Blood platelets (platelets). Platelets in the blood: normal and pathological Video: why platelet levels increase and decrease

Platelets are freely circulating in the blood nuclear-free fragments of the cytoplasm of giant red bone marrow cells - megakaryocytes. The size of platelets is 2-3 microns, their number in the blood is 200-300x10 9 l. Each plate in a light microscope consists of two parts: a chromomere, or granulomer (intensely colored part), and a hyalomer (transparent part). The chromomere is located in the center of the platelet and contains granules, remnants of organelles (mitochondria, EPS), as well as glycogen inclusions.

Granules are divided into four types.

1. a-granules contain fibrinogen, fibropectin, a number of blood coagulation factors, growth factors, thrombospondin (an analogue of the actomyosin complex, involved in platelet adhesion and aggregation) and other proteins. They stain with azure, giving granulomeric basophilia.

2. The second type of granules is called dense bodies, or 5-granules. They contain serotonin, histamine (entering platelets from plasma), ATP, ADP, calcium, phosphorus, ADP causes platelet aggregation when the vessel wall is damaged and bleeding. Serotonin stimulates contraction of the wall of the damaged blood vessel, and also first activates and then inhibits platelet aggregation.

3. λ-granules - typical lysosomes. Their enzymes are released when a vessel is injured and destroy the remains of unresolved cells for better attachment of the blood clot, and also participate in the dissolution of the latter.

4. Microperoxisomes contain peroxidase. Their number is small.

In addition to granules, the platelet has two systems of tubules: 1) tubules associated with the cell surface. These tubules are involved in granule exocytosis and endocytosis. 2) a system of dense tubes. It is formed due to the activity of the Golgi complex of the megakaryocyte.

Rice. Scheme of platelet ultrastructure:

AG - Golgi apparatus, G - A-granules, Gl - glycogen. GMt - granular microtubules, CPM - ring of peripheral microtubules, PM - plasma membrane, SMF - submembrane microfilaments, PTS - dense tubular system, PT - dense bodies, LVS - superficial vacuolar system, PS - perimembrane layer of acidic glycosaminoglycans. M - mitochondria (according to White).

Functions of platelets.

1. Participate in blood clotting and stopping bleeding. Activation of platelets is caused by ADP, released by the damaged vascular wall, as well as adrenaline, collagen and a number of mediators of granulocytes, endothelial cells, monocytes, and mast cells. As a result of adhesion and aggregation of platelets during the formation of a blood clot, processes are formed on their surface, with which they stick to each other. A white blood clot forms. Next, platelets secrete factors that convert prothrombin into thrombin; under the influence of thrombin, fibrinogen is converted into fibrin. As a result, fibrin threads form around the platelet conglomerates, which form the basis of the thrombus. Red blood cells are retained in fibrin threads. This is how a red blood clot is formed. Platelet serotonin stimulates vessel contraction. In addition, due to the contractile protein thrombostenin, which stimulates the interaction of actin and myosin filaments, platelets come close together, the pull is also transmitted to fibrin threads, the clot decreases in size and becomes impenetrable to blood (thromb retraction). All this helps stop bleeding.

2. Platelets, simultaneously with the formation of a blood clot, stimulate the regeneration of damaged tissues.

3. Ensuring the normal functioning of the vascular wall, primarily the vascular endothelium.

There are five types of platelets in the blood: a) young; b) mature; c) old; d) degenerative; d) gigantic. They differ in structure.

Lifespan

platelets is equal to 5-10 days. After this, they are phagocytosed by macrophages (mainly in the spleen and lungs). Normally, 2/3 of all platelets circulate in the blood, the rest are deposited in the red pulp of the spleen. Normally, some platelets may be released into the tissue (tissue platelets).

Impaired platelet function can manifest itself in both hypocoagulation and hypercoagulation of the blood. In the nervous case, this leads to increased bleeding and is observed with thrombocytopenia and thrombocytopathy. Hypercoagulation is manifested by thrombosis - the closure of the lumen of blood vessels in organs by blood clots, which leads to necrosis and death of part of the organ.

