STAGES OF ERYTHROPOIESIS

STAGES OF ERYTHROPOIESIS

Various stages between CFU-E cells and matured RBCs are :

1. Proerythroblast

2. Early normoblast

3. Intermediate normoblast.

4. Late normoblast

5. Reticulocyte

6. Matured erythrocyte.

1. Proerythroblast (Megaloblast)

Proerythroblast or megaloblast is the first cell derived from CFU-E. It is very large in size with a diameter of about 20 μ. Its nucleus is large and occupies the cell almost completely. The nucleus has two or more nucleoliand a reticular network. Proerythroblast does not contain hemoglobin. The cytoplasm is basophilic in nature. Proerythroblast multiplies several times and finally forms the cell of next stage called early normoblast. Synthesis of hemoglobin starts in this stage. However, appearance of hemoglobin occurs only in intermediate normoblast.

2. Early Normoblast

The early normoblast is little smaller than proerythroblast with a diameter of about 15 μ. In the nucleus, the nucleoli disappear. Condensation of chromatin network occurs. The condensed network becomes dense. The cytoplasm is basophilic in nature. So, this cell is also called basophilic erythroblast. This cell develops into next stage called intermediate normoblast.

3. Intermediate Normoblast

Cell is smaller than the early normoblast with a diameter of 10 to 12 μ. The nucleus is still present. But, the chromatin network shows further condensation. The hemoglobin starts appearing. Cytoplasm is already basophilic. Now, because of the presence of hemoglobin, it stains with both acidic as well

as basic stains. So this cell is called polychromophilic or polychromatic erythroblast. This cell develops into next stage called late normoblast.

4. Late Normoblast

Diameter of the cell decreases further to about 8 to 10 μ. Nucleus becomes very small with very much condensed chromatin network and it is known as ink-spot nucleus. Quantity of hemoglobin increases. And the cytoplasm becomes almost acidophilic. So, the cell is now called orthochromic erythroblast. In the final stage of late normoblast just before it passes to next stage, the nucleus disintegrates and disappears. The process by which nucleus disappears is called pyknosis. The final remnant is extruded from the cell. Late normoblast develops into the next stage called reticulocyte.

5. Reticulocyte

Reticulocyte is otherwise known as immature RBC. It is slightly larger than matured RBC. The cytoplasm

contains the reticular network or reticulum, which is formed by remnants of disintegrated organelles. Due to the reticular network, the cell is called reticulocyte. The reticulum of reticulocyte stains with supravital stain. In newborn babies, the reticulocyte count is 2% to 6% of RBCs, i.e. 2 to 6 reticulocytes are present for every 100 RBCs. The number of reticulocytes decreases during the first week after birth. Later, the reticulocyte count remains constant at or below 1% of RBCs. The number increases whenever production and release of RBCs increase. Reticulocyte is basophilic due to the presence of remnants of disintegrated Golgi apparatus, mitochondria and other organelles of cytoplasm. During this stage, the

cells enter the blood capillaries through capillary membrane from site of production by diapedesis.

6. Matured Erythrocyte

Reticular network disappears and the cell becomes the matured RBC and attains the biconcave shape. The cell decreases in size to 7.2 μ diameter. The matured RBC is with hemoglobin but without nucleus.

It requires 7 days for the development and maturation of RBC from proerythroblast. It requires 5 days up to the stage of reticulocyte. Reticulocyte takes 2 more days to become the matured RBC.

 

FACTORS NECESSARY FOR

ERYTHROPOIESIS

Development and maturation of erythrocytes require varie -ty of factors, which are classified into three categories:

1. General factors

2. Maturation factors

3. Factors necessary for hemoglobin formation.

GENERAL FACTORS

General factors necessary for erythropoiesis are:

i. Erythropoietin

ii. Thyroxine

iii. Hemopoietic growth factors

iv. Vitamins