damaged was the mitochondria, and that can result in activation of this pathway. DNA damage. Uh, when DNA is damaged, you don't want those cells hanging around. Why don't you want those cells hanging around? Because you could get cancer from a a cell that has damaged DNA. And so if the D the DNA damage is severe in a cell, um it can actually undergo apoptosis to protect the rest of the organ from from the formation of an eventual cancer. And decreased hormonal stimulation. Uh, for example, example, um the endometrial cycle. We know that when um progesterone comes in, estrogen is removed, and progesterone comes in, eventually we get shedding of the endometrium, and that occurs via apoptosis.
Now, the examiners like you like to go after the following principle, and that is that the mechanism, sort of the key molecule that mediates this intrinsic mitochondrial pathway, is cytochrome C. All right, let's highlight that with a drawing. So this is the cell, and let's pretend that within this cell we've got this mitochondria, and we know that within the mitochondria here is cytochrome C. And so um cytochrome C is very carefully guarded within that mitochondria. In fact, there's a molecule here called Bcl2, and Bcl2's role is to stabilize the mitochondrial membrane so that cytochrome C cannot leak out. If there is a stimulus that um is going to result in apoptosis—for example, let's say that there's DNA damage, or there is cellular injury, or there is a decrease in hormonal stimulation—what will happen is that Bcl2 will get knocked out, and when Bcl2 gets knocked out, cytochrome C can leak out from the cytoplasm from the mitochondria into the cytoplasm and can activate the caspases necessary to drive apoptosis further. So this idea of cytochrome and Bcl2 is um a relatively high-yield principle.
The next way by which apoptosis can be activated, or in particular the caspases can be activated, is the extrinsic receptor-ligand pathway. Um, the basic idea here is that something from the outside world binds a receptor on the cell, which then activates apoptosis. And the classic example, and the one that probably we should focus on, is Fas ligand binding Fas death receptor on the target cell. Uh, why don't we—the the best example I think of this is to, and it requires a little review of um immunology, is to go back and remind ourselves that T cells are actually born in the bone marrow, and once they're produced in the bone marrow, they go to the thymus to be educated. And when they're educated in in the thymus, they actually have to go through two layers of testing. The first is called positive selection. Positive selection—they get asked the question: Do you, I.E. the T cell, have the ability to bind self-antigen plus MHC? If the T cell has has the ability to bind self-antigen plus MHC, it survives positive selection and goes on to the next sort of test. The next test is called negative selection, and in negative selection the question is asked: Do you, I.E. the T cell, have the ability to bind self-antigen too avidly, too strongly? And if the T cell binds self-antigen too strongly, the T cell will then be destroyed during the process of negative selection. You want to get rid of the T cells that are are going to bind um to self-antigen because you don't want to have autoimmune problems. And so the mechanism by which the cells die in negative selection is actually by the Fas ligand Fas death receptor pathway; it's apoptosis basically. And so um if the cells too avidly bind self-antigen, then Fas ligand is expressed, uh, and it hits the death receptor on the T cell and causes the T cell to undergo apoptosis. And this becomes is a very high-yield way by which this pathway can be tested. Um, another example would be tumor necrosis factor binding tumor necrosis factor receptor on a target cell, but for me this really is the high-yield Fas ligand Fas death receptor in the context of negative selection of T cells in the thymus.
And the third pathway by which uh caspases can be activated is the cytotoxic CD8 positive T-cell pathway. Now recall that CD8 positive T cells, they recognize antigen present on MHC class one, and when they bind and recognize that antigen on MHC class one, they kill the cell that expresses the antigen, um, and the way by which they kill the cell is apoptosis. So what happens is the CD8 positive T cell, once it recognizes its antigen and gets the appropriate signal, will actually secrete something called a perforin. The perforin perforates a hole in the membrane of the target cell. Granzyme, also from the CD8 positive T cell, then enters into the pore or the hole and then activates the caspases. And you're very well aware of the fact that caspases then mediate apoptosis. So that's another important pathway by which apoptosis can be activated, I.E., the caspases can be activated.