Erythropoiesis is the process of the origin, development and maturation of erythrocytes. Hemopoiesis or hematopoiesis is the process of origin, development and maturation of all the blood cells.
SITE OF ERYTHROPOIESIS
IN FETAL LIFE
In fetal life, the erythropoiesis
occurs in three stages:
1. Mesoblastic
Stage
During the first two
months of intrauterine life, the RBCs are produced from mesenchyme of yolk sac.
2. Hepatic Stage
From third month of intrauterine life,
liver is the main organ
that produces RBCs. Spleen
and lymphoid
organs are also involved in
erythropoiesis.
3. Myeloid Stage
During the last three
months of intrauterine life, the RBCs are produced from red bone marrow and liver.
IN NEWBORN BABIES,
CHILDREN AND ADULTS
In newborn babies, growing
children and adults, RBCs are produced only from the red bone marrow.
1. Up to the age of 20 years: RBCs
are produced from red bone marrow of all bones (long bones and all
the flat bones).
2. After the age of 20 years: RBCs
are produced from
membranous
bones like
vertebra, sternum,
ribs, scapula, iliac bones and skull
bones and from the ends of long bones. After 20 years of age,
the shaft of the long
bones becomes yellow bone marrow because of fat deposition and looses the
erythropoietic function. In adults,
liver and spleen may produce the blood cells if the bone marrow is
destroyed or fibrosed. Collectively bone marrow is almost equal
to liver in size and weight. It is also as active as liver. Though bone marrow
is the site
of production of all blood cells, comparatively 75%
of the bone marrow is
involved in the production of leukocytes and only 25% is involved in the production
of erythrocytes. But
still, the leukocytes are less in number than the erythrocytes, the ratio being 1:500.
This is mainly because of the lifespan of these cells. Lifespan of erythrocytes
is 120 days whereas the lifespan of leukocytes is veryshort ranging from
one to ten days. So the leukocytes need larger production than erythrocytes to
maintain the required number.
PROCESS OF ERYTHROPOIESIS
STEM CELLS
Stem cells are the
primary cells capable of self-renewal and differentiating into specialized cells.Hemopoietic stem
cells are the primitive cells in the bone marrow, which give rise
to the blood cells.
Hemopoietic stem cells
in the bone marrow are called uncommitted pluripotent hemopoietic stem cells
(PHSC). PHSC is defined as a cell that can give rise to all types of blood
cells. In early stages, the PHSC are not designed to form a particular
type of blood cell. And it is also not possible to determine
the blood cell to be developed from these cells: hence, the name
uncommitted PHSC. In adults, only a few number of these cells are present. But
the best source of these cells is the umbilical cord blood. When the cells are
designed to form a particular type of blood cell, the uncommitted
PHSCs are called committed PHSCs. Committed PHSC is defined as a cell, which is
restricted to give rise to one group of blood
cells. Committed PHSCs are of
two types:
1. Lymphoid stem cells
(LSC) which give rise to lymphocytes and natural killer (NK) cells
2. Colony forming blastocytes,
which give rise to myeloid
cells. Myeloid cells are the blood cells other
than lymphocytes. When
grown in cultures, these cells
form colonies hence the name colony forming
blastocytes. Different units of colony
forming cells are:
i. Colony forming
unit-erythrocytes (CFU-E) – Cells of this unit develop into erythrocytes
ii. Colony forming
unit-granulocytes/monocytes (CFU-GM) – These cells give rise to granulocytes
(neutrophils,
basophils and eosinophils) and monocytes
iii. Colony forming
unit-megakaryocytes (CFU-M)
– Platelets are developed
from these cells.
CHANGES DURING
ERYTHROPOIESIS
Cells of CFU-E pass
through different stages and finally become the matured RBCs. During these
stages four important
changes are noticed.
1. Reduction in size
of the cell (from the diameter of 25 to 7.2 μ)
2. Disappearance of nucleoli and
nucleus3. Appearance of hemoglobin
4. Change in the
staining properties of the cytoplasm.
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