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Hey guys, it's Medicos. Is perfection elsewhere? Medicine makes perfect sense. Today is the second video in our topic about bleeding and coagulation disorders. In the previous video, we have discussed platelets or thrombocytes. Today, let's get into the nitty-gritty and talk about the platelets' structure and the platelet granules—the alpha granules and the dense granules. So let's get started. [Music]
Here is the previous video. Make sure to watch it before this one. I've told you that the normal platelet count is 150,000 to 400,000. Less than 150,000 is called thrombocytopenia; more than 400,000 is called thrombocytosis or thrombocythemia. But we don't care about thrombocytopenia unless the platelet count is less than 50,000. And clinically, we don't care about thrombocytosis unless it's more than 750,000. So when Grandma comes in and her platelet count is 600,000 with no symptoms, I just send her home. Don't worry about it, provided that everything else is normal.
Thrombocytes—the cells of thrombus. Less platelets: thrombocytopenia; more platelets: thrombocytosis, from thrombocythemia. Synthesized by the glorious, magnificent megakaryocytes in the bone marrow. While squeezing out, these megakaryocytes are destroyed into thousands of pieces. Each piece is called a platelet. That's why platelets don't have a nucleus and they cannot divide. They are not cells; they are just pieces. Platelets are just pieces. Where did they get the membrane from? From the membrane of the megakaryocyte. Where did they get the cytoplasm from? From the cytoplasm of the megakaryocyte. They just took a piece. So, bone marrow biopsy—you don't get to see platelets. Platelets are only seen in the peripheral smear.
Let's talk about platelet structure first. Platelets are biconvex. If you remember the red blood cells, the red blood cells were biconcave. The two surfaces are concave. Platelet is the opposite: the two surfaces are convex. However, they are similar in another thing: both don't have a nucleus; both of them are non-nucleated. Platelets are pieces of the megakaryocyte. And the structure of platelets: they have the plasma membrane or the cell membrane, which they got from the megakaryocytes, and the cytoplasm that they get also from the megakaryocyte. The plasma membrane or the lipid bilayer—if you remember our discussion about the cell membrane—any cell membrane is a lipid bilayer, and it's covered by a glycocalyx. Glycocalyx is just carbohydrate. Platelets are slightly different. Yes, they have the lipid membrane, but they are covered by not carbohydrate, but carbohydrate plus protein. Why protein? Because the active thing in your body is protein, and platelets need receptors because they want to bind and adhere to the endothelial tissue. So they need receptors; if they need receptors, they have to be protein. That's why it's not just carbohydrate; it's glycoprotein. And without these receptors, you'll bleed to death. So that's the first layer: glycoprotein coat, and it has the receptors—two receptors to remember: GP 1b and GP IIb/IIIa. Some people will write it GP IIb/IIIa like this. Doesn't matter to me. The lipid bilayer, including phospholipid, and the phospholipid will produce the famous arachidonic acid—the pro-inflammatory—actually, it's one of the most pro-inflammatory things in your body ever. Steroids inhibit the formation of arachidonic acid; that's why steroids are the best, the strongest anti-inflammatory known to man.
Calcium can attack your system because platelets need to contract. Why contract? In order to release—no pun intended—cytoplasm. They have actin and myosin, again for contraction and release. Thrombosthenin. "In" means protein; "stenos" is Greek for strength. So, thrombosthenin—the protein that's strong in the platelets. Again, it's on a contract and release. Release what? Release the granules. Wait, wait. Good things happen to those who wait. Platelets have residuals of Golgi and rough endoplasmic reticulum—why? For enzyme synthesis. The mighty mitochondria to produce ATP, and of course, they need ADP. Fibrin-stabilizing factor, also known as factor XIII. Platelet-derived growth factor, and it helps repair the vessel after the thrombus. Platelets or thrombocytes—they form the thrombus, but like a good cat, they clean after themselves. After the thrombus is over, your cat defecates in the ground, but then it cleans up after itself—exactly like the platelet.
Platelet granules—the ones in the cytoplasm: alpha granules and dense granules. Alpha granules—they are alpha; they are protein; they are strong; they're active. And they include factor XIII, the fibrin-stabilizing factor; the platelet-activating factor; platelet-derived growth factor to help with regeneration and repair; von Willebrand factor; fibrinogen. But isn't fibrinogen a coagulation factor supposed to be formed by the liver? I didn't say that platelets form or create fibrinogen; they just happen to store it. Platelet factor 4, and it will be involved in heparin-induced thrombocytopenia, and I'll talk about that later.
Dense granules, on the other hand, they are not proteins—only the alpha are proteins—and they include ADP and calcium. Why do you need calcium? Calcium is the hero of contraction—contract and release. Calcium—binding agent for vitamin K-dependent factors. What are the vitamin K-dependent factors? Okay, remember when we studied the autonomic nervous system, or when you studied the autonomic nervous system, there were four nerves coming out of the brain called cranial autonomic nerves, and those cranial autonomic nerves happen to be parasympathetic. These are 3, 7, 9, and 10. I know that 3 doesn't come from the brainstem; it comes from the midbrain, but forget it. 3, 7, 9, and 10. Vitamin K-dependent factors: in the US, just put 2, so we have 2, 7, 9, and 10. You don't want to say—to say prothrombin. So, prothrombin, 7, 9, 10. Any mediocre student will just say 2, 7, and 10. But for pros like you, Medicos viewers, you say 2, 7, 9, and 10. Protein C and protein S. Never ever forget this: prothrombin, 7, 9, 10, protein C, protein S. So, in cases of vitamin K deficiency, which factors are deficient? 2, 7, 9, 10, C, S.
This quick review: we find platelets in the peripheral blood, but some of them are hiding in the spleen. That's why splenomegaly decreases the platelet count, causing thrombocytopenia. That's why one of the treatment options for thrombocytopenia is to remove the spleen, forcing all of the platelets to gather around together in the peripheral blood, raising the platelet count back to normal. In the next video, we'll talk about the arachidonic acid. You don't want to miss it. Half of pharmacology is in the next video, believe it or not. So make sure to subscribe. Thanks for watching. Subscribe and smash like. If you'd like to get all of my notes and to ask me any question, go to patreon.com/scishow--ss. Thank you so much for watching. As always, be safe, stay happy, and study hard. Medicos is perfection elsewhere. Medicine makes perfect sense.