Figure 6-16.

Hematopoietic branching lineage tree

Multipotential stem cell Committed precursors Maturing cells

Monoblast Promonocyte Monocyte

Granulocyte- Macrophage
macrophage
Hematopoietic stem cell Peripheral tissues
CFU
Myeloblast Promyelocyte Myelocyte Metamyelocyte

Neutrophil
Myeloid stem cell Eosinophil CFU

Eosinophil
Lymphoid stem cell
Myeloblast Promyelocyte Myelocyte Metamyelocyte
Basophil CFU
Basophil
Mast cell

Myeloblast Promyelocyte Myelocyte Metamyelocyte Mast cell

precursor
Megakaryocyte CFU Peripheral tissues
T cell Platelets
B cell
precursor
precursor Megakaryoblast Megakaryocyte
Erythroid CFU Primitive/mature
progenitor
B cell basophilic
T cell erythroblast

Red
Natural killer Plasma Basophilic Polychromatophilic Orthochromatic
Proerythroblast Reticulocyte blood
cell cell erythroblast erythroblast erythroblast
cell
Thymus Bone marrow

The bone marrow consists of: (1) Hematopoietic stem cells basophils and mast cells (basophil CFUs), and eosinophils
(HSCs), multipotential cells capable of self-renewal. (2) (eosinophil CFUs). Monocytes and neutrophils derive from a
Committed precursor cells (myeloid stem cell and lymphoid common committed progenitor cell (granulocyte-macrophage
stem cell). (3) Maturing cells. Maturing cells develop from cells CFU). The lymphoid stem cell generates the B cell progeny in
called colony-forming units (CFUs). The myeloid stem cell the bone marrow and T cell progenies in the thymus. They are
gives rise to CFUs responsible for the regeneration of red blood discussed in detail in Chapter 10, Immune-Lymphatic System.
cells (erythroid CFUs), platelets (megakaryocyte CFUs),

approximate volume of 1.7 L of marrow contains 1012 rounded by a distinct population of non-hematopoi-
hematopoietic cells, producing about 1 x 109 RBCs etic stromal cells, including mesenchymal stem cells,
and about 1 x 108 leukocytes every hour. adipose cells, endothelial cells, reticular stromal cells,
The bone marrow consists of two microenviron- and macrophages (Figures 6-14 to 6-16).
mental domains, called niches: The cytokines secreted by these cells can regulate
1. The vascular niche. HSCs. The perivascular space contains extracellular
2. The endosteal niche. matrix proteins, such as type IV collagen, fibro-
Niches provide physical support, soluble factors, nectin, fibrinogen and von Willebrand factor, that
and cell-mediated interactions to regulate cell self- in conjunction with cytokines, regulate the HSC
renewal, differentiation and quiescence of hemato- population.
poietic stem cells (HSCs). The vascular niche provides a microenvironment
Under normal conditions, niches enable the for the short-term proliferation and differentiation
balanced, or homeostatic, cell self-renewal and dif- of HSCs. As discussed in Chapter 10, Immune-
ferentiation of HSCs. Under pathologic conditions, Lymphatic System, progenitors of B cells develop
such as myelodysplasia, aging or bone marrow in immune cell niches, with the participation of
malignancies, niches can alter or restrain normal osteoblasts, CAR cells (see below), reticular stromal
hematopoiesis. cells and sinusoidal endothelial cells.
The bone marrow is highly vascularized. It is sup-
The vascular niche plied by the central longitudinal artery, derived from
The vascular niche consists of blood vessels sur- the nutrient artery. Medullary capillary plexuses

Bone marrow 6. BLOOD AND HEMATOPOIESIS 197

 Figure 6-17. Erythroid lineage

Erythrocytes are the most abundant cells of the blood. They contain hemoglobin (_2`2
Pluripotent stem cell chains in the adult) and none of the typical organelles and cytomembranes is observed
in the cytoplasm. Erythrocytes have a lifespan of about 120 days and aged red blood
cells are phagocytosed by macrophages in the liver and spleen.
A lack of oxygen (hypoxia) or a decrease of erythrocytes in circulating blood (anemia;
Myeloid progenitor caused by excessive destruction of red blood cells, bleeding, iron or vitamin B12
deficiency) stimulates interstitial cells in the renal cortex to synthesize and release into
blood the glycoprotein erythropoietin (51 kd). Erythropoietin (EPO) stimulates the
Erythroid CFU early stages of the erythroid colony-forming unit (CFU) to proliferate and differentiate
into basophilic, polychromatophilic, and orthochromatic erythroblasts.

