Bone marrow (see Fig. 10.5), a gelatinous, exten- sively vascular, cell-rich connective tissue, occupies the marrow cavities of long bones and the intertra- becular spaces of spongy bones and provides a microenvironment that is conducive for hematopoi- esis, formation of blood cells and platelets. in infants and young individuals, all marrow is hematopoieti- cally active, and because most of the forming cells are erythrocytes, it is known as red marrow. As an individual approaches 20 years of age, much of the marrow housed in the diaphysis of long bones accu- mulates so much fat that it becomes hematopoieti- cally inactive and is known as yellow marrow. the marrow’s vascular supply arises from:
• Arteries that enter the marrow cavity via nutrient canals
• A system of large sinusoids that eventually deliver their blood into the central longitudinal vein, which delivers its blood into many veins that exit the marrow through nutrient canals.
in contrast to most veins, the veins of the mar row are smaller than their arterial counterparts, and hydro- static pressure within the marrow is high enough to maintain the patency of the sinusoids. the marrow’s
vascular compartment comprises the blood vessels and sinusoids, and the interstices are populated by clusters of hematopoietic cells (islands of hemato- poietic cells), constituting the hematopoietic com- partment. the adluminal surfaces of the endothelial cells of the sinusoids are surrounded by a:
• Basal lamina
• Fine mesh of reticular fibers
• Adventitial reticular cells that contact the basal lamina, covering most of the sinusoidal surfaces cytoplasmic extensions of these adventitial reticu- lar cells extend away from the sinusoids and estab- lish contacts with cytoplasmic extensions of other adventitial reticular cells enclosing spaces that house hematopoietic islands (hematopoietic cords). these clusters of hematopoietic cells are present in various stages of their development but most commonly are composed only of one specific cell lineage. in addi- tion to the various maturing cells, macrophages are also present to destroy extruded nuclei, phagocytose discarded cytoplasm, and provide iron to cells of the erythrocytic series. Adventitial reticular cells control how much of the bone marrow volume is available for hematopoiesis; as they amass lipid in their cyto- plasm, they increase in size and decrease the volume of the hematopoietic compartment.
Chapter Blood and Hematopo Ies Is
10
143
Figure 10.5 Diagram of rabbit bone marrow that was injected with lithium carmine and india ink. (Modified from Fawcett DW:
Bloom and Fawcett A Textbook of Histology, 12th ed. New York, Chapman & Hall, 1994, p 238.)
Primitive reticular cell
Small lymphocyte Polychromatophilic erythroblasts
Polychromatophilic erythroblasts in
mitosis
Orthochromatophilic erythroblast
Erythrocytes Orthochromatophilic
erythroblast Heterophilic leukocyte Lining cells Migrating macrophage
Fixed macrophage
Free macrophage Basophilic
erythroblasts
Fixed macrophage (lining cell) Venous sinus
Arteriole Plasma cell Small lymphocyte
Eosinophilic myelocytes Orthochromatophilic
erythroblast Adventitial
reticular cells Megakaryocyte
Primitive reticular cell Small lymphocyte
Megakaryocyte
Chapter Blood and Hematopo Ies Is
10
144 HEMAToPoIESIS
Hematopoiesis has a prenatal and a postnatal com- ponent. Prenatal hematopoiesis begins around the 14th day of development and has four phases:
• the mesoblastic phase is initiated when blood islands form in the yolk sac; cells at the border of the blood island become endothelial cells, forming blood vessels, whereas most of the cells differentiate into erythroblasts that form nucleated RBCs.
• the hepatic phase replaces the mesoblastic phase at the end of the fifth week after fertilization; RBcs are still nucleated, and leukocytes begin to be formed at the eighth week after fertilization.
• the splenic phase begins during the fourth month of development, and the spleen and liver continue their hematopoietic function until parturition.
• the myeloid phase (bone marrow phase) starts around the sixth month of development and increases in importance as the fetus reaches
parturition; after birth, all hematopoiesis occurs in the bone marrow, although the liver and the spleen can resume hematopoiesis if necessary.
Postnatal hematopoiesis begins at birth and con- tinues throughout the individual’s life and produces an inordinate number of cells. An individual’s bone marrow manufactures and replaces 1 billion blood cells every day through the process of hematopoiesis.