Blood platelets (platelet in animals) have the appearance of small colorless bodies of round, oval or spindle shape, measuring 2-4 microns.

Their amount in the blood is from 2.0·10 9 /l to 4.0·10 9 /l. Blood plates are nuclear-free fragments of cytoplasm that are separated from the giant cells of the bone marrow - megakaryocytes.

Blood platelets have a lighter peripheral part - the hyalomere - and a darker part with grains - the granulomere.

There are five main types in the blood platelet population:

1) Young – basophilic hyalomer, single azurophilic granules (1-5%);

2) Mature – with oxyphilic hyalomer and well-developed azurophilic granularity (88%);

3) Old – denser hyalomer, dark purple granularity (4%);

4) Degenerative - with a grayish-blue hyalomere and a dense dark purple granulomere (2%);

5) Giant forms of irritation - with a pinkish-lilac hyalomere and violet granulomere (2%).

In diseases, the ratio of various forms changes. More youthful forms in newborns.

In cancer, the number of old platelets increases.

The plasmalemma of blood platelets is covered with a glycocalyx and contains glycoproteins - surface receptors involved in the processes of adhesion and aggregation of blood platelets. In the cytoplasm there are actin microfilaments and bundles of microtubules, as well as two systems of tubules.

The first is an open system of channels associated with invaginations of the plasmalemma. Through it, the contents of blood platelet granules are released into the plasma.

Special granules (α-granules) contain various proteins (lamellar factor 4, β-thromboglobin, fibrinogen, thromboplastin) and glycoproteins (fibronectin and thrombospondin - for platelet adhesion).

Heparin (blood thinner) binding proteins include factor 4 and β-thromboglobulin.

Another type of granules - delta granules (δ) - contain serotonin, histamine, adrenaline, Ca 2+, ADP, ATP.

The third type of granules is lysosomes.

The main function of blood platelets is to participate in the process of blood clotting - the body’s protective response to damage and preventing blood loss.

Platelets contain about 12 factors involved in blood clotting. When the vessel wall is damaged, the plates quickly aggregate and adhere to the resulting fibrin strands, resulting in the formation of a blood clot that closes the wound.

An important function of platelets is participation in the metabolism of serotonin.

Platelets are the most important component of blood. The role of platelets in peripheral blood analysis is not clear to the average person, but this indicator can tell a doctor a lot. Blood is not a homogeneous liquid running through the vessels; erythrocytes, leukocytes, and different types, circulate in it. Platelets and other blood components are essential for the human body. Each of the elements plays an important role.

Concept of cells

We can simply and easily say that platelets are red blood cells that do not have a nucleus. Such plates look like biconvex round or oblong disks. Under a microscope, you can see that such a formation looks heterogeneous in color, lighter on the periphery than in the center.

The cell size ranges from 0.002-0.006 mm, that is, they are quite small. The structure of platelets is complex and is not limited to the simple formation of a flat plate.

The lifespan of platelets is about 10 days, after which they die in the spleen or bone marrow. Platelets in the blood can live from 1 to 2 weeks, the time depends on a number of factors. The formation of red cells occurs continuously. Their classification implies division into young, mature, and old populations. Young forms are larger than older specimens.

Throughout life, the rate of production and replacement of platelets and other blood cells varies. With age, the production of stem cells slows down, there are fewer of them, and, consequently, the number of derivatives also. This is why there are different age-adjusted norms for indicators. In children, this figure is highest; in adulthood, it stabilizes and remains at an average value, and then decreases.

Platelets in a blood test with a normal value have different indicators: adults have 150-375 billion platelets per unit volume of blood, in children this amount is 150-250 billion.

Platelets are produced by red bone marrow and mature within a week. The place of formation of human platelets is the thickness of spongy, that is, non-hollow, bones. These are the ribs, pelvic bone, vertebral bodies. The mechanism of cell formation is as follows: the spongy substance produces stem cells. As is known, they do not have differentiation, that is, a tendency towards one structure or another. Under the influence of a number of factors, this cell is formed into a platelet.

