In postnatal life and in adults

In newborn babies, growing children and adults, the red blood cells are produced only from the red bone marrow.

1. Up to the age of 5 to 6 years: The red blood cells are produced in red bone marrow of all bones.

2. From the 6th year up to the 20th year: The red blood cells are produced by red bone marrow of long bones and all the membranous (flat) bones.

3. After the age of 20 years: The red blood cells are produced from all membranous bones like vertebra, sternum, ribs, scapula, iliac bones and skull bones and from the ends of long bones. After 20 years of age, the shaft of the long bones becomes yellow bone marrow because of fat deposition and looses the erythropoietic function.

During disorders of bone, the red blood cells are produced in spleen.

FIGURE 4: Stem cells. L-Lymphocyte. R-Red blood cell. N-Neutrophil. B-Basophil. E-Eosinophil. M-Monocyte. P-Platelet

STAGES OF ERYTHROPOIESIS

The various stages between stem cell and matured red blood cell are as follows (Fig. 5):

1. Proerythroblast

2. Early normoblast

3. Intermediate normoblast

4. Late normoblast

5. Reticulocyte and

6. Matured erythrocyte.

12. 

Erythrocyte Platelets Neutrophils Eosinophil Basophil Monocyte Lymphocyte

FIGURE 5: Stages of erythropoiesis. CFU-E = Colony forming unit—Erythrocyte, CFU-M = Colony forming unit—Megakaryocyte, CFU-GM = Colony forming unit—Granulocyte/Monocyte.

RETICULOCYTE

This is otherwise known as immature red blood cell. It is slightly larger than matured red blood cell. The cytoplasm contains the reticular network or reticulum formed by remnants of disintegrated organelles. Due to the reticular network, the cell is called reticulocyte. The reticulum of reticulocyte is stained by supravital stain.

In newborn babies, the reticulocyte count is 2 to 6%, i.e. 2 to 6 reticulocytes are present for every 100 red blood cells. The number of reticulocytes is reduced during the first week after birth. Later, the reticulocyte count remains constant at or below 1 % of red blood cells. The number may increase whenever there is increased production and release of red blood cells into the circulation.

The reticulocyte is also basophilic due to the presence of remnants of Golgi apparatus, mitochondria and other organelles of cytoplasm. During this stage, the cells can enter the capillaries through the capillary membrane from source of production. The cells enter the blood through the capillary membrane by means of a process called diapedesis.

MATURED ERYTHROCYTE

Now, the reticular network disappears and the cell becomes the matured red blood cell. The matured red blood cell is biconcave and it is smaller in size with a diameter of 7.2 microns. It attains the biconcave shape. It is with hemoglobin and without nucleus.

It requires seven days for the development of matured red blood cell from proerythroblast. It takes five days for the development of reticulocyte. The reticuloctye takes two more days to become the matured red blood cell.

FACTORS NECESSARY FOR ERYTHROPOIESIS

Various substances are necessary for the development and maturation of erythrocytes. These factors are classi­fied into 3 categories, namely:

a) General factors

b) Maturation factors and

c) Factors necessary for hemoglobin formation.

According to other classification, there are 2 main erythropoiesis ways:

1) specific – only due to erythropoietin action;

2) non-specific – due to:

• vitamins;

• microelements;

• hormones.

Specific and noon-specific regulatory ways belong to erythropoiesis humoral regulation.

GENERAL FACTORS

Erythropoiesis is influenced by a variety of general factors namely:

1. Erythropoietin

2. Hormones

3. Hemopoietic growth factors

4. Colony stimulating factors and

5. Vitamins

1. Erythropoietin

This is complex polypeptide. Its amount is increased at:

• bleedings;

• low oxygen partial pressure;

• ascent to height (in the mountains);

• muscular activity.