EPO

Primitive/mature basophilic
erythroblast
progenitor

Basophilic Polychromatophilic Orthochromatic Red blood

Proerythroblast Reticulocyte
erythroblast erythroblast erythroblast cell

Nucleolus
The proerythroblast is the first stage
of the red blood cell lineage that can be
recognized. It derives from a mature
progenitor following stimulation with
erythropoietin. Nucleoli are present.
The cytoplasm contains abundant free
polyribosomes involved in the synthesis
of hemoglobin.
The synthesis of hemoglobin
proceeds into basophilic,
polychromatophilic, and
orthochromatophilic erythroblasts.
As hemoglobin accumulates in the
cytoplasm, the nucleus of the
differentiating erythroblasts is reduced
in size, chromatin condenses, and free
ribosomes decrease. The
orthochromatophilic erythroblast
displays maximum chromatin
condensation.

Proerythroblasts Orthochromatic erythroblasts

and periosteal capillary plexuses are interconnected. ized endothelial cells with significant phagocytic
Medullary sinusoids drain into the central longitu- activity and a capacity to produce growth factors
dinal vein before leaving through the nutrient vein that stimulate the proliferation and differentiation
(see Figure 6-14). of hematopoietic cells.
Mature hematopoietic cells translocate through the Marrow reticular stromal cells produce hematopoi-
sinusoid wall by active transendothelial migration, etic growth factors and cytokines that regulate the
into the sinuses (see Figure 6-15) before entering the production and differentiation of blood cells.
circulation through the central vein. Immature he- Adipose cells provide a local source of energy as
matopoietic cells lack the capacity of transendothelial well as synthesize growth factors. The population of
migration and are retained in the extravascular space adipose cells increases with age and obesity and fol-
by the endothelial cells. lowing chemotherapy. Adipose cells exert a negative
The sinusoids of the marrow are lined by special- regulatory effect on HSCs function.

198 6. BLOOD AND HEMATOPOIESIS Hematopoiesis

 Figure 6-18. Erythropoietin

Oxygen-dependent prolyl hydroxylase

Low O2 tension Erythropoietin and the JAK-STAT signaling pathway
(sensor in renal interstitial cells) is inactive

Hypoxia-inducible factor 1_ 1 Erythropoietin (EPO), produced by interstitial

(transcription factor) is not hydroxylated Erythropoietin is
cells in the renal cortex, is transported to the bone
and activates the erythropoietin gene produced
marrow by the blood circulation.

2 In the bone marrow, EPO binds to the dimerized

1 Erythropoietin (EPO)
erythropoietin receptor, present in early stages of the
EPO receptor erythroid CFU progeny, and induces binding of
Plasma membrane cytosolic STAT 5 (signal transducers and activators of
JAK2 STAT 5
transcription 5) protein to JAK2 (Janus kinase 2), a
tyrosine kinase bound to the intracellular domain of the
receptor.
2 Phosphorylated
(activated) STAT 5 3 The inactive (nonphosphorylated) form of STAT 5
contains an SH2 (Src homology 2) domain. STAT 5 is
3 Recruitment of inactive recruited by JAK2 and binds to it through the SH2
STAT Inactive STAT
domain. STAT 5 becomes phosphorylated and
4 Phosphorylated homodimerizes.
SH2 domain (activated) STAT 5
homodimer 4 The phosphorylated STAT 5 homodimer

Nuclear envelope translocates into the nucleus.

Cytoplasm
5 After binding to DNA, the phosphorylated STAT 5
homodimer activates the transcription of specific genes
Nucleus
required for erythropoiesis.

DNA
5 Gene activity

Marrow macrophages remove apoptotic cells, re- migration and localization of HSCs in bone marrow.
sidual nuclei from orthochromatic erythroblasts and CAR cells, a subpopulation of mesenchymal stem
megakaryocytes, and exclude particles from entering cells, are closely associated with HSCs.
the marrow. Osteoblasts also express angiopoietin-1, a positive
regulator of HSCs, and thrombopoietin (also syn-
The endosteal niche thesized in liver and kidney) and osteopontin, that
The endosteal niche, located at the endosteum– promote HSCs quiescence by stimulating osteoblasts
bone marrow interface, consists of preosteoblasts to produce integrins and cadherins to enhance attach-
(osteoprogenitor cells), osteoblasts and osteoclasts ment of HSCs to the endosteal surface.
interacting with HSCs. Type I collagen is the most
abundant extracellular component of the endosteal Hematopoietic cell populations
niche. The bone marrow consists of three major populations
The endosteal niche is regarded as a site for long- (see Figure 6-16):
term storage of quiescent HSCs. 1. HSCs, capable of self-renewal.
Osteoblasts produce multiple hematopoietic 2. Committed precursor cells, responsible for the
cytokines, including G-CSF (granulocyte-colony generation of distinct cell lineages.
stimulating factor), M-CSF (macrophage-colony 3. Maturing cells, resulting from the differentiation
stimulating factor}, GM-CSF (granulocyte-macro- of the committed precursor cell population.
phage-colony stimulating factor), IL-1, IL-6, and HSCs can self-renew and produce two commit-
IL-7. Osteoblasts produce CXC-chemokine ligand ted precursor cells that develop into distinct cell
12 (CXCL12) with binding affinity to CXCR4 progenies:
(for chemokine receptor type 4). Perivascular re- 1. The myeloid stem cell.
ticular stromal cells, called CAR cells (for CXCL12- 2. The lymphoid stem cell.
abundant cells) are a major source of CXCL12. The Self-renewal is an important property of HSCs.
CXCL12-CXCR4 complex is a regulator of the Self-renewal preserves the pool of stem cells and is