Stem cells, the least differentiated of the hematopoi- etic cells, undergo cell division to form more differ- entiated cells, known as progenitor cells, which also proliferate to form precursor cells (table 10.6). stem cells and progenitor cells do not have histologic char- acteristics that differentiate them from one another.
Precursor cells can be identified as belonging to a specific cell line, however, and each cell is identified by a specific name. some precursor cells are able to proliferate, whereas others are postmitotic cells even though they continue to mature to become a circulat- ing blood cell. the process of hematopoiesis is closely monitored and controlled by cytokines and growth factors.
Chapter Blood and
HematopoIes
Is 10
145Table 10.6 CELLS OF HEMOPOIESIS
Modified from gartner lP, hiatt Jl, strum J: histology. Baltimore, Williams & Wilkins, 1988.
CFU-Ly PHSC
CFU-GEMM Stem cells
Progenitor cells
Precursor cells
BFU-E CFU-E Proerythroblast
Basophilic erythroblast Polychromatophilic
erythroblast
Orthochromatophilic erythroblast Reticulocyte Erythrocyte Mature
cells
CFU-Meg
Megakaryoblast
Megakaryocyte
CFU-Eosinophil
Myeloblast Promyelocyte
Eo. myelocyte
Eo.
metamyelocyte Eo. stab Eosinophil
CFU-Basophil
Myeloblast Promyelocyte
Ba. myelocyte
Ba.
metamyelocyte Ba. stab Basophil
CFU-GM CFU-G
Myeloblast Promyelocyte
Neutro.
myelocyte Neutro.
metamyelocyte Neutro. stab
Neutrophil
CFU-M Promocyte
Monocyte
CFU-LyT T lymphocyte
T lymphoblast
T lymphocyte
CFU-LyB B lymphocyte
B lymphoblast
B lymphocyte Ba., basophil; BFU, burst-forming unit (e, erythrocyte); cFU, colony-forming unit (e, erythrocyte); eo., eosinophil; g, granulocyte; geMM, granulocyte, erythrocyte, monocyte, mega-
karyocyte; gM, granulocyte-monocyte; ly, lymphocyte; lyB, B cell; lyt, t cell; M, monocyte; Meg, megakaryoblast; neutro., neutrophil; Phsc, pluripotential hemopoietic stem cell.
Chapter Blood and Hematopo Ies Is
10
146 Stem cells, Progenitor cells, and Precursor cells the antecedents of all blood cells and platelets are stem cells, known as pluripotential hematopoietic stem cells (PHSCs), that reside in the bone marrow, forming approximately 0.1% of the entire nucleated cells of the marrow, and resemble lymphocytes.
Although Phscs seldom enter the cell cycle, occa- sionally they experience sudden spurts of mitotic activity, forming more Phscs. Although stem cells are morphologically indistinguishable, they possess differing cell membrane markers that permit them to be recognized. Phscs differentiate into two catego- ries of stem cells (see table 10.6) known as multi- potential hematopoietic stem cells (MHSCs):
• colony-forming unit–granulocyte, erythrocyte, monocyte, and megakaryocyte cells (cFU- geMMs) are responsible for the formation of progenitor cells that give rise to the myeloid cell lines.
• BFU-E (burst-forming unit–erythrocytes) gives rise to CFU-E and then to erythrocytes.
• CFU-Meg give rise to megakaryocytes that form platelets.
• CFU-Eosinophil give rise to eosinophils.
• CFU-Basophil give rise to basophils.
• CFU-GM give rise to CFU-G and CFU-M, cells that give rise to neutrophils and monocytes.
• Colony-forming unit–lymphocyte cells (CFU- Ly) are responsible for the formation of progenitor cells that give rise to the lymphoid cells lines, cFU-lyt (t lymphocytes) and cFU-lyB (B lymphocytes).
• in contrast to stem cells and progenitor cells, precursor cells cannot regenerate themselves (i.e., they cannot produce more precursor cells), but they do possess definite histologic features permitting their identification as the predecessor of specific circulating blood cells (Fig. 10.6, see table 10.6). the first precursor cell of each cell lineage is:
• Proerythroblasts, which give rise to erythrocytes
• Megakaryoblasts, which give rise to platelets
• Myeloblasts—an exception to the rule because they are recognizable only as the precursors of neutrophils, eosinophils, or basophils
• Promonocytes, which give rise to monocytes
• Naïve T lymphocytes
• Naïve B lymphocytes
Chapter Blood and Hematopo Ies Is
10
147
cLINIcAL coNSIDERATIoNS
Myelofibrosis is a condition in which fibroblasts of the bone marrow manufacture an abundance of fibrous connective tissue, instead of producing just slender collagen fibers to support the blood vessels, sinusoids, and blood islands of bone marrow. As more and more fibrous tissue is formed, the marrow becomes heavily inundated with this fibrous material, and the volume formerly available for hematopoiesis is reduced to such an extent that erythrocyte formation is reduced, and the patient becomes anemic. WBC formation may decrease or increase, whereas platelet formation decreases. This rare disorder, affecting 1 out of 50,000 people in the United States, is usually limited to individuals 50 to 70 years old.