It is also called hemopoietin or erythrocyte stimulating factor. Erythropoietin belongs to the substances with relatively slow metabolism. Its half-duration life period in blood is more than 1,5 hours. About 10% of circulating erythropoietin is released from organism wih urine. Erythropoietin daily excretion with urine comprises 0,9-4,0 Activity Units.

Chemistry

Erythropoietin is a glycoprotein containing syalic acids. Its molecular weight is 46000 Da. It is synthesized in the form of pro-erythropoietin (193 amino acids) without specific activity. It comes in plasma in unactive state wher under specific enzyme – erythrogenin – action – is transformed into active erythropoietin (it consists of 168 amino acids).

Source of Secretion

Erythropoietin is secreted by peritubular capillaries of kidneys (juxta-glomerular apparatus mainly but epitheliocytes as well), uterus, salivary glands (especially submandibular ones), monocytes-macrophages, liver (during embryogenesis). Macrophagal erythropoietin is of huge importance in erythropoiesis regulation because of central position of macrophages-monocytes in bone marrow erythroid insulas. Macrophage feeds erythroid insula with erythropoietin, ferritin, iron, vitamins and other substances. Such erythroid insulas begin their presence from the term of erythropoiesis in yolk sac.

Stimulant for Secretion

Hypoxia is the stimulant for the secretion of erythropoietin. Hypoxy is accompanied by enzymes activation in kidney structures (they are sensitive to hypoxy). Phospholipase A2 releasing leads to prostaglandins E1 and E2 increasing, than adenylatecyclase level rising up and finally cAMP concentration and activity increasing in kidney peritubular cells producing erythropoieitn. Epinephrin and norepinephrin also increase cAMP and cGMP level in kidneys.

Actions of Erythropoietin

Erythropoietin causes formation and release of new red blood cells into circulation. After secretion, it takes 4 to 5 days to show the action. The hormone promotes the following processes:

1. Production of proerythroblasts from the stem cells in CFU-E of the bone marrow.

2. Development of proerythroblasts into matured red blood cells through the normoblastic stages—early, intermediate and late normoblasts and reticulocyte.

3. Release of matured erythrocytes into blood through the capillary membrane from bone marrow. Even some reticulocytes (immature erythrocytes) are released along with matured red blood cells.

4. It stimulates synthesis of DNA-dependent RNA.

5. Rhibosomal RNA synthesis starts in 15 min after the cell contact with erythropoietin, DNA – in 2 hors, ferritin-containing proteins – in 4 hours.

6. The result of cellular metabolism change is erythroid cells proliferative and hemoglobin-synthesizing ability enforcement.

7. Increasing blood stream in vessels surrounding erythropoietic tissue in bone marrow.

8. Reticulocytes exit increasing from bone marrow sinusoids into blood.

9. Mitosis number increasing in erythroid cells row.

10. One or several mitotic cycle excluding.

It is very important that the specific erythropoietin-binding receptor structure was deshiphrated by the scientists.

Synthesis regulation

It is realized at the genetic level. The 7^th chromosome is responsible for this. Kidney structure sensitive to hypoxy represents hem-containing protein (hemoprotein) of peritubular cells binding oxygen molecule. Hemoprotein oxyform inhibits transcription of genes responsible for erythropoietin at sufficient oxygenation. On the contrary, hemoprotein deoxyform lacks its oxygen molecule, its affinity to the gene-operator is decreased and erythropoietin synthesis is activated.

Main regulator of erythropoietin production is oxygen level in blood or, if to be more exact, blood oxygen availability for tissues depending, in turn, on:

16. - oxygen level in blood;

17. - hemoglobin ability to give oxygen;

18. - tissues increased needs.

Erythropoietin production is stimulated in mountains where pO₂ is decreased in atmospheric air as well as bleeding decreasing blood oxygenous capacity.

Erythropoietin content is 0,01-0,08 IU/ml in a human being plasma. But it can rise in 1000 and more times at hypoxy. Erythropoetin has 2 forms alpha- and beta- than are differed only by carbohydrates number and possessing practically equal biological activity.