Erythropoietin 6. BLOOD AND HEMATOPOIESIS 199

 Figure 6-19. Erythroid lineage
Basophilic cytoplasm
Proerythroblast See Figure 6-17.
Basophilic erythroblast

A large cell (12 to 16 +m in diameter) with intensely basophilic cytoplasm as an

indication of a large number of polyribosomes. The nucleus contains coarsely clumped
chromatin and nucleoli are not usually seen. This cell can divide by mitosis.
Basophilic erythroblasts derive from the proerythroblast.
Nucleolus absent

Hemoglobin
Polychromatophilic erythroblasts

These cells may range in diameter from 9 to 15 +m. The nucleus exhibits dense
chromatin patches separated by lighter areas. No nucleolus is visible. The
cytoplasm may contain clumps of polyribosomes (light-blue staining) involved in the
synthesis of hemoglobin (light pink-to-gray staining).
Polyribosomes No cell division takes place after the polychromatophilic erythroblast.
Nucleolus absent

Hemoglobin (pink
staining predominates)
Orthochromatic erythroblast

This cell is approximately 8 to 10 +m in diameter. The cytoplasm is pink, much the

same as the reticulocyte. These cells have an extremely dense (pyknotic),
eccentrically located nucleus. Orthochromatic erythroblasts are postmitotic.
Gradual reduction in cell
The transition to reticulocyte is preceded by the extrusion of the condensed nucleus
diameter and increasing
that carries with it a rim of cytoplasm. The extruded nucleus is engulfed by a
nuclear condensation
macrophage.
Eccentric pyknotic nucleus

Reticulocyte

These anucleated cells measure approximately 7 to 8 +m in diameter. The

cytoplasm is pink like the orthochromatic erythroblast. In regular preparations,
these cells appear identical to mature erythrocytes. With supravital stains, such
as methylene blue or cresyl blue, a filamentous (reticular) network of
polyribosomes becomes visible.
Reticulocytes remain in the bone marrow for 1 or 2 days and then are released
into the peripheral blood. Following 1 day of circulation, reticulocytes mature into
erythrocytes.
Residual polyribosomes

critical for feeding common myeloid progenitor and of committed precursor cells.
common lymphoid progenitor into the differentia- Myeloid and lymphoid stem cells are multipoten-
tion or maturation pathway. tial cells (see Figure 6-16). They are committed to the
HSCs are difficult to identify, mainly because formation of cells of the blood and lymphoid organs.
they represent approximately 0.05% of total hema- Five colony-forming units (CFUs) derive from the
topoietic cells (about 106 to 107 stem cells). In bone myeloid stem cell:
marrow transplantation, only 5% of the normal 1. The erythroid CFU, that produces red blood
hematopoietic stem cells are needed to repopulate cells.
the entire bone marrow. 2. The megakaryocyte CFU, that generates plate-
HSCs cannot be identified by morphology; they lets.
can be recognized by specific cell surface markers 3. The granulocyte-macrophage CFU, that pro-
(c-kit receptor and Thy-1). Instead, CD34+ com- duces monocytes and neutrophils.
mitted precursor cell populations, also containing 4. The eosinophil CFU.
CD34– HSCs, are generally used for transplantation 5. The basophil CFU, that in addition to baso-
in the clinical treatment of malignant diseases with phils, produces non-granulated mast cell precursors
chemotherapeutic agents that deplete a certain group that become granulated mast cells when recruited

200 6. BLOOD AND HEMATOPOIESIS Erythroid lineage

 Figure 6-20. Myeloid lineage

Multipotential stem cell Committed precursors Maturing cells

4 Monoblast Promonocyte Monocyte

3 Granulocyte-
macrophage Macrophage
1 Hematopoietic stem cell (HSC) CFU
Peripheral tissues
5 Neutrophilic
myeloblast Promyelocyte Myelocyte Metamyelocyte

