ERYTHROPOIESIS ( FORMATION OF RED BLOOD CELLS)

ERYTHROPOIESIS ( FORMATION OF RED BLOOD CELLS)

1. Pronormoblast 

2. Basophilic (early) normoblast 

3. Polychromatic (intermediate) normoblast  

4. Orthochromatic (late) normoblast 

5. Reticulocyte 

6. Erythrocyte  

Function of Red Blood Cells

Elimination of Red Blood Cells

ERYTHROPOIESIS

The production of red cells is regulated by the kidneys with the help of a hormone, erythropoietin. When the oxygen tension is reduced, it stimulates the pluripotential stem cell to undergo mitosis and differentiation into a blast cell called pronormoblast or proerythroblast.

 1. Pronormoblast 

The earliest recognisable erythroid cell in the marrow is the pronormoblast. It is a large round or oval cell, 15-17 microns in diameter. The nucleus is large and occupies most of the cell. It has a delicate membrane of fine chromatin, and 1 to 3 nucleoli are clearly visible. The cytoplasm stains blue and may show small, thick, rounded projections at the periphery. The pronormoblast divides by mitosis to produce basophilic normoblast.

2. Basophilic (early) normoblast 

This cell has an average diameter of 13-17 microns. The nucleus is slightly smaller than that of the pronormoblast and nucleoli may not be visible. The cytoplasm stains deep blue due to the increase in the concentration of ribonucleic acid (RNA) in the cytoplasm. The early normoblast divides by mitotic division. 

3. Polychromatic (intermediate) normoblast  

It is smaller in size, the average diameter being 11 to 15 microns. The nucleus is also reduced considerably in size and shows a dense clumping of chromatin. The cytoplasm changes colour to purple and then to grayish-pink due to gradual disappearance of RNA and formation of haemoglobin. The cell divides by mitosis only in early stage.

 4. Orthochromatic (late) normoblast 

The diameter of this cell is about 8 to 10 microns. The nucleus is very small in size and starts slowly to disappear from the cytoplasm. The cytoplasm stains pinkish as the RNA is scanty and haemoglobinisation is nearly complete.

5. Reticulocyte 

 This cell has an average diameter of 8 microns. It is an immature erythrocyte. The nucleus has completely disappeared at this stage and some remnants of ribosomes are still present, giving the cell a bluish tinge. The ribosomes may be seen as a reticular st the cytoplasm when stained supra-vitally with brilliant cresyl blue. The reticulocyte may be released into the peripheral blood.

 6. Erythrocyte  

After 24 to 48 hours in the peripheral blood, the reticulocyte matures into an adult red cell. A normal red blood cell (RBC) is a biconcave disc, with an average diameter of 7.3 microns, thicker at the edges and thinner at the centre. It is non-nucleated and contains haemoglobin. 

The whole sequence of maturation from the pronormoblast to a mature RBC takes about 3-5 days; and each RBC has a total life span of approximately 120 days. There are about 5 million erythrocytes per mm3 ( 5 x 10%) of peripheral blood in a healthy individual. Figure 2.2 shows the process of maturation of erythrocytes.

Note

All the cells described above are according to their appearance when stained with a Romanowsky stain, e.g., Leishman stain.

Function of Red Blood Cells

The main function of the RBC is to carry oxygen to the cells of the body. Haemoglobin within each RBC acts as the oxygen carrier. 

The concentration of haemoglobin in blood is thus an indication of its capacity to carry oxygen, on which all cells are absolutely dependent for life and energy. 

maturation of erthrocyte

Elimination of Red Blood Cells

At the end of its life span of 120 days, the worn-out RBC is removed from circulation by the reticulo endothelial system (RES) by a process called phagocytosis. 

The RES cells(The reticulo-endothelial cells) are located in sinusoids (small blood vessels) in the liver, spleen, bone marrow, lymph nodes, etc. The old RBCs show some changes on their surface which are recognised by the phagocytes in the RES and removed by engulfing and digesting them. All the re-usable components, such as protein and iron are recovered and re-circulated, while non-reusable components are eliminated.