William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
Continuous formation of the cells, corpuscles, and platelets of the blood
is necessary to keep their numbers relatively constant as they wear out
or are lost from the body. The formation is called haemocytopoiesis or
haemopoiesis for short.
A DIVISIONS OF HAEMOPOIESIS
l Myelopoiesis - formation of granular leucocytes (granulopoiesis),
monocytes (monopoiesis), erythrocytes (erythropoiesis), and platelets (thrombopoiesis).
2 Lymphopoiesis - formation of lymphocytes and plasma cells. (Plasma
cells are not normally seen in the blood.)
B SITES OF MYELOPOIESIS
l Embryonic: mesenchyme gives rise to:
- (a) blood islands in the yolk sac;
- (b) fetal liver haemopoietic tissue;
- (c) bone marrow in cavities of the developing bones;
- (d) marrow in the spleen and lymph nodes.
- (a) Red/myeloid marrow remains mostly in the axial skeleton - skull
diploë, ribs, sternum, parts of the vertebrae and pelvis.
- (b) Ectopic/extramedullary myelopoiesis may be seen in adults,
usually in disease, e.g., fibrosis of marrow, and at the sites of
myelopoiesis in the fetus.
C THEORETICAL CONSIDERATIONS OF HAEMOPOIESIS
l Granular leucocytes and RBCs are specialized end products in being
unable to divide, and living for only a few weeks. Since their numbers in the
blood stay constant, new cells must be forming from less specialized ones.
2 Bone marrow, stained as for a blood smear, has cells, construed from their
granularity, eosinophilia, nuclear morphology, etc, as members of
developmental sequences, apparently starting with a large,
undistinguished weakly basophil, primitive cell, and ending as one of the
clear-cut specialized kinds.
3 If all the primitive marrow cells multiplied and then turned into blood
cells, when the blood cells were spent, no primitive ones would exist to
replace them. Thus, the primitive cells must act as stem cells able to divide,
and with two possible fates: some to stay as primitive stem cells,
others to differentiate into special forms.
4 Since there are several specialized blood cells, are there separate,
but histologically indistinguishable, stem cells: one for each blood cell
type? - The polyphyletic theory of committed progenitors for
each lineage. Yes, but the monophyletic theory also survives,
because rare multipotent/pluripotent stem cells exist, and can
replenish the restricted stem cells, e.g., those for
5 CFU-S denotes the pluripotent cell in mouse, and forms the basis for naming
progenitor cells in humans. Colony-forming unit - spleen/CFU-S was the
cell that could give rise to an island/colony of complete haemopoiesis in the
spleen of the mouse, after splenic and other sites of haemopoiesis had been
totally destroyed by irradiation. Where, then, did the rescuing cell come
from to form the colony? The CFU-S was obtained from infant mice and injected
just after the irradiation. (A convenient human source for equivalent stem
cells is blood from the umbilical cord.)
6 All cell divisions and differentiations need controlling growth factors
(cytokines), not only to maintain the stem cell population, but to persuade
some of them to fill precisely the ranks of the various blood cells.
7 After a stem cell becomes a committed precursor/progenitor for a
certain cell line, a period elapses when histology, without immunostaining,
cannot identify the line. Later, perceptible morphological changes make the cell a
recognizable precursor, say a pro-erythroblast. Thereafter, the development
of the cell is divided into named stages, each based on a significant change
in appearance from the previous stage.
The potential for confusion exists, since workers have differed in the number
of stages chosen, e.g., omitting pre-stages, and their names for a given cell
type, e.g., rubriblast/normoblast for erythroblast.
8 The ability of the few stem cells to divide does not preclude proliferation
by committed precursors, and by cells at later, recognizable, stages of
development, for continued amplification of cell numbers.
9 Fig. 8 omits: how the early elements shown match the committed
precursor-recognizable precursor classification; typical population figures
for each cell kind; how recognizable the kinds are; and the controlling
factors for proliferation and differentiation (G below).
Fig. 8 Pathways of blood cell differentiation - Stained marrow-smear view
. Haemocytoblast/Pluripotent stem cell
.~ / | \ \
H .~ / | \ \
A .~ / | \ \
E .~ / | \ \
M Lymphoblast Monoblast Myeloblast Pro-erythroblast Megakaryoblast
O | | | | |
P | | | | |
O | | | | |
I | | | Basophil |
E | | Pro-myelocyte Erythroblast |
T | | | | |
T | | | | |
I | | | | |
C | | | Polychromatic |
T | | Myelocyte Erythroblast |
| | | | |
T | | | | |
I | | | | |
S | | | | |
S | | Metamyelocyte Orthochromatic |
U | | | Erythroblast |
E | | | | |
S | | Band granulocyte Reticulocyte Megakaryocyte
- - - - -| - - - - - - | - - - - - - -| - - - - - - - - - | - - - - - - - - | - - - - -
B | | | | |
L | | | | |
O Lymphocyte Monocyte Granulocyte Erythrocyte Platelets
O | | |
D | | |
- - - - -| - - - - - - | - - - - - - -| - - - - - - - - - - - - - - - - - - - - - -
C | | Tissue
T Plasma cell Macrophage Granulocyte
D CHANGES IN DEVELOPING BLOOD CELLS
l Large, weakly basophilic pro-erythroblast increases the free
ribosomes in its cytoplasm to become a basophil erythroblast.