The resulting platelet goes through several stages of formation:

• the stem cell becomes a colony-forming megakaryocytic unit;
• megakaryoblast stage;
• the proplatelet becomes a promegakaryocyte;
• the last stage is the platelet.

The process of plate formation looks like the “untying” of cells from a large “parent” - a megakaryocyte.

The resulting clone of platelets circulates in the blood in a free state; there is a structure where a cell depot is formed. This is necessary in order to ensure, if necessary, a certain number of cells in the right place. They are necessary until the urgent synthesis of new populations is established. Such a storage site is the spleen, release occurs by contraction of the organ.

As a percentage, about a third of the cells are stored in the spleen, and the process of platelet release from it is controlled by adrenaline.

Structure and properties of the plate

Modern technologies have made it possible to determine the structure and functions of red blood platelets. They consist of several layers, each of which represents functional areas.

When cutting the plate, it was revealed that the formation of platelets occurs with the formation of microstructures (microfilaments, tubes and organelles).

Each performs its own function:

1. The outer layer is represented by a three-layer membrane, that is, a shell. It has receptors that are responsible for adhesion to other platelets and attachment to body tissues. In order to ensure the main function of the plates, the thickness of the membrane also contains the enzyme phospholipase A, which is involved in the process of clot formation. The membrane or plasmalemma has dimples that connect to a system of channels in the thickness of the shell.
2. Under the membrane there is a lipid layer represented by glycoproteins. There are several types; they bind platelets to each other. The first type is responsible for forming bonds between the surface layers of two platelets. Next, glycoproteins enter into a reaction, ensuring further “gluing” of cells to each other. Type five allows platelets to remain stuck together for a long time.
3. The next layer is microtubules, which ensure contraction of the structure and movement of the contents of the granules outward.
4. Even deeper inside there is a zone of organelles, these are mitochondria, dense bodies, glycogen granules, etc. These components become sources of energy (ATP, ADP, serotonin, calcium and norepinephrine). Thanks to these components, wound healing becomes possible.

Microtubules and microfilaments are the cytoskeleton of cells, that is, they allow it to have a stable shape.

The characteristics of platelets allow them to provide the following properties: adhesion, activation and aggregation.

Adhesion is the ability of bodies to adhere to the wall of a damaged vessel.

This is possible due to the presence of appropriate receptors for damaged endothelium. The connection can be formed by gluing the cell to the collagen of the vessel.

Another property of a platelet is activation, which implies an increase in the area and volume of the cell to provide a larger area of ​​interaction. Additional functions of the platelet include the production and release of growth factors and vasoconstrictor components, as well as coagulation ones.

Aggregation is the ability of plates to stick to each other through fibrinogen through receptors. The reversible phase of the process is about 2 minutes. The further course of the reaction is controlled by prostaglandins and the concentration of nitric oxide to avoid excessive aggregation outside the site of damage.

Functions

Platelets are of greatest importance to the human body when bleeding occurs. What are platelets needed for?

The functions of platelets can be represented by the following list:

• The plates contain biologically active substances released after cell destruction and death. With this substance, the role of platelets is to release growth factors.

• The main function of platelets is hemostatic. To realize this, cells are grouped into large and small groups. Platelets have 12 factors that influence the blood clotting process. Most often, this need arises when there is damage that results in bleeding.
• Regenerative (with minor damage, active substances in cell granules promote healing of the vascular wall).
• Metabolism of serotonin.
• Protective (plates can capture foreign agents and destroy them through their own death).

Platelets are responsible for stopping bleeding in the body through several mechanisms:

• the body's primary reaction is the migration of platelets from the depot and peripheral blood to the site of damage, their subsequent aggregation: this causes the formation of a platelet plug;
• blood platelets contain substances (adrenaline, norepinephrine) that are released at the site of bleeding to provide a vasoconstrictor effect. This ensures that blood circulation in the affected area is limited;
• secondary hemostasis is the start of the process of fibrin clot formation at an accelerated pace.

Blood platelets accumulate at the site of vessel injury, and active substances are released from their granules. Stopping bleeding occurs not only with the participation of blood cells, but also components of the vessel wall.