Frequently, this disease has no known cause (idiopathic myelofibrosis), or it may accompany other bone marrow disorders or bone marrow infections. Many patients with this disease have been exposed to ionizing radiation or benzene. In
its early stages, myelofibrosis is asymptomatic and remains so until the patient becomes so anemic that he or she experiences a decline in energy levels, loses weight, and has weakness and general malaise. If the leukocyte and platelet counts are also depressed, the individual becomes susceptible to infection, petechiae, and hematomas. Because the bone marrow is incapable of sustaining normal hematopoiesis, the spleen and liver begin to assume the function of blood cell formation, and they both increase in size, causing abdominal pain. The only way to confirm that the patient has myelofibrosis is by obtaining a bone marrow biopsy specimen.
Patients with this disorder may live for 10 years or more, but in certain cases the disease progresses rapidly (acute or malignant myelofibrosis). There is no cure for this condition, although bone marrow transplants have been successful in ameliorating the disease.
Figure 10.6 Precursor cells of the erythrocytic and granulocytic series. (From Gartner LP, Hiatt JL: Color Textbook of Histology, 3rd ed. Philadelphia, Saunders, 2007, p 240.)
ERYTHROCYTIC
EOSINOPHILIC
NEUTROPHILIC
BASOPHILIC Proerythroblast Basophilic
erythroblast Polychromatophilic erythroblast
Eosinophilic myelocyte
Myeloblast Promyelocyte
Neutrophilic myelocyte
Basophilic
myelocyte Basophilic
metamyelocyte Neutrophilic
metamyelocyte Neutrophilic stab cell
Basophilic stab cell Basophil Neutrophil Eosinophilic
metamyelocyte Eosinophilic stab cell Eosinophil Orthochromatophilic
erythroblast Reticulocyte Erythrocyte
Chapter Blood and Hematopo Ies Is
10
148 Hematopoietic Growth Factors (colony-Stimulating Factors)
certain cells of the body produce numerous gly- coproteins that stimulate hematopoiesis. these hem atopoietic growth factors (colony-stimulating fac tors) reach their target cells as endocrine hor- mones, paracrine hormones, or via cell-to-cell contact.
each of these factors stimulates a specific stem cell, progenitor cell, or precursor cell to proliferate or dif- ferentiate or both so that the level of a particular blood cell attains its normal concentration in the circulating blood (table 10.7):
• Steel factor (stem cell factor), granulocyte-
monocyte colony-stimulating factor, IL-3, and IL-7 induce Phsc, cFU-geMM, and cFU-ly to undergo mitosis to maintain their population density
• Granulocyte colony-stimulating factor,
monocyte colony-stimulating factor, IL-2, IL-5,
IL-6, IL-11, IL-12, macrophage inhibitory protein-a, and erythropoietin induce Phsc, cFU-geMM, and cFU-ly to give rise to progenitor cells (see table 10.7).
Additionally, colony-stimulating factors induce uni- potential precursor cells to form neutrophils, eosin- ophils, basophils, and monocytes; erythropoietin induces the formation of erythrocytes; and thrombo- poietin induces the formation of platelets.
steel factor, produced by stromal cells of the bone marrow, is expressed on the plasma membrane of these cells. For Phsc, cFU-geMM, and cFU-ly cells to become activated, they must contact the steel factor in the stromal cell plasmalemma. hematopoi- esis can occur only in areas where stromal cells express steel factor on their membranes. if hemato- poietic cells are not contacted by hematopoietic growth factors, they enter into apoptosis, die, and are eliminated by macrophages.
