FACTORS NECESSARY FOR ERYTHROPOIESIS
Development and
maturation of erythrocytes require variety 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.
i. Erythropoietin
Most important general factor for
erythropoiesis is the hormone called erythropoietin. It is also called
hemopoietin or erythrocyte stimulating
factor.
Chemistry
Erythropoietin is a
glycoprotein with 165 amino acids.
Source of secretion
Major quantity of
erythropoietin is secreted by peritubular capillaries of kidney. A small
quantity is also secreted from liver and brain.
Stimulant for
secretion
Hypoxia is the stimulant for the
secretion of erythropoietin.
Actions of
erythropoietin
Erythropoietin causes
formation and release of new RBCs into circulation. After secretion, it takes 4
to 5 days to show the action.
Erythropoietin
promotes the following processes:
a. Production of
proerythroblasts from CFU-E of the bone marrow
b. Development of proerythroblasts
into matured RBCs through the several stages – early normoblast,
intermediate normoblast, late
normoblast and reticulocyte
c. Release of matured
erythrocytes into blood. Even some reticulocytes (immature erythrocytes) are
released along with
matured RBCs.
Blood level of
erythropoietin increases in anemia.
ii. Thyroxine
Being a general
metabolic hormone, thyroxine accelerates the process of erythropoiesis at many levels.
So, hyperthyroidism and polycythemia are
common.
iii. Hemopoietic
Growth Factors
Hemopoietic growth factors
or growth inducers are the interleukins and stem cell factor (steel factor).
Generally these factors induce the proliferation of PHSCs. Interleukins (IL)
are glycoproteins, which belong to the cytokines family.
Interleukins involved in
erythropoiesis:
a. Interleukin-3
(IL-3) secreted by T-cells
b. Interleukin-6
(IL-6) secreted by T-cells, endothelial cells and macrophages
c. Interleukin-11
(IL-11) secreted by osteoblast.
iv. Vitamins
Some vitamins are also necessary for
the process of erythropoiesis. Deficiency of these vitamins
cause
anemia associated with
other disorders. Vitamins
necessary for erythropoiesis:
a. Vitamin B: Its
deficiency causes anemia and pellagra (disease characterized by skin lesions,
diarrhea, weakness,
nervousness and dementia).
b. Vitamin C: Its
deficiency causes anemia and scurvy (ancient disease characterized
by impaired collagen synthesis
resulting in rough skin, bleeding gum, loosening of teeth,
poor wound healing,
bone pain, lethargy and emotional
changes).
c. Vitamin D: Its
deficiency causes anemia and rickets
d. Vitamin E: Its
deficiency leads to anemia and malnutrition.
MATURATION FACTORS
Vitamin B12, intrinsic
factor and folic acid are necessary for the maturation of RBCs.
1. Vitamin B12
(Cyanocobalamin)
Vitamin B12 is the maturation
factor necessary for erythropoiesis.
Source
Vitamin B12 is called extrinsic
factor since
it is obtained mostly from diet. Its absorption from intestine
requires
the presence of intrinsic factor of Castle. Vitamin
B12 is
stored mostly in liver and in small quantity in muscle. When
necessary, it is transported to the bone marrow to promote
maturation of RBCs. It is also produced in the large intestine by the
intestinal flora.
Action
Vitamin B12 is
essential for synthesis of DNA in RBCs. Its deficiency leads to failure in
maturation of the cell and
reduction in the cell division. Also, the cells are larger
with fragile and weak cell membrane resulting in macrocytic anemia.
Deficiency of vitamin
B12 causes pernicious
anemia.
So, vitamin B12 is
called antipernicious factor.
2. Intrinsic
Factor of Castle
Intrinsic factor of castle is produced
in gastric mucosa by the parietal cells of the gastric glands.
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