Neutrophil

2 Myeloid stem cell

c-kit receptor
6 Eosinophil CFU
Eosinophil
Eosinophilic Promyelocyte Myelocyte Metamyelocyte
myeloblast
7 Basophil CFU
Basophil
Mast cell

Promyelocyte Myelocyte Metamyelocyte Mast cell

Basophilic
precursor
myeloblast

Bone marrow Peripheral tissues

1 A hematopoietic stem cell (HSC; c-kit positive, CD34 3 The granulocyte-macrophage CFU gives rise to the monoblast
negative) gives rise to a myeloid stem cell. and neutrophilic myeloblast.
4 Monoblasts produce monocytes leading to macrophages.
2 The myeloid stem cell produces five committed precursor
5 The neutrophilic myeloblast produces neutrophils.
cells: (1) The granulocyte-macrophage colony-forming unit
6 The eosinophilic CFU generates the eosinophil cell progeny.
(CFU). (2) The eosinophilic CFU. (3) The basophilic CFU. (4)
The megakaryocyte CFU (not shown). (5) The erythroid CFU 7 The basophil CFU gives rise to basophils and mast cell

(not shown). precursors. Mast cells mature (granulate) in peripheral tissues.

to connective tissue and mucosae (see Chapter 4, 2. Erythropoietin (Figure 6-17) and thrombopoi-
Connective Tissue). etin (Greek thrombos, clot; poietin, to make).
The lymphoid stem cell derives from the hema- 3. Cytokines (primarily interleukins).
topoietic stem cell and gives rise to T cell and B cell Colony-stimulating factors are so named because
precursors. We study the development and matura- they are able to stimulate committed precursor cells to
tion of T cells and B cells in Chapter 10, Immune- grow in vitro into cell clusters or colonies. Interleukins
Lymphatic System. are produced by leukocytes (mainly lymphocytes)
and affect other leukocytes (paracrine mechanism)
Clinical significance: Hematopoietic growth factors or themselves (autocrine mechanism).
Hematopoietic growth factors control the prolifera- Hematopoietic cells express distinct patterns of
tive and maturational phases of hematopoiesis. In growth factor receptors as they differentiate. Binding
addition, they can extend the life span and function of the ligand to the receptor leads to a conformational
of a number of cells produced in the bone marrow. change, activation of intracellular kinases, and the
Several recombinant forms are available for clinical final induction of cell proliferation (see Chapter 3,
treatment of blood disorders. Cell Signaling).
Hematopoietic growth factors, also known as he- We discuss the roles of specific hematopoietic
matopoietic cytokines, are glycoproteins produced growth factors when we analyze each cell lineage.
in the bone marrow by endothelial cells, stromal
cells, fibroblasts, developing lymphocytes, and mac- Erythroid lineage
rophages. Hematopoietic growth factors are also Erythropoiesis includes the following sequence (see
produced outside the bone marrow. Figure 6-17): proerythroblast, basophilic erythro-
There are three major groups of hematopoietic blast, polychromatophilic erythroblast, orthochro-
growth factors: matic erythroblast, reticulocyte, and erythrocyte.
1. Colony-stimulating factors. The major regulator of erythropoiesis is erythropoi-

Agranulocytes 6. BLOOD AND HEMATOPOIESIS 201

 Figure 6-21. Myeloid lineage

Cytoplasmic granules Myeloblast

are absent
Throughout the granulocytic differentiation process (the neutrophilic series is
shown), major changes occur in the structure of the nucleus and the content
of the cytoplasm. For example, in the myeloblast (10 to 20 +m; a cell usually
difficult to identify in Wright-stained preparations), the nucleus is round with
uncondensed chromatin and a visible nucleolus. As the cell progresses
through the subsequent stages of differentiation, the nucleus becomes
indented, then segmented, and the chromatin increases its condensation.
Nucleoli are present The cytoplasm of the myeloblast is essentially granule-free. Primary
granules appear in the promyelocyte stage, while specific or secondary
granules are synthesized by myelocytes.

Promyelocyte
This cell measures approximately 15 to 20 +m in diameter. It has a large,
round nucleus with uncondensed chromatin and one or more oval nucleoli.
The synthesis of primary granules, stained red or magenta, occurs
Nucleoli and exclusively at this stage. The cytoplasm is basophilic due to the presence
primary granules of abundant rough endoplasmic reticulum. Promyelocytes give rise to
are present neutrophilic, eosinophilic, or basophilic myelocytes. It is not possible in
conventional preparations to determine which type of granulocyte will be
produced by a given promyelocyte.
Golgi region
Myelocyte
This cell, measuring 12 to 18 +m, has a round or oval nucleus that may be
slightly indented; nucleoli are not present. The basophilic cytoplasm
contains primary granules produced in the promyelocyte stage as
well as some specific granules, whose synthesis is detected in the
myelocyte. Consequently, the myelocyte cytoplasm begins to resemble
Both primary and specific that of the mature basophil, eosinophil, or neutrophil. The myelocyte is
granules are seen the last stage capable of mitosis. Myelocytes produce a large number of
specific granules, but a finite number of primary granules (produced in the
Nucleoli are not present promyelocyte) are distributed among daughter myelocytes.

