Haemopoiesis (Production of Blood Cells)
During the
foetal life, production of blood cells occurs in various body tissues, but
mainly in the liver and the bone marrow. Shortly after birth, under normal
conditions, the marrow is the only tissue that continues to produce red cells,
granulocytes, monocytes and platelets. Until about 5 years of age, the marrow
in all the bones is red and cellular.
However, this red marrow is gradually replaced by yellow inactive bone marrow and by the age of 18 to 20 years the active red marrow remains only in the sternum, the ribs, the vertebrae, the skull, the pelvic bones and the proximal ends of long bones.
However, this red marrow is gradually replaced by yellow inactive bone marrow and by the age of 18 to 20 years the active red marrow remains only in the sternum, the ribs, the vertebrae, the skull, the pelvic bones and the proximal ends of long bones.
Lymphocytes
also originate in the bone marrow, but the lymphoid tissue (lymph nodes,
spleen, thymus, etc.) has a prime function in their maturation. These formed
elements of blood go through a series of developmental steps consisting of
duplication, differentiation and maturation in the marrow or the lymphoid
tissue. They are released in the circulating peripheral blood only when they
are sufficiently developed or mature.
All these blood cells have a predetermined life span. The plasma components, mainly proteins, also have a fixed rate of turnover, which is often expressed as Tso, i.e., the time during which half the substance is removed from the circulation and replaced.
The loss of cells through the normal process of aging, physiological consumption and some times by loss from the body, is normally compensated for by proliferation, maturation and release of new cells. This process is referred to as haemopoiesis.
The constituents of the blood are in a state of balance between loss, regeneration and replacement. This state of equilibrium is known as homeostasis.
MAIN CELL LINES
In the
marrow, a common pluripotential or multipotential stem cell, after a number of
cell divisions and differentiation steps, gives rise to three main cell lines.
(a)
Erythroid series
(b)
Granulocytic, monocytic, as well as lymphocytic series
(c)
Megakaryocytic series
The
mechanism by which the multipotential stem cells are induced to become stem
cells (progenitor cells) for the various cell lines is not clearly understood.
The stem cells have the capability of self-renewal and of responding to various stimuli and hormonal messages with increased production of one or the other cell line when the need arises. There are very few pluripotential stem cells in the body.
Their number is regulated and kept constant by a feedback mechanism and in a healthy individual, they are capable of supplying the blood cells according to requirements.
The stem cells have the capability of self-renewal and of responding to various stimuli and hormonal messages with increased production of one or the other cell line when the need arises. There are very few pluripotential stem cells in the body.
Their number is regulated and kept constant by a feedback mechanism and in a healthy individual, they are capable of supplying the blood cells according to requirements.
The development of a mature blood cell from a pluripotential cell takes place in three phases.
1.
Differentiation phase On receiving a stimulus, the pluripotential stem cell
divides by mitosis to produce two daughter cells. One of these cells retains
the properties of the pluripotential cell while the other starts development to
produce a blood cell as directed by the stimulus. This cell is called the blast
cell of that particular cell line, e.g., myeloblast of the leucocyte series.
2.
Proliferation phase. In this phase, the blast cell divides and re-divides to
produce a large number of immature cells. With each division, the cell
progresses towards maturity, while still increasing in number.
3. Maturation phase During this phase, each cell starts developing characteristic properties which will be necessary to perform its assigned physiological function. When the blood cells are almost mature, they are released in the peripheral blood circulation.
Figure 2.1
shows a summary of haemopoiesis.
Note
Production
of platelets is different from that of the red and the white blood cells. The
process is known as endomitosis or endoreduplication where the nuclear division
is not accompanied by cell division. As a result, the developing cell,
megakaryocyte, continues to increase in size.
ERYTHROPOIESIS
The
production of red cells is regulated by the kidneys with the help of a hormone,
erythropoietin. When the oxygen tension is reduced, it stimulates the
pluripotential stem cell to undergo mitosis and differentiation into a blast
cell called pronormoblast or proerythroblast.
ERYTHROPOIESIS ( FORMATION OF RED BLOOD CELLS)
LEUCOPOIESIS
Under normal
conditions, five types of mature white blood cells (WBCs) or leucocytes are
found in the peripheral blood. Three of these types contain granules in their
cytoplasm, and are therefore called granulocytes.
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