Chapter Blood and Hematopo Ies Is
10
Table 10.7 HEMATOPOIETIC GROWTH FACTORS 149
Factors Principal Action Site of Origin
stem cell factor Promotes hematopoiesis stromal cells of bone marrow
gM-csF Promotes cFU-gM mitosis and differentiation;
facilitates granulocyte activity t cells; endothelial cells g-csF Promotes cFU-g mitosis and differentiation; facilitates
neutrophil activity Macrophages; endothelial cells
M-csF Promotes cFU-M mitosis and differentiation Macrophages; endothelial cells il-1 in conjunction with il-3 and il-6, it promotes
proliferation of Phsc, cFU-geMM, and cFU-ly;
suppresses erythroid precursors
Monocytes; macrophages, endothelial cells
il-2 stimulates activated t cell and B cell mitosis; induces
differentiation of nK cells Activated t cells
il-3 in conjunction with il-1 and il-6, it promotes proliferation of Phsc, cFU-geMM, and cFU-ly and all unipotential precursors (except for lyB and lyt)
Activated t cells and B cells
il-4 stimulates t cell and B cell activation and development
of mast cells and basophils Activated t cells
il-5 Promotes cFU-eo mitosis and activates eosinophils t cells il-6 in conjunction with il-1 and il-3, promotes
proliferation of Phsc, cFU-geMM, and cFU-ly;
also facilitates ctl and B cell differentiation
Monocytes and fibroblasts
il-7 Promotes differentiation of cFU-ly; enhances
differentiation of nK cells stromal cells
il-8 induces neutrophil migration and degranulation leukocytes, endothelial cells, and smooth muscle cells
il-9 induces mast cell activation and proliferation;
modulates ige production; promotes t helper cell proliferation
t helper cells
il-10 inhibits cytokine production by macrophages, t cells, and nK cells; facilitates ctl differentiation and proliferation of B cells and mast cells
Macrophages and t cells
il-12 stimulates nK cells; enhances ctl and nK cell
function Macrophages
γ-interferons Activate B cells and monocytes; enhance ctl differentiation; augment the expression of class ii hlA
t cells and nK cells
erythropoietin cFU-e differentiation; BFU-e mitosis endothelial cells of peritubular capillary network of kidney; hepatocytes thrombopoietin Proliferation and differentiation of cFU-Meg and
megakaryoblasts Unknown
BFU, burst-forming unit (e, erythrocyte); ctl, cytotoxic t cell; cFU, colony-forming unit (eo, eosinophil; g, granulocyte; geMM, granulocyte, erythrocyte, monocyte, megakaryocyte; gM, granulocyte-monocyte; ly, lymphocyte; s, spleen); csF, colony- stimulating factor (g, granulocyte; gM, granulocyte-monocyte; M, monocyte); il, interleukin; nK, natural killer; Phsc, pluri- potential hematopoietic stem cell.
From gartner lP, hiatt Jl: color textbook of histology, 3rd ed. Philadelphia, saunders, 2007, p 242.
Chapter Blood and Hematopo Ies Is
10
150
Table 10.8 CELLS OF THE ERYTHROPOIETIC SERIES
Cell Size (µm) Nucleus* and
Mitosis Nucleoli Cytoplasm* Electron
Micrographs Proerythroblast 14–19 Round, burgundy-
red; chromatin network is fine;
mitosis
3–5 gray-blue, peripheral
clumping scant ReR; many
polysomes, few mitochondria;
ferritin Basophilic
erythroblast 12–17 same as above, but chromatin network is coarser; mitosis
1–2? similar to above but slight pinkish background
similar to above;
some hemoglobin is present Polychromatophilic
erythroblast 12–15 Round and densely staining; very coarse chromatin network; mitosis
none yellowish pink in
bluish background similar to above but more hemoglobin is present orthochromatophilic
erythroblast 8–12 small, round, dense;
eccentric or is being extruded; no mitosis
none Pink in a slight bluish
background Few mitochondria and polysomes;
much hemoglobin
Reticulocyte 7–8 none none similar to mature
RBc, but stained with cresyl blue;
display bluish reticulum
clusters of ribosomes; cell is filled with hemoglobin
erythrocyte 7.5 none none Pink cytoplasm only hemoglobin
*colors as appear using Romanovsky-type stains.
From gartner lP, hiatt Jl: color textbook of histology, 3rd ed. Philadelphia, saunders, 2007, p 246.
ReR, rough endoplasmic reticulum.
