7recombine in more than one type of stromal cell in the bone marrow (Zhang and
Link 2016 ).
Imaging the interaction of HSC with candidate niche cells: The discovery
that the SLAM family markers CD48 and CD150 could be used to image mouse
HSC (as Lin−CD48−CD150+ cells) in long bones revolutionized the HSC niche field
(Kiel et al. 2005 ). Experiments measuring the interaction of this “SLAM” HSC with
different types of candidate niche cells and structures led to the discovery that most
HSC are located in perivascular areas of the bone marrow and the discovery of peri-
arteriolar cells, megakaryocytes and osteolineage cells, among others, as candidate
niche cells (Asada et al. 2017a; Bruns et al. 2014 ; Kunisaki et al. 2013 ; Mendez-
Ferrer et al. 2008 ; Nakamura-Ishizu et al. 2014 ; Nombela-Arrieta et al. 2013 ;
Silberstein et al. 2016 ; Zhao et al. 2014 ). HSC could also be labeled as CD117+α-
catulin- GFP+ cells using α-catulin-GFP reporter mice (Acar et al. 2015 ) or Hoxb5+
cells using Hoxb5-cherry reporter mice (Chen et al. 2016 ) although these models
have not yet been widely used. The main limitation of imaging approaches is that
they are correlative: they can determine whether HSC are in the proximity of a can-
didate niche cell but additional functional validation is required. Another concern is
how to define proximity. Different groups have used different cut-offs to define
HSC proximity to a candidate niche cell and a consensus has yet to emerge. The
current state of the art to assess for specific interactions is based on testing whether
the HSC distribution observed in vivo is statistically different from a computer-
generated random distribution (Acar et al. 2015 ; Kunisaki et al. 2013 ).
2.3 Cellular Composition and Organization of the Murine
Bone Marrow HSC Niche
Several excellent recent reviews have extensively described the role of each of the
known niche cell types (Asada et al. 2017b; Birbrair and Frenette 2016 ; Crane et al.
2017 ; Ramalingam et al. 2017 ; Sanchez-Aguilera and Mendez-Ferrer 2017 ; Yu and
Scadden 2016 ). We will thus focus on describing the cellular composition and orga-
nization of the HSC niche. Figure 2.1 summarizes discoveries from many different
laboratories over the last 15 years and highlights the complexity of the HSC niche.
Niche cells associated with the vasculature: The bone marrow is a highly vas-
cularized organ where arteries and arterioles enter the tissue and transform into a
sinusoidal network that is drained by a central vein (Kunisaki et al. 2013 ). The vast
majority of HSC are located adjacent to, or in close proximity (less than 5 μm) of, a
blood vessel (Kiel et al. 2005 ). HSC arise from an hemogenic endothelium during
development and remain associated with blood vessels through life (Ramalingam
et al. 2017 ). Endothelial cells are an indispensable component of the HSC niche;
they produce factors like CXCL12, SCF, E-SELECTIN and NOTCH ligands that
regulate HSC self-renewal and trafficking HSC (reviewed in Ramalingam et al.
( 2017 )). Endothelial-cell derived Notch ligands also regulate BM angiogenesis and
2 The Bone Marrow Microenvironment for Hematopoietic Stem Cells