They contribute to the formation of a blood clot:

• platelets become active thromboplastin;
• in the presence of this substance, a conversion occurs from prothrombin in an inactive state to thrombin;
• in the presence of thrombin, fibrinogen triggers the formation of fibrin filaments.

These reactions take place under the obligatory condition of the presence of calcium ions.

The third stage of the hemostatic process is characterized by thickening of the clot due to the contraction of actin and fibrin. Since the number of cells decreases during thrombus formation, the accumulation of thrombopoietin reminds the body that it is necessary to synthesize new plates.

A decrease in cell population is called thrombocytopenia, and an increase is called thrombocytosis. The cause of such a change is determined by the doctor individually.

The functions of platelets are most realized when stopping external and internal bleeding, although they also have a number of auxiliary purposes.

The blood platelets that are designed to fight sudden blood loss are called platelets. They accumulate in places where any vessels are damaged and clog them with a special stopper.

Appearance of records

Under a microscope, you can examine the structure of platelets. They look like disks, the diameter of which ranges from 2 to 5 microns. The volume of each of them is about 5-10 µm 3 .

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 select 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 special receptors 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 adhesion (attachment to the subendothelium) and aggregation (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 an important component is phospholipase A. It is this that forms the indicated acid necessary for the synthesis of prostaglandins. They, in turn, are intended to form thromboxane A 2, which is necessary for powerful platelet aggregation.

Glycoproteins

The structure of platelets is not limited by 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 the attachment of these blood cells to the subendothelial collagen. It ensures the adhesion of the plates, their spreading and their binding to another protein - fibronectin.

Glycoprotein II is intended for all types of platelet aggregation. It ensures that fibrinogen binds to these blood cells. It is thanks to this that the process of aggregation and contraction (retraction) of the clot continues unhindered.

But glycoprotein V is designed to maintain platelet connection. It is hydrolyzed by thrombin.

If the content of various glycoproteins in this 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 tubes are their contractile apparatus. Thus, when these plates are stimulated, the ring contracts and moves the granules towards the center of the cells. As a result, they shrink. All this causes the secretion of their contents outward. This is possible thanks to a special system of open tubules. This process is called granule centralization.

When the microtubule ring contracts, 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, and 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 functions of these cells ensure adhesion and Thus, ADP is produced when platelets attach to the walls of blood vessels, and it is also responsible for ensuring that these plates from the bloodstream continue to attach to those that have already adhered. Calcium regulates the intensity of adhesion. Serotonin is produced by the platelet when it releases granules. It is he who provides the lumen at the site of rupture.

Alpha granules located in the organelle zone promote the formation of platelet aggregates. They are responsible for stimulating the growth of smooth muscles, restoring the walls of blood vessels and smooth muscles.

Cell formation process

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 It is divided into several stages. First, a colony-forming megakaryocyte unit is formed. Over several stages, it transforms into a megakaryoblast, a promegakaryocyte and ultimately into a platelet.

Every day the human body produces about 66,000 of these cells per 1 μl of blood. In an adult, the serum should contain from 150 to 375, in a child from 150 to 250 x 10 9 / l of platelets. Moreover, 70% of them circulate throughout the body, and 30% accumulate in the spleen. If necessary, this one releases blood platelets.

Basic functions

In order to understand why blood platelets are needed in the body, it is not enough to understand the structural features of human platelets. They are intended primarily to form a primary plug, which should close the damaged vessel. In addition, platelets provide their surface to accelerate plasma coagulation 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 any damaged cells.

It is noteworthy that they can quickly and irreversibly transition 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, their lifespan ranges from 6.9 to 9.9 days. After the end of the specified period, they are destroyed. This process mainly takes place in the bone marrow, but it also occurs to a lesser extent in the spleen and liver.

Experts distinguish five different types of blood platelets: young, mature, old, forms of irritation and degenerative. Normally, the body should have more than 90% mature cells. Only in this case will the structure of platelets 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 causes bleeding that is difficult to stop. And an increase in their number causes the development of thrombosis - the appearance of blood clots. They can clog blood vessels in various organs of the body or block them completely.

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. If the activity of these cells is impaired, thrombasthenia is diagnosed.