Golgi region
Metamyelocyte
This postmitotic cell measures 10 to 15 +m in diameter. The eccentric, bean-
shaped nucleus now contains some condensed chromatin. The cytoplasm
closely resembles that of the mature form. The specific granules outnumber
the primary granules.

Band form
Band form
This cell has a diameter of about 9 to15 +m. The nucleus is U-shaped with
rounded ends. Its cytoplasm resembles that of the mature form. Two band
form neutrophils are shown together with a myelocyte and a metamyelocyte
neutrophil.
Golgi region
The Golgi region can be distinguished in the myelocyte and
metamyelocyte.

Metamyelocyte

Myelocyte with Golgi region

202 6. BLOOD AND HEMATOPOIESIS Myeloid lineage

 Figure 6-22. Myeloid lineage: Cell types

Nucleolus
Polychromatophilic
erythroblast

Primary granule

Nucleolus
Golgi region

Promyelocyte

Band form
neutrophil

Early promyelocyte Promyelocyte

A distinctive feature of promyelocytes is the primary As promyelocytes advance in their development, primary granules become
granules (azurophilic in the neutrophilic lineage). more abundant. Promyelocytes have a diameter of 15 to 20 +m, contrasting
Several nucleolar masses can be seen within an with the much smaller band form cell (9 to 15 +m) and polychromatophilic
eccentric or central nucleus. erythroblasts (12 to 15 +m) present in the field. A nucleolus is still visible.

Nuclear lobes

Band-shaped
nucleus

Golgi region
Primary
granule

Secondary or
specific granules are Primary granule
smaller and less
dense than primary Secondary granule
granules.

Band form Polymorphonuclear neutrophil

Both primary and secondary or specific granules can be Both primary and secondary granules can be seen in the
seen in the cytoplasm of this band form neutrophil. cytoplasm of this cell displaying a multilobulated nucleus.

Myeloid lineage 6. BLOOD AND HEMATOPOIESIS 203

 Figure 6-23. Myeloid lineage: Basophil

Neutrophil contains smaller Basophil contains larger granules

cytoplasmic granules

Nucleus Cytoplasmic granules

Basophils display large cytoplasmic granules
containing substances that are released to mediate
allergic and inflammatory reactions, in particular to affect
vascular permeability.
An increase in basophils is seen in myeloproliferative
disorders. An acute nonlymphocytic leukemia with
Band form basophil basophil-like cells is associated with symptoms caused
by the release of histamine.

etin (EPO) (Figure 6-18), a glycoprotein produced can be monitored by an increase of reticulocytes in
primarily (90%) in the kidneys (juxtatubular inter- circulating blood. Reticulocytes can be identified by
stitial cells in the renal cortex) in response to hypoxia the supravital stain of residual polyribosomes forming
(a decrease in oxygen level in inspired air or tissues). a reticular network (Figure 6-19).
Renal juxtatubular interstitial cells sense oxygen Note in Figure 6-17 that polychromatophilic eryth-
levels through oxygen-dependent prolyl hydroxylase, roblasts are erythropoietin-independent, mitotically
a protein that hydroxylates the transcription factor active, and specifically involved in the synthesis of
hypoxia-inducible factor 1_ (HIF-1_) to repress the hemoglobin. Derived orthochromatic erythroblasts,
activity of the erythropoietin gene. Under conditions reticulocytes, and mature RBCs are postmitotic cells
of low oxygen tension, the hydroxylase is inactive (not involved in mitosis).
and nonhydroxylated HIF-1_ can drive the produc-
tion of erythropoietin. Leukopoiesis
Erythropoietin stimulates the proliferation of Leukopoiesis (Greek leukos, white; poietin, to make)
erythroid progenitor cells by decreasing the levels of results in the formation of cells belonging to the
cell cycle inhibitors and increasing cyclins and the granulocyte and agranulocyte series.
antiapoptotic protein BclxL. Erythropoietin is also In the current branching lineage tree model of he-
produced by neurons and glial cells in the central matopoiesis (see Figure 6-16), the myeloid stem cell
nervous system and in the retina. The administration generates the granulocytic neutrophil, eosinophil
of erythropoietin exerts a protective effect on neurons and basophil progenies, in addition to megakaryocyte
after ischemia (stroke). and erythroid progenies.
Erythropoietin synthesis in chronic renal diseases The granulocyte lineage (Figure 6-20) includes
is severely impaired. Recombinant erythropoietin can the myeloblast, promyelocyte, myelocyte, meta-
be administered intravenously or subcutaneously for myelocyte, band cell, and mature form. In the bi-
the treatment of anemia caused by a decrease in the nary lineage tree model, the granulocyte-macrophage
production of erythropoietin by the kidneys. precursor gives rise to neutrophils and monocytes.
The effectiveness of erythropoietin treatment Agranulocytes include lymphocytes and monocytes.

