The smallest cells of blood are platelets. They look like non-nuclear disks. Their diameter is only up to 4 microns, and the thickness is up to 0.75 microns. In each liter of human blood, the platelet count is between 180 and 350 x 109.
The way of the appearance of platelets
Blood plates appear during the fragmentation of the cytoplasm. They are separated from the huge cells of the bone marrow - megakaryocytes. They, in turn, appear as a result of endomitosis. After separation, 3-5 cycles occur, during which the chromosomes are doubled, but the cytoplasm is not separated. Experts call this process incomplete division.
It is important to understand that the place of formation of platelets is the red bone marrow. From it they exit and enter the bloodstream. But a third of them are immediately deposited in the spleen. This is due to a slower movement through the sinuous cords of the spleen. The remaining 2/3 circulate in the blood on average seven days. The maximum duration of their life can reach 12 days.
The process of nucleation of
plates To understand how the smallest blood cells appear, one must know not only the place of platelet formation. It is important to clarify the process of their development. So, the ancestor is megakaryoblast. This is a large cell, whose size reaches 20 microns with the nucleus. It contains nucleotides. The megakaryoblast is transformed into promegakaryocyte. This structure is prone to polymorphism of the nucleus, its cytoplasm is non-grained, basophilic.
The next stage of development is the megakaryocyte. This is a huge cell in the bone marrow, whose diameter is from 60 to 120 microns. It contains a coarse core, which can take very bizarre forms. Its cytoplasm is large in size, it is granular with inclusions of pink-violet.
The penultimate stage is the appearance of thrombocytopenic megakaryocyte. After him, the platelets are immediately isolated. They are separated from the cytoplasm of the megakaryocyte. All this takes place in the bone marrow, this is the main place of platelet formation.
Isolation of
platelets Thrombocytic megakaryocytes in the bone marrow are located on the surface of the endothelium, called the sinus. Their cytoplasmic processes pass through it. Some of them penetrate the lumen of the sinus 1-2 μm and fix the megakaryocyte on the endothelium. They serve as anchors. Other processes are long cytoplasmic bands, the length of which reaches 120 microns. They pass through the lumen of the sinus. They are called prothrombocytes. Each megakaryocyte can be from 6 to 8.
In the lumen of the sinus, a local contraction of the cytoplasm begins. As a result, it bursts and appears around 1000 individual platelets. This place of formation of platelets is not the last. This is due to the fact that prothrombocytes also enter the bloodstream. They reach the microcirculatory bed of the lungs, full platelets are released there. That is why pulmonary veins have a greater number of them than in the corresponding arteries. It is noteworthy that 7 to 17% of all thrombocytes passing through the blood stream can form in them.
Types of cells
Specialists distinguish several forms of the smallest blood cells. Separately, allocate young platelets. In comparison with mature forms, they are distinguished by a relatively large size, scanty graininess and bluish hyalomer. Their formation is affected by the increased work of the bone marrow, it is often associated with a significant loss of blood.
In appearance, the old platelets also differ. Photo under the microscope allows you to consider the presence of a narrow rim, a large number of vacuoles and granules. They are distinguished by uneven outlines, a dense granulomer, which can occupy the entire platelet. Excessive number of them indicates the appearance of a malignant tumor.
Separate the forms of irritation. These are the ligaments from the megakaryocyte that appear when the separation process is broken. Their presence indicates a blood disease. They often look like small or, on the contrary, giant blood platelets of a person.
Normally, blood should contain more than 90% of mature platelet forms. They have a discoid shape with clear boundaries, centrally located granulomera, which consists of 5-20 azurophilic grains, lilac hyalomer.
The process of vital activity
Appearing in the bone marrow, cells leave the main place of platelet formation. By the way, their production is regulated by thrombocytopoietins. They, in turn, appear not only in the bone marrow, but also in the spleen, the liver. Specialists distinguish two groups of thrombocytopoietins. They can be long or short-term. The first of them contribute to the full maturation and differentiation of megakaryocytes, while the latter enhance the cleavage of these small plates and accelerate their entry into the blood.
Thrombocytes are destroyed, photos of which can be seen at multiple magnifications, on average after a week of functioning. This process takes place mainly in the bone marrow, but these blood cells also disintegrate in the spleen and liver. Destruction takes place in special cells of the system, called macrophages.
The structure of the
plates Each mature platelet can be divided into three zones. The peripheral part is represented by a three-layer membrane. It contains receptors for collagen, serotonin, thrombin, epinephrine, ADP.On the outer part of the membrane there is a special amorphous layer consisting of factors responsible for coagulation of the blood plasma.
The average is a layer of sol-gel. It contains special channels with an outlet to the outer part of the membrane. Also, the layer contains microfilaments, which are responsible for the disk-like shape of the plates. The properties of this part of the platelets depends on how the retraction of the blood clot will take place.
A zone of organelles is also distinguished. It contains glycogen and alpha-granules, dense bodies, mitochondria.
Functions After learning how the described small particles of blood appear, many are beginning to wonder what the platelets do. They are designed to participate in hemostasis, help to repair damaged blood vessels. This is made possible by the fact that they can be attached to the damaged walls and restore them. Also, these cells are necessary to prevent bleeding and release of the body's main body fluid from the bloodstream.
These functions are performed by the properties of platelets: adhesion and aggregation. So they are called their ability to attach to the subendothelium and stick together.