2 Cell size decreases, and organelles are lost.
3 Nucleus, initially large and pale, with nucleoli, gets smaller and
stains more darkly.
4 Cytoplasm acquires haemoglobin at the expense of ribosomal
ribonucleprotein (RNP) - thus its staining affinity changes from basophilia
to acidophilia; the mixed-hued halfway stage is the
5 Small cell, with orange cytoplasm and a round dark nucleus, is the
6 Nucleus, in a little cytoplasm, is extruded for phagocytosis.
7 Reticulocyte/polychromatophil erythrocyte is an RBC that is released
into the blood still with RNP in its cytoplasm. Supravital staining with
brilliant cresyl blue causes this material to clump as a blue network
(reticulum) in around 2 per cent of the RBCs of normal blood.
l Myeloblast/granuloblast develops into a
2 promyelocyte synthesises non-specific azurophil granules (lysosomes)
in the cytoplasm, and with its nucleus getting smaller and darker.
3 Myelocyte, after a pause, then makes additional granules specific
for one of the three kinds of granulocyte in their staining affinity.
4 Nucleus elongates and indents, and chromatin becomes coarser, giving the
metamyelocyte (now unable to divide).
5 More granules form and the nucleus becomes sausage-shaped - band/juvenile
granulocyte. Then the nucleus starts segmenting, as the cell becomes the mature granulocyte.
l Haemocytoblast enlarges to become a megakaryoblast.
2 The nucleus experiences several rounds of DNA replication, but each time
with reassembly of a single nuclear envelope and no segregation into
separate nuclei. Thus the nucleus takes on a distinctive lumpy, polyploid form.
(The single, large, lumpy nucleus is the criterion for distinguishing megakaryocytes from
nearby osteoclasts in bone sections.)
3 Fine cytoplasmic azurophil granules accumulate as the cell becomes a very
large granular megakaryocyte.
4 Many paired membranes of smooth ER (demarcation membranes) appear
and contribute plasmalemma to the formation of
5 pseudopodia, which are extended into the lumen of a sinusoid,
where they cast off in the blood as platelets.
6 Megakaryocyte cytoplasm might also serve as a transcellular migration
pathway for some new leucocytes passing from the marrow into the blood.
4 Agranular leucocytes
l In developing, they do not become so strikingly different from their stem
cells as do granulocytes and RBCs.
2 Monocytes form from monoblast/pre-monocyte precursors in bone marrow.
3 Lymphocytes develop from lymphoblasts in bone marrow and lymphoid organs.
4 Some circulating lymphocytes appropriately stimulated can also
E BONE MARROW
l The naked-eye appearance of fresh, unstained marrow may be red from
many developing RBCs, or yellow from mainly fat cells.
2 Red marrow has many elements, see
3 Microscopic methods for marrow include sections, and smears of
aspirated sternal marrow stained with a blood stain.
- (a) Blood sinusoids are lined by endothelial cells on an
incomplete BL. Collagen fibrils (reticular fibres) support these,
- (b) adventitial stromal/reticular cells, similar to fibroblasts,
but extending processes between, and greatly influencing, the haemopoietic
- (c) Macrophages cleanse blood, and detect and destroy worn-out
RBCs and other elements. The iron recovered is stored, combined with
protein as ferritin granules, before release to the labile pool and reuse.
- (d) Blood cells develop extravascularly, are stored, then
released through the sinusoidal wall into the circulation.
- (e) Megakaryocytes form and release platelets.
- (f) Fat cells are present, large and empty of fat in embedded
- (g) Bone surface cells act as an enclosing sac for the marrow.
F CELL COMPARTMENTS, POPULATIONS AND KINETICS
l Although the marrow smear picture appears static, it represents part of a
dynamic system of cell populations. For example an RBC may be: (a) in the
marrow developing, (b) in the marrow being stored, or (c) circulating in the
blood. The population of each compartment at any time represents the balance
between the numbers entering and leaving.
2 Most cells of the RBC sequence/erythroid complex/erythron are circulating;
whereas the granular leucocyte system has most of the cells developing or
being stored in the marrow, and only a minority in circulation. Many of these
leucocytes in the vessels hug the wall as a barely moving, marginated reserve.
3 Some factors affecting blood cell formation
l Bacterial infection increases the number of circulating granulocytes
(a leucocytosis) and their rate of formation.
2 Erythropoietin is a humoral factor, released from the kidney in
response to hypoxia, that increases RBC production. Thrombopoietin
controls platelet formation, but has multiple sources, including liver
3 RBC formation requires adequate dietary elements, e.g., folic acid,
iron, vitamin B12.
