Okay, our final formed element is our platelet. So technically, formed elements are blood cells. And in a very strict sense, platelets are not cells because they don't contain the things required to be considered a cell.
Why? Because they're actually little fragments or pieces of a much larger cell called a megakaryocyte. So they're really, really small. But even though they're small, they are... really, really important in our blood and they contain a bunch of granules that house things like serotonin or ADP or enzymes needed to help assist the clotting process.
So they are essential if we rupture our blood vessel in any way. So again, their function, temporary plug to help seal breaks and blood vessels. So we have platelets that are constantly circulating in our bloodstream. So how do we prevent them from forming a clot if we don't need them to form a clot?
Well, if we have healthy cells, these healthy cells release nitric oxide and prostacyclin. And if as long as we have nitric oxide and prostacyclin being released from the healthy cells, then we don't have our platelets activated. So they remain inactivated if we have cells releasing nitric oxide and prostacyclin.
So the process of forming... Platelets is called thrombopoietin because platelets are also called thrombocytes. That's the more correct term, just like our erythrocytes and our leukocytes. They're formed also by the myeloid line from a megakaryoblast, which is considered a stage one megakaryocyte. How do we get such a large cell, the megakaryoblast or the site?
Basically what happens is this cell... undergoes continuous rounds of mitosis, but does not undergo cytokinesis. So it's constantly replicating, duplicating everything it has.
but it's not dividing its cytoplasm and pinching off into two individual daughter cells. So as a result, this really, really, really, really, really large size that keeps, has pretty much replicated and gone through several rounds of mitosis, but never actually divided itself. Until eventually we really reach what's called a stage 4 megakaryocyte. And this really, really large stage 4 megakaryocyte, since some of its plasma...
membrane out into the lumen of capillary. So we call those little projections cytoplasmic projections that kind of sneak through the capillary wall. And when they sneak through the capillary wall, they actually break off.
And the little break off pieces into our bloodstream are what we refer to platelets. They age very quickly because again, they don't have any nutrient supply or the ability to divide or regenerate any. anything. So they only last about 10 days.
So here's our picture. Again, we're starting from the same hematopoietic stem cell line right here. We're going to become a megakaryoblast, then eventually a megakaryocyte that goes through, you know, four different stages. Once we reach our stage four megakaryocyte, these are the little cytoplasmic extensions that are going to sneak through the capillary wall and break off.
And when they break off, we form these platelets. What you're seeing in here are those. granules with the really important chemicals that we need in order to help assist with the clotting process.
So here is just a summary of everything that we talked about so far in regards to the formed elements. You don't have to know the exact duration or lifespan of each specific form element. Just in general, I would expect you to know that erythrocytes are formed in the form of erythrocytes. typically have a longer lifespan than leukocytes because leukocytes die trying to fight infections.
Platelets don't have a long lifespan as well. I would understand the role of each specific type of leukocyte as well as which leukocyte is a granulocyte or a granulocyte. And again, the role of a platelet and what it is derived from, which is the stage four megakaryocyte. And erythrocyte and the function.