© 2006 Nature Publishing Group

*Ludwig Institute for Cancer
Research, Lausanne Branch,
University of Lausanne,
Chemin des Boveresses 155,
1066 Epalinges, Switzerland.
‡Genetics and Stem Cell
Laboratory, Swiss Institute
for Experimental Cancer
Research (ISREC) and School
of Life Sciences, Ecole
Polytechnique Federale de
Lausanne (EPFL), Chemin des
Boveresses 155, 1066
Epalinges, Switzerland.
Correspondence to A.T.
e-mail:
Andreas.Trumpp@isrec.ch
doi:10.1038/nri1779
Self-renewal
The capacity of a stem cell to
divide in such a way that one
or both daughter cells retain
the stem-cell fate.
Steel-Dickie mice
(Sl/Sld). A spontaneous mouse
mutant with a defect in the
production of membrane-
bound stem-cell factor (SCF),
although secreted SCF is
produced normally
Adult stem cells are present in most self-renewing tis-
sues, including the skin, the intestinal epithelium and
the haematopoietic system. On a single-cell basis, they
have the capacity both to produce more stem cells of
the same type (that is, to self-renew) and to give rise to a
defined set of mature differentiated progeny to maintain
or repair their host tissue1–3. The best-characterized adult
stem cell is the haematopoietic stem cell (HSC)4,5. Since
HSCs were first identified6, advances in technology have
made it possible to purify adult mouse HSCs close to
homogeneity. Several groups have achieved long-term
reconstitution of the haematopoietic system of a lethally
irradiated mouse by transplantation of a single purified
bone-marrow HSC, providing functional proof of the
existence of adult HSCs2,7–9. Maintenance of HSCs and
regulation of their self-renewal and differentiation in vivo
is thought to depend on their specific microenvironment,
which has been historically called the haematopoietic-
inductive microenvironment10 or ‘stem-cell niche’11. The
crucial role of the microenvironment for HSC function
has long been recognized because a mutation in the
gene encoding membrane-bound stem-cell factor (SCF;
also known as KIT ligand) that is present in Sl/Sld mice
(steel-Dickie mice) causes changes in the HSC niche and
leads to the failure of bone-marrow HSC maintenance
in vivo12–14. Nevertheless, the structure and localization, as
well as the molecular and cellular basis for niche activity,
have long remained a ‘black box’. It is only recently that
the concept of a stem-cell niche has been supported by
data on the molecules and cell types that are involved in
its formation, first in invertebrates and more recently in
mammals1,15–17. Many of the different types of signals that
are exchanged between stem cells and niche cells, as well
as some of the signalling pathways that control stem-
cell maintenance, self-renewal and differentiation, have
recently been identified. In this Review, we discuss models
for the different types of bone-marrow HSC niches that
might exist, particularly focusing on the molecules that are
known to coordinate HSC function in vivo.
The adult HSC
Murine HSCs were initially identified on the basis of
their ability to form colonies in the spleens of lethally
irradiated mice following bone-marrow transfer6,18.
Subsequently, a number of assays have been developed
to monitor HSC activity in vivo and in vitro (BOX 1). The
most widely accepted assay is the capacity of HSCs to
provide lifelong reconstitution of all blood-cell lineages
after transplantation into lethally irradiated recipients.
The strictest version of this long-term repopulating
(LTR) assay, known as serial transplantation, requires
that HSC-containing donor bone marrow can be
re-transplanted into secondary, and even tertiary,
recipients while retaining both self-renewal and multi-
lineage differentiation capacity19. These functional assays
have been used to establish the cell-surface phenotype
of mouse HSCs, allowing their prospective isolation by
fluorescence-activated cell sorting (FACS) (BOX 1).
