Haemopoiesis
During foetal development, the formation of blood
cells (haemopoiesis) commences in wall of the
yolk sac. After the second month of foetal development, the liver, and,
slightly later, the spleen, become the dominant sites of haemopoiesis. From the
6th month, and dominating from the 7th month onwards, the formation of blood
cells occurs in bone marrow, which is the major site of formation of blood
cells in normal adult humans.
Yellow bone marrow, which harbours mainly
adipocytes, dominates in the hollow of the diaphysis of adult long bones.
Haemopoiesis occurs in red bone marrow, which is
typically found between the trabeculae of spongy bone in the epiphysis of adult
long bones. Both age and demands on haemopoiesis may effect the relative
amounts of red and yellow bone marrow. Haemopoietic cells surround the vascular
sinusoids and are supported by reticular connective tissue. In addition to the
endothelial cells of the sinusoids and the reticulocytes of the connective
tissue, macrophages are frequent in red bone marrow.
Haemopoietic Cells
The basis of haemopoiesis is a small population of self-replicating stem cells, which ultimately can
generate all types of blood cells. Their progeny may develop into either lymphocytic stem cells or pluripotent
haemal stem cells (colony-forming unit - stem cell - CFU-S). The latter type gives rise to stem
cells which can form the major groups of blood cells other than lymphocytes.
Depending on their progeny it is possible to differentiate
- burst-forming unit of the erythroid line
(BFU-E),
- colony-forming unit - granulocytes and macrophages (CFU-G/M), and
- colony-forming unit - megakaryocytes (CFU-Mk).
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Erythrocytes
The first identifiable
stage of erythropoiesis is the proerythroblast
- a large, slightly basophilic cell, which contains a large, lightly stained
nulceus. Proerythroblasts proliferate to generate a sequence of cells which
show a gradual decrease in size and condensation of their chromatin. They are named after changes in the staining characteristic
of their cytoplasm (basophilic erythroblast, polychromatophilic and
orthochromic normoblasts). The nucleus is finally extruded from the
normoblast. The cell enters circulation as a reticulocyte,
which still contains some organelles. Reticulocytes remain for a few
days in either the bone marrow or the spleen to mature to erythrocytes.
In some blood smears reticulocytes may be recognisable
because of a very slight basophilic staining - either homogeneous or in the
form of a basophilic stippling.
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Granulocytes
Myeloblast appear light-microscopically similar to
proerythroblast. They proliferate to generate promyelocytes.
Promyelocytes begin to accumulate nonspecific granules, but they are still able
to divide. The maturation of their progeny, the myelocytes,
is characterised by the accumulation of specific granules and changes in
nuclear morphology. Metamyelocytes have a
C-shaped nucleus.
Blood
Platelets (Thrombocytes)
are, as
mentioned above, fragments of the cytoplasm of megakaryocytes. Megakaryocytes
are very large cells (up to 160 µm in diameter), which contain very large,
highly lobulated, polyploid nuclei. Megakaryocytes are in turn the product of
the differentiation of basophilic megakaryoblasts.
Precursors of
blood cells which are usually only found in the bone marrow can be found in
peripheral blood in a variety of pathological conditions.
If a Rh-negative mother has been immunised by
erythrocytes of a Rh-positive foetus, a condition called Erythroblastosis fetalis may
develop during subsequent pregnancies. It would show itself in the foetus or
newborn by the presence of erythrocyte precursors in peripheral blood -
although other, more severe symptoms should be obvious. Chronic myeloid leukemia is
another condition - in this case showing itself by the presence of all types of
granulocyte precursors in peripheral blood.
The nomenclature employed for
haemopoietic cells (but not the number of stages recognized) is somewhat
variable across texts. Note also that these cell types refer to stages of
development along a morphologically more or less continuous spectrum.
sections of red bone marrow - H&E or a bone marrow smear - Leishman, Wright's, Giemsa or
May-Grünwald-Giemsa stains
Red Bone Marrow, rabbit - H&E
Most of the haemopoietic cells visible will be of the
erythroblastic line. The only cell type of this line which is easy to
distinguish in H&E stained sections are normoblasts.
A very condensed nucleus is seen in late (orthochromic)
normoblast. Granulocyte and erythrocyte precursors will mostly intermingle, but
may be distinguished by nuclear morphology and/or size. A bent nucleus is found
in metamyelocytes - this shape is very pronounced in the last, immature form of
neutrophils, which are also called stab or band cells. If the cell (1) is
large, with a distinct "clearing" in the otherwise pink cytoplasm and
(2) has an ovoid or slightly indented nucleus, it is likely to be a myelocyte.
Cells with large light nuclei and almost unstained cytoplasm are either
reticulocytes or macrophages.
Macrophages are frequently
associated with normoblasts, and together these cells form erythroblastic
islands. The name for macrophages in these islands, nurse cells, may tell you a
bit about their function in addition to the scavenging of the expelled nuclei.
Identify normoblasts, myelocytes and metamyelocytes and
include them in your drawing of the megakaryocyte/blast.
![thumbnail]( "Haemopoietic Cells")
Title:
Haemopoietic Cells
by:
om
at
2013-02-15T03:23:00+07:00
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Haemopoietic Cells