Page 124 - Hematology_ Basic Principles and Practice ( PDFDrive )

C H A P T E R 9

HEMATOPOIETIC STEM CELL BIOLOGY

Marlies P. Rossmann, Stuart H. Orkin, and John P. Chute

Hematopoietic stem cells (HSCs) are characterized by their unique over a century ago. 23,24 In support of this hypothesis, a spontaneous
ability to self-renew and give rise to the entirety of the blood and zebrafish mutant, cloche (named for its bell-shaped heart because of
1–3
immune system throughout the lifetime of an individual. HSCs the loss of endothelium), lacks both vasculature and hematopoietic
are very rare cells, representing approximately one in 100,000 bone cells but no other mesodermal lineages such as cardiac progeni-
4
marrow (BM) cells in the adult. The concept of the existence of an tors. 25,26 The gene mutated in cloche was recently cloned and encodes
HSC that is capable of reconstituting hematopoiesis in vivo was first a PAS (PER-ARNT-SIM)-domain-containing basic helix-loop-helix
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introduced more than 60 years ago, when Jacobson et al demon- (bHLH) transcription factor (npas4l), which belongs to the same class
26a
strated that lead shielding of the spleen protected mice from otherwise that also includes the aryl hydrocarbon receptor and HIF-1α. Also,
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lethal γ-irradiation. Subsequently, Jacobson and colleagues demon- mice lacking FLK1 (VEGFR2, a receptor for vascular endothelial
strated that similar radioprotection of mice could be achieved via growth factor), expressed on endothelial (progenitor) cells, fail to
shielding of one femur. Shortly thereafter, it was demonstrated that develop both vascular endothelium and blood islands during embryo-
intravenous injection of BM cells also provided radioprotection of genesis. 27,28 Indeed, gene tracing studies in mouse and human
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lethally irradiated mice. Interestingly, investigators initially hypoth- embryonic stem cell cultures identified a progenitor with both
esized that the radioprotected spleen or BM provided soluble factors hematopoietic and endothelial potential. 29–31
8,9
that mediated radiation protection. However, subsequent experi- Primitive hematopoiesis encompasses the generation of primarily
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ments by Nowell et al and Ford et al critically demonstrated that large erythroid cells and primitive macrophages. 32–34 Following this
transplanted BM cells provided radioprotection directly via cellular initial wave, beginning at mouse E8.25, erythromyeloid progenitors
reconstitution of the blood system. The historical significance of these are generated as prodefinitive progenitors. 35,36 Both waves arise
studies cannot be overestimated because they provided the basis for transiently in the YS during a time comparable to the first trimester
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not only the ultimate isolation and characterization of HSCs but also in humans, but the cells lack the capacity for self-renewal and
for the field of hematopoietic cell transplantation. multilineage differentiation present in definitive HSCs. The first
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Subsequent landmark studies by Till and McCulloch demon- definitive HSCs capable of long-term, multilineage reconstitution of
strated that transplantation of limiting doses of BM cells gave rise to irradiated adult recipient mice appear at E10.5 in the intraembryonic
myeloid and erythroid colonies in the spleens of irradiated recipient region encompassing the aorta, gonads, and mesonephros (AGM), in
mice. Importantly, Till and McCulloch showed that the numbers of particular in hematopoietic intraaortic clusters in the ventral wall of
colonies detected in recipient mice was proportional to the numbers the dorsal aorta. 20,38–40 Then, within a remarkably short period of 1.5
of BM cells injected into the irradiated mice, suggesting that a par- days during embryonic development virtually all HSCs are born that
ticular population of hematopoietic cells was capable of reconstituting will replenish the hematopoietic system throughout fetal and adult
hematopoiesis in vivo. 12–14 The clonogenic nature of a subset of BM life. 41,42 Several complementary studies using lineage tracing experi-
cells was definitively shown when these investigators irradiated BM ments in both mice and zebrafish have demonstrated that within the
cells and then transplanted the cells into lethally irradiated mice. dorsal aorta, hemogenic endothelial cells (ECs) are the direct precur-
Persistent chromosomal aberrations were demonstrated in spleen sors of definitive HSCs. 42–47 In a process known as endothelial-to-
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colonies in recipient mice. It was subsequently shown that cells hematopoietic transition, HSCs bud off the hemogenic endothelium
within the spleen colonies were radioprotective of lethally irradiated to form intraaortic hematopoietic clusters from which they are
mice and contained myeloid, erythroid, and lymphoid cells. 12,16 released into circulation. Interestingly, while the AGM gives rise to
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Taken together, these data strongly suggested the presence of hema- HSCs, it is not the site of hematopoietic differentiation. Rather,
topoietic stem or progenitor cells that were capable of in vivo engraft- HSCs colonize the fetal liver where they expand and then differenti-
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ment and generation of multilineage progeny from a small number ate ( and references therein).
of parent cells. 17 Evidence from studies in mice suggests that some adult hemato-
poiesis also occurs at sites other than the AGM. By E12 the fetal liver
contains more HSCs than can be accounted for by HSCs generated
EMBRYONIC ORIGIN OF HEMATOPOIETIC STEM CELLS in the AGM alone. Quantitative analysis of HSC distribution
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showed that both YS and placenta 51,52 generate definitive HSC that
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Mammalian hematopoiesis occurs in several waves, which are sepa- migrate to the liver and other hematopoietic sites. Lastly, a c-Myb-
rated temporally and spatially and produce different cell types: a and thus HSC-independent cell lineage that emerges between E8.5
transient first “primitive” is followed by a “prodefinitive” and then a and E9.5 in the YS has recently been shown to give rise to YS mac-
“definitive” wave, which is lasting through life. 18–20 While most of the rophages and later on to tissue macrophages in brain (microglia), liver
evidence is derived from the mouse, data from humans, albeit limited, (Kupffer cells), and skin (Langerhans cells). 54
point to a very comparable hematopoietic program. 21,22
During embryogenesis, the hematopoietic cells of the first, primi- DEFINITION AND CHARACTERIZATION OF
tive wave are formed when cells from the epiblast that constitute the
prospective mesoderm ingress and migrate through the primitive HEMATOPOIETIC STEM CELLS
streak between the endoderm and ectoderm, both in the embryo
proper and in the extraembryonic yolk sac (YS). In the latter, meso- Phenotype
dermal cells aggregate to form blood islands surrounded by visceral
endodermal cells on mouse embryonic day (E) 7–7.5. The close Murine HSCs
proximity of erythroid cells and vascular endothelium in YS blood
islands, their origin from mesoderm and their simultaneous differen- The HSC is the most well-defined somatic, multipotent stem cell in
tiation led to the proposal of a common precursor, the hemangioblast, the body. With the emergence of antibody technology and flow
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