All functional HSCs are found in the population of
bone-marrow cells that does not express the cell-surface
Bone-marrow haematopoietic-
stem-cell niches
Anne Wilson* and Andreas Trumpp‡
Abstract | Adult stem cells hold many promises for future clinical applications and
regenerative medicine. The haematopoietic stem cell (HSC) is the best-characterized
somatic stem cell so far, but in vitro expansion has been unsuccessful, limiting the future
therapeutic potential of these cells. Here we review recent progress in characterizing
the composition of the HSC bone-marrow microenvironment, known as the HSC niche.
During homeostasis, HSCs, and therefore putative bone-marrow HSC niches, are located
near bone surfaces or are associated with the sinusoidal endothelium. The molecular
crosstalk between HSCs and the cellular constituents of these niches is thought to
control the balance between HSC self-renewal and differentiation, indicating that future
successful expansion of HSCs for therapeutic use will require three-dimensional
reconstruction of a stem-cell–niche unit.
NATURE REVIEWS | IMMUNOLOGY VOLUME 6 | FEBRUARY 2006 | 93
REVIEWS
F O C U S O N E A R LY LY M P H O C Y T E D E V E LO P M E N T

© 2006 Nature Publishing Group

Lin–
SCA1+
KIT+
Thy1.1low
FLT3–
Long-term
self-renewal
potentialLT-HSC
SP+
N-cad+
TIE2+
CD38+
CD150+
Endoglin+
MYClow
Rholow
Lin–
SCA1+
KIT+
Thy1.1low
FLT3–
ST-HSC
CD34+
CD11blow
Lin–
SCA1+
KIT+
Thy1.1–
FLT3+
MPP
CD34+
CD11blow
CD48+
MYChi
Rhohi
CD4low
markers normally present on lineage (Lin)-committed
haematopoietic cells but does express high levels of
stem-cell antigen 1 (SCA1) and KIT. Therefore, this
HSC-containing subset of bone-marrow cells is known
as the LSK (Lin–SCA1+KIT+) subset. Because only some
phenotypic LSK HSCs have LTR activity, they can be
further subdivided into long-term (LT)-HSCs, which
are CD34– fms-related tyrosine kinase 3 (FLT3)–CD150+
and have LTR activity, and short-term (ST)-HSCs,
which are CD34+FLT3– and have only limited self-
renewal activity9,20–22 (BOX 1). Although it has been
shown that 100 LSK HSCs can provide protection from
lethal irradiation23, several groups have succeeded in
reconstituting all haematopoietic lineages from a single,
purified HSC (BOX 1). These data clearly show that at the
clonal-level HSCs fulfill the characteristics of true adult
stem cells — multi-lineage reconstitution and long-
term self-renewal. Recent gene-profiling studies have
begun to establish a transcriptional signature of purified
HSCs, which is the first step to elucidating the molecu-
lar mechanisms of HSC function9,24–27. Furthermore,
the number of functional HSCs in vivo is altered in
a large number of mutant mice (see Supplementary
information S1 (table)), implicating several of these
gene products in the regulation of self-renewal and
differentiation of stem cells.
Asymmetric self-renewing division in stem cells
The vast majority of cell divisions are symmetrical,
producing identical daughter cells and leading (in the
absence of apoptosis) to increased numbers of cells. This
process is readily observed for cells in culture and also
occurs during organogenesis, where substantial cellular
expansion (including stem cells) occurs during embryo-
genesis. By contrast, under homeostatic conditions in
the adult, the number of tissue stem cells in a particular
organ remains relatively constant, despite the fact that
they proliferate, because they not only self-renew but also
produce differentiated progeny.
This balance could be achieved if the number of stem
cells dividing symmetrically to generate two identical
daughter cells with stem-cell function was equivalent to
the number of stem cells giving rise to two differentiated
daughter cells. However, because this mechanism does not
function at the single-cell level, and would require close
coordination of two separate stem-cell populations, it is
commonly assumed that an individual stem cell can give
rise to two non-identical daughter cells, one maintaining
stem-cell identity and the other becoming a differenti-
ated cell. There are two mechanisms by which this asym-
metry can be achieved, depending on whether it occurs
pre- (divisional asymmetry), or post- (environmental
asymmetry) cell division (FIG. 1).
