Embryonic fetal hematopoiesis and morphology of blood cells

March 27, 2018 20:45 | Diagnostics
Familiarity with embryonic hematopoiesis is of great importance for the pediatrician, because in prenatal and pathological hematology of a newborn and infants, prenatally formed morphological and functional blood elements are very common. These elements occur more often, the less time passed since birth.

Blood tissue is the youngest tissue in the human body. It occurs from the mesenchymal part of the middle germinal leaf - the mesoblast. In the development of blood tissue in humans, there are three periods: mesoblast, hepatic and medullary.

The first period - mesoblast - begins at the 2nd - 3rd week of embryonic life and ends at the end of the second or the beginning of the third month. Between the three embryonic leaves( ecto-, endo- and mesodermal), islets of mesenchymal tissue are formed, the so-called blood islands. Peripheral cells of these islets are isolated into endothelial cells and form cavities, which are the first blood vessels. Associated blood vessels are formed later. The central cells of the islets are separated from the cell group. These are the first blood germ cells( primary blast cells).In the protoplasm of primary power cells, hemoglobin is deposited and embryonic nucleus-bearing red cells( megaloblasts) appear. The morphology of the embryonic megaloblast is the same as the megaloblast of malignant anemia. With further development, the embryonic megaloblast loses the nucleus and becomes a non-nuclear megalocyte.

In the first period of hemopoiesis there are no white-cell cells.

The second period of the - hepatic - begins in a person in the second month of the womb and ends in the fifth to the sixth month. Hematopoietic tissue, which initially is in the body of the embryo wherever there is mesenchymal tissue, is now limited only to the liver.

In the hepatic period, in addition to the megaloblastic series( erythroblasts of the first generation), cells of the ordinary erythroblast series( second generation erythroblasts) appear, which gradually replace megaloblasts in the third month. At the same time, there are also myeloid cells, monocytes, megakaryocytes and platelets. Consequently, at the beginning of the third month of uterine life, only lymphocytes are absent in the blood.

In the fourth month, the thymus gland and spleen begin to play the role of hemopoietic organs, but still do not form lymphocytes. The spleen and thymus never reach the leading role in hemopoiesis. Their hematopoietic function after the fifth month is rapidly weakening.

The first lymph nodes are formed in the fourth month. At this time, there are also lymphocytes.

Beginning with the fifth month, the hematopoietic function of the liver gradually decreases and completely disappears at birth.

The third - medullary( bone marrow) or myeloid period - begins on the fifth month of embryonic life and continues throughout the life of a person.

The bone marrow is formed already by the third month of embryonic life, but the leading role is obtained only by the fifth-sixth month, gradually replacing the extramedullary hematopoietic foci.

By the nature of the formed elements, the blood is becoming more mature and, in the course of time, approaches qualitatively and quantitatively the composition of the blood of the newborn. But even after birth in the body of an infant, the tendency to return extramedullary hemorrhage and to rejuvenate blood( the appearance of erythroblasts, myelocytes, myeloblasts, megaloblasts) remains for a long time.

The number of red blood cells in the third month of embryonic life is 500,000, the fourth month is 1,000,000-2,000,000, the fifth month 2,500,000-3,500,000 and the sixth month 3,000,000-4,000,000 in mm3.The concentration of hemoglobin in the blood rises from 10 g% in the fourth month to 12-15 g% after the sixth month.

The number of leukocytes in the embryonic period varies, without special patterns, between 1600 and 1800 in mm3.

The origin of the blood cells

From what has been said above, it follows that the blood cells originate from the mesenchyme, in particular from the mesenchyme residues in the postembryonic life of the so-called reticuloendothelial system.

There was much debate about whether all blood cells originate from one common germ cell of reticulo-endothelial origin( unitary theory), or for each blood series( granulocytes, lymphocytes, erythroblasts, monocytes, megakaryocytes) there is a separate mesenchymal embryonic cell( polyphyletic theory).There is also partial polyphyletism, according to which blood cells come from two( dualism), three( triage) and more germ cells.

Now most researchers are inclined to the opinion that all blood cells are formed from one common embryonic cell of reticuloendothelial origin, hemogistioblast( Ferrate), and the differentiation of individual genera of cells occurs later.

Hemogystioblast is closely related to the rest of the elements of the reticuloendothelial system, forming from them or passing into them.

General morphology of blood cells

Blood cells, like other cells in the human body, consist of a nucleus and protoplasm. Only red blood cells - red blood cells - do not have a nucleus, due to the special function that they perform.

When determining blood cells, the following factors are important:

1) cell size;

2) cell shape;

3) the relationship between the magnitude of the nucleus and the magnitude of protoplasm( the ratio of nucleus to protoplasm);

4) the shape of the core;

5) chromatin structure of the nucleus;

6) the presence or absence of nucleoli in the nuclei;

7) the number and size of the nucleoli;

8) color of protoplasm;

9) protoplasmic structure;

10) the presence of granules in the protoplasm( their shape, color, magnitude), vacuole and phagocytic elements.