204 6. BLOOD AND HEMATOPOIESIS Myeloid lineage

 Figure 6-24. Origin and fate of monocytes

Monocytes are recognized by the indented nucleus. The cytoplasm Lysosomes in a promonocyte Golgi region Nucleolus
displays lysosomes that increase in number when the monocyte
becomes a macrophage. Monocytes are the largest cells found in
peripheral blood. They circulate for about 14 hours and then migrate
into tissues where they differentiate into a variety of tissue-specific
macrophages.

Monoblast Promonocyte

Bone marrow Monocyte

Blood vessel

Bone: Osteoclast Kupffer cell Alveolar Peritoneum (8%)

Skin: Langerhans cell Liver (56%) macrophage
Brain: Microglia Lung (15%) Other tissues (21%)
Spleen (red pulp)

Granulocytes as neutrophils. Eosinophil-specific granules are larger

Neutrophil and macrophage cell lines share a than neutrophil granules and appear refractile under
common precursor cell lineage: the granulocyte- the light microscope. Eosinophilic granules contain
macrophage CFU (see Figure 6-20). eosinophil peroxidase (with antibacterial activity)
Eosinophils and basophils derive from indepen- and several cationic proteins (major basic protein,
dent eosinophil and basophil CFUs. Neutrophil, and eosinophil cationic protein, with antiparasitic
eosinophil, and basophil granulocytes follow a similar activity). See Figure 6-5 for a listing of proteins as-
pattern of proliferation, differentiation, maturation, sociated with eosinophils.
and storage in the bone marrow. Details of these The basophil CFU produces basophils and mast
processes are better recognized for neutrophils, the cell precursors, a lineage specification that is regu-
most abundant granulocyte in the bone marrow lated by the expression of the transcription factors
and blood. It takes 10 to 14 days for neutrophils to GATA-binding protein 2 (GATA2) and CCAAT/
develop from early precursors, but this timing is ac- enhancer-binding protein-_ (C/EBP_). Deletion of
celerated in the presence of infections or by treatment C/EBP_ favors mast cell development, whereas
with granulocyte colony-stimulating factor (CSF) or its overexpression induces the development of the
granulocyte-macrophage CSF (see below). basophil lineage. In addition, signalling mediated
Myeloblasts, promyelocytes, and myelocytes are by STAT5 (for signal transducer and activator of
mitotically dividing cells; metamyelocytes and band transcription 5) is essential for the development of
cells cannot divide but continue to differentiate (see basophil precursors in bone marrow.
Figure 6-20). Basophils are distinguished by their large, coarse,
A typical feature of the maturation of granulocytes and metachromatic granules that fill the cytoplasm
are the cytoplasmic primary (azurophilic) and sec- and often obscure the nucleus (Figure 6-23). Like
ondary (specific) granules (Figures 6-21 and 6-22). neutrophils and eosinophils, basophils complete their
Myeloblasts are undifferentiated cells lacking maturation in bone marrow. The granules contain
cytoplasmic granules. Promyelocytes and myelocytes peroxidase, heparin, and histamine as well as kalli-
display primary granules in cells of the neutrophil, krein, a substance that attracts eosinophils. See Figure
eosinophil, and basophil series. Primary granules 6-6 for additional structural and functional features
persist as such throughout the cell differentiation of basophils.
sequence (see Figure 6-22). Secondary granules ap- Mast cells leave the bone marrow as immature
pear in myelocytes. precursor cells rather than granule-containing mature
Eosinophils exhibit the same maturation sequence cells like basophils. Mast cells are found close to blood

Myeloid lineage 6. BLOOD AND HEMATOPOIESIS 205

 Figure 6-25. Hematopoietic growth factors regulating the myeloid lineage
Multipotential stem cell Committed precursors Maturing cells

Monoblast Promonocyte Monocyte

Granulocyte-
Macrophage
macrophage
Hematopoietic stem cell CFU
M-CSF

Neutrophilic
myeloblast Promyelocyte Myelocyte Metamyelocyte

Myeloid stem cell Neutrophil

c-kit receptor
G-CSF
Stem cell factor
(c-kit ligand), Eosinophil CFU
thrombopoietin,
IL-1, IL-3, and Eosinophil
IL-6, Flt3 ligand Eosinophilic Promyelocyte Myelocyte Metamyelocyte
myeloblast
IL-5
Basophil CFU
Basophil
Mast cell