4 Androgenic steroid hormones stimulate erythropoiesis.
5 Stromal cells release cytokines and, with the matrix, create a
microenvironment favourable for haemopoiesis.
G . A LITTLE CLINICAL HAEMATOPOIESIS
The scheme below depicts the cell types and early events in haemopoiesis,
detected mostly by cell-surface antigens. The usual histological chart
telescopes these events and, instead, concentrates on later visible changes,
which lie on the right side of this Figure, and are under the control of the
last-acting specific factors such as EPO and IL-5. (Growth factors causing
progress in a stage of a lineage are in italics or a symbol, e.g., *.)
Fig. 9 Haematopoiesis
LP - - - - - - - - - - - - - - - - T lymphocytes
/ BP - - - - - - - - - - - - - - - - B lymphocytes
PSC----CSC Meg-CSF TPO
^ | #\ CFU-Meg - - - - - - MegaK - - - - - - Platelets
|__| @\ /
*\ **/ ** EPO
\ / - - - BFU-E - - - - - CFU-E - - - - - Red blood cells
~ \ M-CSF
~ \ CFU-M - - - - - - Monocytes
~ \** /
~ \ ** /
~ \ / ** G-CSF
~ \CFU-GM - - - - CFU-G - - - - - - Neutrophils
IL-3~ ** \
Key ~ \ IL-5
# SCF ~ \ CFU-Eo - - - - - Eosinophils
@ IL-6 ~
* IL-1 ~
** IL-3 & ~ IL-3
GM-CSF ~ CFU-Mast - - - - - -Basophils
PSC Pluripotent stem cell
CSC Committed stem cell
MPC Myeloid progenitor cell; LPC Lymphoid progenitor cell
CFU Colony-forming unit; BFU-E Burst-forming unit-erythroid
CFU-GM Colony-forming unit-Granulocytes & Monocytes;
CFU-M Colony-forming unit-Monocyte, etc.
The above diagram is modified from Kenneth Kaushansky's Fig.
1 in Proteins1992;12:1-9, with the kind permission of John Wiley &
Sons, Inc, New York, holders of the 1992 copyright ©, and of the author
[ The ideas and terminology of the diagram are up-to-date. A full 1998
version would include only more growth factors and separate lineages for
natural killer and dendritic cells.]
1 Tasks & solutions
- To keep the process going - stem cells
- To arrive at precise numbers of special cell types - proceed in steps
that can be separately controlled, and give precursor cells some options
of what they are going to turn into.
- To control the necessary events of cell division, choice, specialization
/differentiation, and migration into the blood - cytokines and certain hormones
- collectively called haemopoietic growth factors
- To make the cells responsive to needs, e.g., bacterial infection (more
neutrophils), low O2 (more RBCs) - use feedback from
defensive reactions or in hormonal loops
2 Ideas of haemopoiesis
.. The controlling microenvironment, with stromal cells;
.. self-renewal of a cell population;
.. cell differentiation;
.. restricted versus wide-ranging potential/competence for differentiation;
.. progression through stages of differentiation;
.. lineage or sequence of precursors, as the ancestry of a particular cell type;
.. early versus late events and controls;
.. cascades and combinations of factors (signals);
.. colonies of cells grown in culture, thought to mimic clusters/nests in marrow;
.. the clonal colony derived from one original cell.
Lots of them. Welcome to medicine! Here the difficulties are that:
- The same letter means more than one thing, e.g., M - myeloid, monocyte; C - cell, committed, colony;
P - pluripotent, progenitor/precursor.
- Where a factor causes selective differentiation, the convention is to put
the abbreviation of the target cell type first, before the one specifying that
it is a growth factor, e.g., M-CSF.
- Cell-type abbreviations are combined, where the factor acts on less
specialized, i.e., multipotent progenitors, e.g., GM-CSF for a factor
stimulating differentiation to a precursor of neutrophils (and eosinophils)
- 'Unit' (U) is a term for 'cell'.
4 Feasible clinical uses for growth factors
Erythropoietin - to combat anaemia from renal disease
.. to boost marrow performance after chemotherapy
.. to help injected stem cells or grafted marrow 'take' and perform
.. to restore PMN numbers in AIDS, certain anaemias, and neutropenia
.. as a differentiation therapy for leukaemia
5 Histological accounts
1 Focus on the very late events, discernible with conventional stains of smears,
(nuclear and cell-size changes, acquiring granules, etc).
2 Can be wrong, e.g., too closely relating basophil formation to that of
3 Leave vague the early events that clinicians need to know for untangling
types of leukaemia, stimulating greater numbers of a deficient cell type,
and finding stem cells to transplant, instead of marrow.
However, haematologists and pathologists use extensively the knowledge and
terms, e.g., reticulocyte, band cell, etc, derived from the simple and available techniques of histology.