Functions and structure of platelets

Blood plates, which are designed to fight sudden blood loss, are called platelets. They accumulate in places of damage to any vessels and block them with a special stopper.

Appearance of plates

Under a microscope, one can consider the structure of platelets. They look like disks whose diameter varies from 2 to 5 microns. The volume of each of them is of the order of 5-10 μm ³ .

By its structure, platelets are a complex complex. It is represented by a system of microtubules, membranes, organelles and microfilaments. Modern technology has made it possible to cut a flattened plate into two parts and to separate several zones in it. This is how we could determine the features of the structure of platelets. Each plate consists of several layers: the peripheral zone, sol-gel, intracellular organelles. Each of them has its own functions and purpose.

Outer layer

Peripheral zone consists of a three-layer membrane. The structure of platelets is such that on its outer side there is a layer containing plasma factors responsible for blood clotting, special receptors and enzymes. Its thickness does not exceed 50 nm. Receptors of this layer of platelets are responsible for the activation of these cells and their ability to adhere (attach to the subendothelium) and aggregation (the ability to connect with each other).

Also, the membrane contains a particular phospholipid factor 3 or a so-called matrix. This part is responsible for the formation of active coagulation complexes, together with plasma factors responsible for blood coagulation.

In addition, it contains arachidonic acid. Its important component is phospholipase A. It is this that forms the acid necessary for the synthesis of prostaglandins. They, in turn, are intended for the formation of thromboxane A ₂ , which is necessary for the powerful aggregation of platelets.

Glycoproteins

The structure of platelets is not limited to the presence of an external membrane. In its lipid bilayer are glycoproteins. They are intended for binding of platelets.

Thus, glycoprotein I is a receptor that is responsible for attaching the subendothelium of the indicated blood cells to the collagen. It ensures the adhesion of the plates, their spreading and their binding with another fiber - fibronectin.

Glycoprotein II is for all types of platelet aggregation. It provides binding on these blood cells to fibrinogen. It is thanks to this that the process of aggregation and contraction (retraction) of the clot continues unchecked.

But glycoprotein V is designed to maintain the connection of platelets. It is hydrolyzed by thrombin.

If the contents of various glycoproteins decrease in this layer of the platelet membrane, this causes increased bleeding.

Sol-gel

Along the second layer of platelets, located under the membrane, is a ring of microtubules. The structure of platelets in the human blood is such that these tubes are their contractile apparatus. Thus, when these plates are stimulated, the ring contracts and shifts the granules to the center of the cells. As a result, they shrink. All this causes the secretion of their contents outside. This is possible thanks to a special system of open tubules. Such a process is called "centralization of granules".

With the contraction of the microtubule ring, it also becomes possible to form pseudopodia, which only favors an increase in aggregation capacity.

Intracellular organelles

The third layer contains glycogen granules, mitochondria, α-granules, dense bodies. This is the so-called zone of organelles.

Dense bodies contain ATP, ADP, serotonin, calcium, adrenaline and norepinephrine. All of them are necessary for platelets to work. The structure and functions of these cells provide adhesion and healing of wounds. Thus, ADP is produced by attaching platelets 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 been stuck. Calcium regulates the intensity of adhesion. Serotonin is produced by a platelet when granules are released. It is he who provides a narrowing of the lumen at the site of the rupture of the vessels .

Alpha-granules, located in the zone of the organelles, contribute to the formation of platelet aggregates. They are responsible for stimulating smooth muscle growth, restoration of vessel walls, smooth muscles.

The process of cell formation

To understand what is 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 megakaryocyte unit is formed. For several stages, it is transformed into a megakaryoblast, promegakaryocyte and eventually into a platelet.

Every day, the human body produces about 66,000 of these cells per 1 μl of blood. An adult in the serum should be between 150 and 375, in a child from 150 to 250 x 10 ⁹ / L platelets. At the same time, 70% of them circulate through the body, and 30% accumulate in the spleen. If necessary, this organ contracts and releases blood platelets.

Main functions

In order to understand why the body needs a blood plate, it is not enough to understand what features of the structure of human platelets. They are designed primarily to form the primary plug, which must close the damaged vessel. In addition, platelets provide their surface in order to speed up the plasma clotting reactions.

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 over 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 last long. On average, the duration of their existence is from 6.9 to 9.9 days. After the end of this period, they are destroyed. Basically this process takes place in the bone marrow, but also to a lesser extent it goes in the spleen and liver.

Specialists distinguish five different types of blood platelets: young, mature, old, 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 the 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 changes 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. When the activity of these cells is violated, thrombastenia is diagnosed.