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268 Part IV: Molecular and Cellular Hematology Chapter 18: Hematopoietic Stem Cells, Progenitors, and Cytokines 269

resulted in slower marrow recovery but ultimately greater levels of HSC that marrow cells (especially macrophages) can fuse with other cells,
expansion than did infusion of larger numbers of cells. These results and spontaneous in vitro fusion of embryonic stem cells with mar-
were interpreted to suggest that the more rapid recovery of marrow row-derived cells yields hybrids that display stem cell function 296,297 ;
function associated with the administration of a larger marrow inocu- further experimentation is required to prove or disprove the concept
lum, with its increased numbers of stem cells, prematurely shut down of HSC plasticity, a proof that will have far reaching implications for
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HSC expansion, calling attention to an extrinsic regulatory mechanism. regenerative medicine.
Moreover, the differences in expansion capacity amongst fetal and adult
stem cells might also reflect the influence of extrinsic factors. When HEMATOPOIETIC PROGENITORS
adult human marrow cells are transplanted, they retain their stem
cell capacity only if quiescent at the time of transfer. In contrast, fetal The loss of one or more developmental potentials of the HSC results in
liver and cord blood stem cells contribute to long-term hematopoiesis a progenitor committed to any number of specific hematopoietic cell
regardless of the phase of the cell cycle in which they reside at the time lineages. Besides the loss of pluripotency, committed hematopoietic
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of harvest. It is postulated that this latter property of fetal stem cells progenitors display a number of characteristics that differ from their
depends on the fetal hematopoietic microenvironment, making it likely parents, including the lack of capacity for self-renewal, a higher frac-
that extrinsic factors play the key role in the decision to self-renew or tion of cells traversing the cell cycle, reduced ability to efflux foreign
differentiate. Strong experimental evidence has been generated indicat- substances, and a change in their surface protein profile. On the genetic
ing that stem cell numbers in an individual are governed by the number level, the transition of HSCs to committed progenitors is marked by the
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of hematopoietic niches. This data strongly suggests that, at the very downregulation of a large number of HSC-associated genes and pro-
least, the availability of stem cell niches places an upper limit upon HSC gressive upregulation of a limited number of lineage-specific genes.
expansion. This section highlights some of the features of specific lineage-commit-
Clues from the developmental biology of lower organisms may ted progenitors that allow for their purification, characterization, and,
also shed important insights into the mechanisms that regulate the deci- potentially, their manipulation for therapeutic benefit. Details of the
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sion between symmetric and asymmetric HSC divisions. Within the morphologic, biochemical, and genetic aspects of the differentiation of
niche of developing Drosophila gonadal tissue exist hierarchies of cells. each of these progenitors is found in the chapters corresponding to their
When female gonadal stem cells divide, the cell directly contacting the mature blood cell types.
niche supportive cells remains a stem cell, the daughter that loses con-
tact differentiates and initiates oogenesis. A similar niche architecture PROGENITOR CELL ASSAYS
also sets the stage for gonadal stem cell retention in the fly testis and Assays for most hematopoietic progenitor cells consist of marrow or
in many tissues of many organisms. The developing principle is that a (occasionally) blood cells, either unfractionated or purified to varying
stem cell in contact with the stem cell determining niche stromal cell, degrees, a semisolid support (either methylcellulose or agar, which pre-
or residing in a region of the niche possessing the highest concentration vents cellular migration), and a source of hematopoietic growth factors.
of a stem cell–determining soluble factor, will remain a stem cell, and The cultures are incubated in a humidified environment at 37°C for 2 to
those removed from contact or soluble factor will differentiate. In such 7 days for murine cells, or 5 to 14 days for human cells, during which
a niche, the axis of stem cell division then determines cell fate; if the axis time the vast majority of the cells that began culture as mature blood
of cell division is parallel to the front of stem cell–determining contact cells die, allowing the few hematopoietic progenitors present to prolifer-
or soluble mediator gradient, the proximal cell will remain a stem cell ate and differentiate into mature blood cells. As the cells in such culture
while the distal cell differentiates; if the axis of cell division is perpen- systems are immobilized by the semisolid supporting matrix, all of the
dicular, both cells will remain under the influence of the “stemness” fac- progeny in the resultant colonies are derived from a single progenitor,
tor(s), and remain stem cells. Consequently, spindle-polarizing signals allowing one to retrospectively determine the developmental capac-
could be responsible for the fate of the daughters of stem cell division, ity of that cell, termed a colony-forming cell (CFC) or unit (CFU). The
a focus of much research, but at present, few established mechanisms.
requirement for a source of hematopoietic growth factors was initially
fulfilled by using cellular underlayers containing fibroblasts, lympho-
STEM CELL PLASTICITY cytes, or monocytes, or tissue culture medium conditioned by a variety
of normal and neoplastic cellular sources, but essentially all the req-
A remarkable observation has been repeatedly made in patients who uisite growth factors are now available in purified recombinant form.
underwent sex-mismatched (male into female) marrow transplanta- Despite substantial progress in our understanding of the developmental
tion, suffered subsequent organ damage, and were carefully studied requirements of committed hematopoietic progenitors, we still do not
until the time of their death. In such settings, Y chromosome-bearing have an adequate in vitro colony-forming assay for some well-charac-
cells were identified at the site of repair of previous myocardial infarc- terized hematopoietic progenitor cells (e.g., those committed to the T
tions, strokes, and other organ damage. These observations suggest that lymphocytic or natural killer [NK] cell lineages) that still require more
hematopoietic cells can contribute to the replacement of damaged cells complex assays (e.g., fetal thymus explant assay).
of multiple organs. More direct experimentation has lent additional sup-
port to this idea; several investigators have found that marrow cells are CHARACTERISTICS OF SPECIFIC PROGENITOR
capable of giving rise to cells of multiple organs, including nerve, 289,290
liver, 291,292 skeletal muscle, and cardiac muscle, in a process termed CELL TYPES
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transdifferentiation. However, direct evidence establishing this conclu- Lymphoid Progenitors
sion is lacking, as most such studies have assayed only partially purified Common Lymphoid Progenitors The existence of a population of cells
cell populations that might also contain alternate types of stem cells, committed to all lymphoid lineages but devoid of myeloid capacity was
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and almost none have been performed using single cells, a requirement theorized to exist based on a number of analyses. For example, patients
for robust proof of their multipotency. An alternate explanation for the with adenosine deaminase deficiency, or mice with genetic elimination
presence of marked hematopoietic cells at nonhematopoietic sites of of the γ receptor, the signaling kinase JAK3, or the transcription factor
C
organ damage has been termed cell fusion. It has long been appreciated Ikaros, lack T and B lymphocytes and have few, if any, myeloid defects,

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