Promyelocyte Myelocyte Metamyelocyte Mast cell

Basophilic
precursor
myeloblast

GM-CSF, SCF

Hematopoietic growth factor Target cells Source Mode of action

Erythropoietin (EPO) Erythroid cell lineage Juxtatubular interstitial cells Induced by hypoxia and heart and
(renal cortex) (90%); facultative lung diseases
production

Granulocyte colony- Neutrophils Endothelial cells, fibroblasts, Induced by inflammatory cytokines

stimulating factor (G-CSF) macrophages in all organs (tumor necrosis factor-_, IL-1, and
(facultative production) IL-6) derived from monocytes

Granulocyte-macrophage colony- Neutrophils, eosinophils, Endothelial cells, T cells, Acts synergistically with EPO to
stimulating factor (GM-CSF) basophils, monocytes, and fibroblasts, and monocytes support the erythroid cell lineage, and
dendritic cells with TPO to stimulate megakaryocyte
progenitors
Thrombopoietin (TPO) Megakaryocyte progenitors Liver (50%; constitutive and Induced by inflammatory cytokines
and hematopoietic stem facultative production), kidney (especially by IL-6) and thrombocyto-
cells (constitutive production), and penia
skeletal muscle
Stem cell factor (SCF or Basophils, mast cells, and primordial Endothelial cells, fibroblasts, Acts synergistically with IL-3, TPO,
c-kit ligand) germ cells; hematopoietic stem cells and marrow stromal cells G-CSF, and other cytokines to
(in the presence of IL-3 and other stimulate hematopoietic stem cells
cytokines)
Flt3 ligand (fms-like tyrosine Hematopoietic stem cells T cells and marrow stromal Blood levels increase in pancytopenia.
kinase; structurally related to cells Acts with IL-3, IL-7, TPO, G-CSF, and
SCF and M-CSF) other cytokines to stimulate
hematopoietic stem cells

vessels and have a significant role in vasodilation dur- immunoglobulin E (Fc¡RI) and the tyrosine kinase
ing hyperemia in acute inflammation. c-kit receptor for stem cell factor.
Immature mast cells in the periphery can be Remember from our discussion in Chapter 4, Con-
identified by their expression of the receptor for nective Tissue, that there are two classes of mature

206 6. BLOOD AND HEMATOPOIESIS Regulation of hematopoiesis

 Figure 6-26. Megakaryocyte and the origin of platelets

Multilobed
nucleus

Cytoplasm

Multilobed
nucleus

Alpha
granule

Nuclear
envelope

Megakaryocyte Dense core granule Invaginated membrane system

The development and maturation of a megakaryocyte (in about 5 During the cytoplasmic maturation of a megakaryocyte,
days) are characterized by the following sequence: the cell membrane invaginates to form channels separating
1. Serial mitotic divisions (reaching a DNA content up to 128n) cytoplasmic islands about 3 to 4 +m in diameter.
without cell division, a process known as endoreduplication. As a These platelet demarcation channels eventually coalesce
result, a tightly packed, multilobed nucleus is observed. to generate proplatelets. Megakaryocytes typically rest
2. Cytoplasmic maturation, characterized by an increase in the next to bone marrow sinusoids (vascular niche) and extend
number of dense core granules, alphagranules, and a network of 10 to 20 proplatelet projections at one time between
membrane channels and tubules known as the Invaginated membrane endothelial cells into the sinusoids where they are shed.
system. S1P, bound to the S1pr1 receptor, mediates the
3. Proplatelet shedding into sinusoids of the bone marrow. extension and shedding of proplatelets.

Thrombopoietin Vascular niche

The multilobed nucleus
S1P (sphingosine 1 is extruded and
phosphate) phagocytosed by
Proplatelet shedding macrophages
into sinusoid

Preplatelet

Platelet
Sinusoid

Multilobed
nucleus The entire cytoplasm is
Invaginated Endothelial cell gradually converted
membrane system into proplatelets
Basal lamina Macrophage
continuous with the
plasma membrane

