Blood platelets, which are designed to deal with sudden blood loss, are called platelets. They accumulate in places of damage to any vessels and clog them with a special stopper.
Record appearance
Under the microscope, you can see the structure of platelets. They look like discs, the diameter of which ranges from 2 to 5 microns. The volume of each of them is about 5-10 microns3.
In terms of their structure, platelets are a complex complex. It is represented by a system of microtubules, membranes, organelles and microfilaments. Modern technologies have made it possible to cut a flattened plate into two parts and to single out several zones in it. This is how they were able to determine the structural features of platelets. Each plate consists of several layers: peripheral zone, sol-gel, intracellular organelles. Each of them has its own functions and purpose.
Outer layer
The peripheral zone consists of a three-layer membrane. The structure of platelets is such that on its outer side there is a layer that contains plasma factors responsible for blood clotting, specialreceptors and enzymes. Its thickness does not exceed 50 nm. The receptors of this layer of platelets are responsible for the activation of these cells and their ability to adhere (attach to the subendothelium) and aggregate (the ability to connect with each other).
The membrane also contains a special phospholipid factor 3 or the so-called matrix. This part is responsible for the formation of active coagulation complexes together with plasma factors responsible for blood clotting.
In addition, it contains arachidonic acid. Its important component is phospholipase A. It is she who forms the indicated acid necessary for the synthesis of prostaglandins. They, in turn, are designed to form thromboxane A2, which is necessary for powerful platelet aggregation.
Glycoproteins
The structure of platelets is not limited to the presence of an outer membrane. Its lipid bilayer contains glycoproteins. They are designed to bind platelets.
Thus, glycoprotein I is a receptor that is responsible for attaching these blood cells to the collagen of the subendothelium. It ensures the adhesion of the plates, their spreading and their binding to another protein - fibronectin.
Glycoprotein II is designed for all types of platelet aggregation. It provides fibrinogen binding on these blood cells. It is thanks to this that the process of aggregation and reduction (retraction) of the clot continues unhindered.
But glycoprotein V is designed to maintain the connectionplatelets. It is hydrolyzed by thrombin.
If the content of various glycoproteins in the specified layer of the platelet membrane decreases, this causes increased bleeding.
Sol-gel
Along the second layer of platelets, located under the membrane, there is a ring of microtubules. The structure of platelets in human blood is such that these tubules are their contractile apparatus. So, when these plates are stimulated, the ring contracts and displaces the granules to the center of the cells. As a result, they shrink. All this causes the secretion of their contents to the outside. This is possible thanks to a special system of open tubules. This process is called “granule centralization.”
When the microtubule ring shrinks, the formation of pseudopodia also becomes possible, which only favors an increase in the ability of aggregation.
Intracellular organelles
The third layer contains glycogen granules, mitochondria, α-granules, dense bodies. This is the so-called organelle zone.
Dense bodies contain ATP, ADP, serotonin, calcium, adrenaline and norepinephrine. All of them are necessary for platelets to work. The structure and function of these cells provide adhesion and wound healing. So, ADP is produced when platelets attach to the walls of blood vessels, it is also responsible for ensuring that these plates from the bloodstream continue to attach to those that have already stuck. Calcium regulates the intensity of adhesion. Serotonin is produced by the platelet when the granules are released. It is he who ensures the narrowing of their lumen at the site of rupture of the vessels.
Alpha-granules located in the zone of organelles contribute to the formation of platelet aggregates. They are responsible for stimulating the growth of smooth muscles, restoring the walls of blood vessels, smooth muscles.
The process of cell formation
To understand the structure of human platelets, it is necessary to understand where they come from and how they are formed. The process of their appearance is concentrated in the bone marrow. It is divided into several stages. First, a colony-forming megakaryocytic unit is formed. Over several stages, it transforms into a megakaryoblast, a promegakaryocyte, and ultimately a platelet.
Daily, the human body produces about 66,000 of these cells per 1 µl of blood. In an adult, serum should contain from 150 to 375, in a child from 150 to 250 x 109/l of platelets. At the same time, 70% of them circulate through the body, and 30% accumulate in the spleen. When needed, this organ contracts and releases platelets.
Main Functions
In order to understand why platelets are needed in the body, it is not enough to understand what are the structural features of human platelets. They are intended primarily for the formation of a primary plug, which should close the damaged vessel. In addition, platelets provide their surface in order to speed up the reactions of plasmafolding.
In addition, it was found that they are needed for the regeneration and healing of various damaged tissues. Platelets produce growth factors designed to stimulate the development and division of all damaged cells.
It is noteworthy that they can quickly and irreversibly change to a new state. The stimulus for their activation can be any change in the environment, including simple mechanical stress.
Features of platelets
These blood cells do not live long. On average, the duration of their existence is from 6.9 to 9.9 days. After the end of the specified period, they are destroyed. Basically, this process takes place in the bone marrow, but also to a lesser extent it occurs in the spleen and liver.
Specialists distinguish five different types of platelets: young, mature, old, forms of irritation and degenerative. Normally, the body should have more than 90% of mature cells. Only in this case, the structure of platelets will be optimal, and they will be able to perform all their functions in full.
It is important to understand that a decrease in the concentration of these blood cells causes bleeding that is difficult to stop. And an increase in their number is the cause of the development of thrombosis - the appearance of blood clots. They can clog blood vessels in various organs of the body or completely block them.
In most cases, with various problems, the structure of platelets does not change. All diseases are associated with a change in their concentration.in the circulatory system. A decrease in their number is called thrombocytopenia. If their concentration increases, then we are talking about thrombocytosis. If the activity of these cells is disturbed, thrombasthenia is diagnosed.
