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
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