Megakaryocyte 6. BLOOD AND HEMATOPOIESIS 207

 mast cells: connective tissue mast cells (CTMCs; (M-CSF) are restricted to the monocyte lineage (see
located around blood vessels) and T-cell dependent Osteoclastogenesis in Chapter 5, Osteogenesis).
mucosal mast cells (MMCs; in the intestinal villi and Monoblasts (14 +m in diameter) are morphologi-
respiratory mucosa). CTMCs and MMCs contain cally similar to myeloblasts. The monoblast is present
subsets of metachromatic granules specifically syn- in the bone marrow and is difficult to identify with
thesized during their maturation in local tissues and certainty. The cytoplasm is basophilic and the nucleus
released upon host response to pathogens. is large and displays one or more nucleoli. The fol-
It is important to stress once more that basophils lowing cell in the series is the promonocyte.
and mast cells are associated with type 2 immunity, Promonocytes (11 to 13 +m in diameter) contain
that develops in the presence of TH2 cells, high a large nucleus with a slight indentation and uncon-
levels of immunoglobulin E and eosinophilia and densed chromatin. A nucleolus may be visualized.
in response to allergens and multicellular parasites The basophilic cytoplasm, due to polyribosomes, con-
(helminths). tains primary granules (lysosomes with peroxidase,
arylsulfatase, and acid phosphatase). The primary
Agranulocytes: Lymphocytes granules are smaller and fewer than in promyelocytes.
Lymphocytes constitute a heterogeneous population Both monoblasts and promonocytes are mitotically
of cells that differ from each other in terms of ori- active cells.
gin, life span, preferred sites of localization within Monocytes (12 to 20 +m in diameter) in the bone
lymphoid organs, cell surface markers and function. marrow and the blood have a large indented nucleus
HSCs gives rise to all hematopoietic cells, includ- found in the central portion of the cytoplasm (Figure
ing lymphocytes of the B and T cell lineage. B cells 6-24; see Figure 6-8). Granules (primary lysosomes)
mature in the bone marrow and then migrate to and small vacuoles are typical features. Lysosomes
other lymphoid organs. T cells complete their matu- contain proteases and hydrolases. Monocytes are
ration in the thymus and then migrate to specific motile in response to chemotactic signals and attach
lymphoid organs. to microorganisms, a function facilitated by special
A lymphoblast gives rise to a prolymphocyte, receptors for the Fc portion of immunoglobulin G
an intermediate stage that precedes the mature and for complement proteins coating the microorgan-
lymphocyte. B and T lymphocytes are nonphago- ism. Monocytes are active phagocytes.
cytic cells. They are morphologically similar but Macrophages (15 to 80 +m in diameter) constitute
functionally different, as discussed in Chapter 10, a population of blood monocytes. After circulating
Immune-Lymphatic System. for 20 to 40 hours, monocytes leave the blood to
Lymphoblasts (8 to 12 +m in diameter) are the enter the tissues (lungs, spleen, liver, lymph node,
precursors of the lymphocytes. A lymphoblast has peritoneum, gastrointestinal tract and bone [osteo-
an uncondensed nucleus with a large nucleolus. clasts]), where they become macrophages in response
The cytoplasm contains many polyribosomes and to local conditions.
a few cisternae of the endoplasmic reticulum (see The structural and functional characteristics of
Figure 6-7). tissue macrophages are discussed in Chapter 4, Con-
Lymphocytes (8 +m in diameter or less) contain a nective Tissue. In Chapter 11, Integumentary System,
round or slightly indented condensed nucleus. The we discuss the antigenic reactivity of monocyte-
nucleolus is not visible. The cytoplasm is moderately derived Langerhans cells in epidermis. In Chapter
basophilic and generally devoid of granules. 17, Digestive Glands, we explore the important role
of Kupffer cells in liver function, and in Chapter 10,
Monocytes Immune-Lymphatic System, we examine the phago-
Monocytes derive from the granulocyte-macrophage cytic properties of macrophages in spleen.
CFU. We have already discussed that the granulocyte-
macrophage CFU gives rise to the neutrophil lineage Pathology: Colony-stimulating factors and
and the macrophage lineage. interleukins
Under the influence of a specific CSF, each precur- G-CSF is a glycoprotein produced by endothelial
sor cell establishes its own hierarchy: the granulocyte cells, fibroblasts, and macrophages in different parts
colony-stimulating factor (G-CSF) takes the granulo- of the body. The synthetic form of G-CSF (known
cyte precursor cell into the myeloblast pathway; the as filgrastim or lenograstim) causes a dose-dependent
granulocyte-macrophage colony-stimulating factor increase of neutrophils in the blood. G-CSF is used
(GM-CSF) guides the monocyte precursor cell into for the treatment of neutropenia (neutrophil +
the monoblast pathway, leading to the production of Greek penia, poverty; small numbers of neutrophils
peripheral blood monocytes and tissue macrophages. in circulating blood) after cancer chemotherapy,
Receptors for the macrophage-stimulating factor after bone marrow transplantation, to facilitate an

208 6. BLOOD AND HEMATOPOIESIS Agranulocytes