Divisional asymmetry. In divisional asymmetry, specific
cell-fate determinants in the cytoplasm (mRNA and/or
proteins) redistribute unequally before the onset of cell
division. During mitosis, the cleavage plane is oriented
such that only one daughter cell receives the determinants.
Therefore, two non-identical daughter cells are produced,
one retaining the stem-cell fate while the other initiates
differentiation (FIG. 1a).
In invertebrate model systems, the establishment
of asymmetry by this mechanism is crucial for vari-
ous developmental processes and the molecular basis
for it has been well documented28. Asymmetrically
localized proteins in Drosophila melanogaster include
members of the partitioning defective (PAR) family
of proteins, such as Inscuteable (INSC) and Partner of
Inscuteable (PINS, the homologue of which is LGN in
mammals), as well as NUMB, a negative modulator of
Notch signalling29. However, only a few examples
of divisional asymmetry have been documented in
higher vertebrates28,30. For example, in the mamma-
lian fetal epidermis, basal cells not only divide sym-
metrically to allow a two-dimensional expansion of the
Box 1 | Characteristics of haematopoietic stem cells
Haematopoietic stem cells (HSCs) are defined functionally by their ability to mediate
long-term repopulation of all blood-cell lineages (known as long-term repopulating
(LTR) activity) and to form colony forming units in the spleen after transfer to lethally
irradiated recipients. Assays to assess HSC activity in vitro include LTC-IC (long-term
culture-initiating cell) and CAFC (cobblestone area-forming cell) assays131.
All LTR HSCs are contained in the lineage-negative (Lin)– stem-cell antigen 1
(SCA1)+KIT+ (LSK) subset that comprises ~0.5% of bone marrow132. 100 LSK cells are
sufficient for multi-lineage LTR activity23. Additional markers to further subdivide the
LSK population into long-term HSCs (LT-HSCs) and short-term HSCs (ST-HSCs), which
have limited self-renewal activity, have been identified and are summarized in the
figure. LTR activity is also enriched in the population of bone-marrow cells with low-
level staining of rhodamine 123 (Rho)133. In addition, functional adult LTR HSCs can also
be isolated by their ability to actively efflux the DNA-binding dye Hoechst 33342. This
characteristic is designated as side-population (SP) ability134,135.
Single-cell reconstitution studies have indicated the following frequencies for multi-
lineage reconstitution and long-term engraftment:
• LSKThy1.1low cells (18%)2,7
• SP+RholowLin– cells (40%)136
• LSKCD150+CD48–CD41– cells (47%)9
• LSKSP+CD34– cells (35%)137 and (96%)8
LT-HSCs divide infrequently because (by DNA content) only ~5% are in the S or G2/M
phases of the cell cycle51,138, and 60-70% of LSK cells are shown to be in G0 by Ki67
staining52. Studies using bromodeoxyuridine (BrdU) uptake have calculated that LSK
HSCs divide every 30–60 days51,138. 3.8% of LSK CD150+ HSCs are in the S or G2/M
phases of the cell cycle9. The low cycling status of HSCs might explain their significant
resistance to cytotoxic drugs in vivo40.
Label-retaining cells (LRCs) are defined by their capacity to retain the DNA label BrdU
long-term (for 70 days). Lin–KIT+ LRCs are enriched for phenotypic HSCs, but due to the
nature of the assay, functional LTR activity cannot be assessed.
LT-HSCs, ST-HSCs and haematopoietic progenitor cells show substantially different
gene expression patterns9,24–27.
FLT3, fms-related tyrosine kinase 3; MPP, multipotential progenitor; N-cad, N-cadherin;
TIE2, tyrosine kinase receptor 2.
R E V I E W S
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