Erythropoiesis, Granulocytopoiesis, Monocytopoiesis, and Lymphopoiesis
the formation of erythrocytes, erythropoiesis, requires two forms of progenitor cells to produce 2.5
× 1011 RBcs on a daily basis. if the number of RBcs in blood is less than the normal amount, endothelial cells of the kidney’s peritubular capillary network and hepatocytes of the liver release erythropoietin.
this factor, in concert with steel factor, il-3, il-9, and granulocyte-macrophage colony-stimulating factor, stimulates cFU-geMM to differentiate into numer- ous BFU-e, which proliferate to form even more cFU-e cells. When these cells are formed, the kidney and liver cells cease the production of erythropoietin, and the low level of this factor induces the cFU-e to form proerythroblasts. the cells of the erythro- blastic series and their properties are presented in table 10.8.
the formation of granulocytes, granulocytopoie- sis, depends on cFU-gM, which gives rise to two other progenitor cells: cFU-M, responsible for the monocyte formation, and cFU-g, responsible for neutrophil formation. eosinophils and basophils arise from cFU-eo and cFU-Ba. the factors il-1, il-6, and tnF-α induce the release of the growth factors granulocyte colony-stimulating factor, granu- locyte-monocyte colony-stimulating factor, and il-5, which function in stimulating the formation of neu- trophils, eosinophils, and basophils. the first mor-
phologically recognizable cell of the granulocytic precursors is the myeloblast, and the second is the promyelocyte. neither myeloblasts nor promyelo- cytes possess specific granules, however, and all three granulocytes share these precursors. the next cell in the lineage has specific granules and can be recog- nized as a neutrophilic, eosinophilic, or basophilic myelocyte. the cells of the neutrophilic series are presented in table 10.9.
the progenitor cell of monocytopoiesis is the bipotential CFU-GM, which gives rise to the uni- potential CFU-M, from which promonocytes are derived. these give rise to monocytes that enter the circulation.
Platelets are derived from CFU-Meg, which give rise to megakaryoblasts that enlarge by undergoing endomitosis, where the cell undergoes mitosis with- out cytokinesis and gives rise to very large cells, known as megakaryocytes. these large cells lie next to sinusoids and extend their cytoplasm into the sinusoidal lumen. the cytoplasmic projections un- dergo fragmentation along demarcation channels and release proplatelets into the sinusoids. the proplate- lets disperse into individual platelets and enter the circulation.
Lymphopoiesis begins with the stem cell CFU-Ly, which gives rise to the progenitor cells cFU-lyt and cFU-lyB. these cells give rise to naïve t cells (cFU- lyt) and naïve B cells (cFU-lyB).
Chapter Blood and
HematopoIes
Is 10
151Table 10.9 CELLS OF THE NEUTROPHILIC SERIES
Cell Size (µm) Nucleus* and Mitosis Nucleoli Cytoplasm* Granules Electron Micrographs
Myeloblast 12–14 Round, reddish blue;
chromatin network is fine; mitosis
2–3 Blue clumps in pale blue setting;
cytoplasmic blebs at cell periphery
none ReR, small golgi, many mitochondria and polysomes
Promyelocyte 16–24 Round to oval, reddish blue; chromatin network is coarse;
mitosis
1–2 Bluish cytoplasm; no cytoplasmic blebs at cell periphery
Azurophilic
granules ReR, large golgi, many mitochondria, numerous lysosomes
neutrophilic
myelocyte 10–12 Flattened, acentric;
chromatin network is coarse; mitosis
0–1 Pale blue cytoplasm Azurophilic and specific granules
ReR, large golgi, numerous
mitochondria, lysosomes, and specific granules
neutrophilic
metamyelocyte 10–12 Kidney-shaped, dense;
chromatin network is coarse; no mitosis
none Pale blue cytoplasm Azurophilic and specific granules
organelle population is reduced, but granules are as above
neutrophilic band
(stab; juvenile) 9–12 horseshoe-shaped;
chromatin network is very coarse; no mitosis
none Pale blue cytoplasm Azurophilic and specific granules
same as above
neutrophil 9–12 Multilobed; chromatin
network is very coarse;
no mitosis
none Pale bluish pink Azurophilic
and specific granules
same as above
*colors as appear using Romanovsky-type stains (or their modifications).
From gartner lP, hiatt Jl: color textbook of histology, 3rd ed. Philadelphia, saunders, 2007, p 248.
ReR, rough endoplasmic reticulum.
152