There are 6 steps under which a matured RBC is formed. These steps are discussed below -
1. Proerythroblast (megaloblast)
It is the first cell which is derived from colony forming unit erythroid (CFU -E). 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.
Characteristics -
a. Large in size.
b. Diameter is about 20 micron.
c. Nucleus is large and occupies the cell completely.
d. Nucleus has two or more nucleoli and a reticular network.
e. Cytoplasm present inside the cell is basophilic in nature.
f. Proerythroblast cells does not contain hemoglobin.
2. Early Normoblast
Characteristics -
a. The early normoblast cell is little smaller than proerythroblast.
b. Diameter of this cell is around 15 µ.
c. In the nucleus, the nucleoli disappear.
d. There is condensation of chromatin network and it becomes dense.
e. The cytoplasm is still basophilic in nature, so this cell is also called basophilic erythroblast.
This cell develops into next stage called intermediate normoblast.
3. Intermediate Normoblast
Characteristics -
a. Cell is smaller than the early normoblast with a diameter of 10 to 12 µ.
b. The nucleus is still intact.
c. Chromatin network is continuously condensed.
d. The hemoglobin starts appearing.
e. Cytoplasm is still basophilic.
Note - 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
Characteristics
a. Diameter of the cell decreases further to about 8 to 10 µ.
b. Nucleus becomes very small with highly condensed chromatin network so it is known as ink-spot nucleus.
c. Quantity of hemoglobin increases.
d. Nature of cytoplasm becomes almost acidophilic. So, the cell is now called orthochromic erythroblast.
Note - In the final stage of late normoblast just before it passes to next stage, the nucleus disintegrates and disappears and the process by which nucleus disappears is called pyknosis. Late normoblast develops into the next stage called reticulocyte.
5. Reticulocyte
Reticulocyte is also known as immature RBC.
Characteristics -
a. Size is slightly larger than matured RBC.
b. The cytoplasm contains the reticular network or reticulum so the cells are also known as reticulocyte.
c. Reticulocyte is basophilic due to the presence of remnants of disintegrated Golgi apparatus, mitochondria and other organelles of cytoplasm.
Note - During this stage, the cells enter the blood capillaries through capillary membrane from site of production by the process of diapedesis.
6. Matured Erythrocyte
Characteristics -
a. Reticular network disappears.
b. The cell becomes the matured RBC and attains the biconcave shape.
c. The cell decreases in size to 7.2 µ diameter.
d. The matured RBC is with hemoglobin but without nucleus.
Note - 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.
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