The size of the blood cells is different. Usually the younger cells are larger than the more mature ones. However, there are exceptions to this rule, for example, promyelocyte is more myeloblast;mature cells of the megakaryocyte series are more young cells.

The value of blood cells undergoes significant individual fluctuations and therefore has little diagnostic significance.

The shape of the blood cells is usually round, less often irregular( cells of the reticulum, megakaryocytes) and has no diagnostic value. Only changes in the form of erythrocytes( poikilocytes, spherocytes, ellipocytes, etc.) have an important diagnostic significance.

The core-protoplasm ratio is usually the larger the younger the cell. Especially significant is its increase in malignant cells( tumor cells, paramyeloblasts, in particular micromyeloblasts).It is due to the correlation of the size of the nucleus to the protoplasm that it is possible to diagnose the most dangerous oncological diseases in the early stages( for example, stomach cancer).

The shape of the cell nucleus is usually round, or slightly concave. Only in mature cells of the myeloid series and megakaryocyte series, the nucleus is elongated or segmented. From other cells, a tendency to nuclear polymorphism under normal conditions manifests a monocyte.

The core contains a chromatin substance. There are two types of chromatin: one is colored darker, the other is colored lighter. Both types of chromatin differ in the content of ribonucleic acids.

The younger the cell, the richer its nucleus is oxychromatin, ie, the lighter it is colored. Simultaneously with maturation of the cell, the amount of baschromatin increases, and in some mature cells( for example, in normoblasts) the nucleus consists almost exclusively of baschromatin.

In young( blast) cells, the basichromatin forms a fine fine porous mesh, located on a homogeneous light oxychromatin background. With the maturation of cells in the nodes of the grid, baschromatic thickenings are formed, which gradually increase.

In the following stages the grid is torn, basichromatic thickenings merge into one another and form various characteristic figures, depending on the type of cell.

Chromatinous lumps as the maturation of the cell more and more merge, forming in some cases, for example, in the orthochromic erythroblast, a compact, structureless dark-colored mass.

Under the pycnosis of the nucleus is meant the dark color of the chromatin and its fusion into compact masses. In pyknotic nuclei, the oxychromatin residue produces cracks, so that white spaces are visible between the dark chromatin lumps. At full pycnosis, cracks( white spaces) are absent - all chromatin merges into a dark structureless spot. Such completely structureless nuclei are found most often in orthochromic erythroblasts. Depending on the degree of pycnotic changes, pycnosis can be mild, moderate and severe.

Nucleoli. The nucleus of the germinal blood cells( blasts) contains round nucleoli - nucleoli. Mature cells do not contain nucleoli. If the size of the nucleolus exceeds a third of the diameter of the nucleus, this indicates a malignant character of the cell.

Protoplasm of young cells( blasts) is basophilic. It pale during maturation and finally, in most bloodlines, it becomes oxyphilic. The oxyphilicity of the erythroblast series is explained by the deposition of hemoglobin.

Cell protoplasm usually does not have a structure, but there are exceptions. For example, in proerythroblasts, the protoplasm is porous, and the lymphocyte has an enlightenment near the nucleus.

Some of the blood cells have vesicles in the protoplasm( vacuoles).Such are plasma cells( peripheral and bone marrow), monocyte, reticulum cells. Vacuolization of other cells is a pathological phenomenon. In pathological conditions, vacuolization may appear in the nucleus.

Some blood cells( reticulum cells, monocytes, megakaryocytes) are active phagocytes. Phagocytic cellular elements( parasites, erythrocytes, leukocytes), pigmented grains, etc. are often installed in their protoplasm.

Grit. Quite often the protoplasm of blood cells is granular. Particularly typical is the granularity of myeloid cells, so they are called granulocytes. The granularity of the myeloid series is neutrophilic, eosinophilic and basophilic, depending on its relation to acidic and alkaline inks. According to the Meu-Grunwald-Romanovsky-Giemsa neutrophilic grains are colored in violet, eosinophilic in red, and basophilic in violet-blue. In addition to mature cells of the myeloid order, also lymphocytes, monocytes and promyelocytes have granularity, which is colored according to the Meu-Grunwald-Romanovsky-Giemsa in a pink-violet color. The granularity of promyelocytes and monocytes is called azurophilic, lymphocytes are azure granularity. The azurophilic and azure granularity can not be distinguished from each other when painting according to Meu-Grunwald-Romanovsky-Giemsa. They differ in their relation to the oxidase reaction: the azurophilic grains give a positive oxidase reaction, the azuric grains give a negative oxidation reaction.
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