9identified using different types of genetic reporters (e.g. CXCL12-reporter (Omatsu
et al. 2010 ; Sugiyama et al. 2006 ), Nestin-GFPdim (Kunisaki et al. 2013 ; Mendez-
Ferrer et al. 2010 ) and LepR-cre-lineage traced cells (Ding and Morrison 2013 ;
Ding et al. 2012 )) and cell surface markers (Chan et al. 2009 ; Pinho et al. 2013 ) but
these largely overlap and likely label the same cell (reviewed in Hanoun and Frenette
( 2013 )). Importantly these cells are enriched in osteoprogenitors and are capable of
giving rise to bone in vivo (Mendez-Ferrer et al. 2008 ; Omatsu et al. 2010 ; Zhou
et al. 2014 ). Bone marrow arterioles are ensheathed by Ng2+ stromal cells. Ng2+
arterioles are enriched in quiescent HSC that also display low levels of reactive
oxygen species (ROS) when compared to sinusoids. Ablation of Ng2+ cells or con-
ditional CXCL12 deletion in Ng2+ cells causes loss of quiescent HSC (Asada et al.
2017a; Itkin et al. 2016 ; Kunisaki et al. 2013 ). Arterioles also provide physical sup-
port and associate with sympathetic nerves that enter the bone marrow from the
periphery (Kunisaki et al. 2013 ). Both sympathetic nerves and associated non-
myelinating Schwann cells are components of the HSC niche. Sympathetic nerves
produce norepinephrine which acts via β3 adrenergic receptors in BM stromal cells
to control CXCL12 release and thus HSC trafficking from the bone marrow to the
blood (Katayama et al. 2006 ; Mendez-Ferrer et al. 2008 , 2010 ). GFAP+ non-
myelinating Schwann cells ensheath BM sympathetic nerves and are major sources
of active TGFβ that promotes HSC quiescence (Yamazaki et al. 2011 ). Another
component of the niche associated with the vasculature are megakaryocytes. These
are multinucleated hematopoietic cells that reside in the sinusoids where they pro-
duce platelets. Imaging studies showed that approximately 35% of HSC are in con-
tact with a megakaryocyte (Bruns et al. 2014 ; Zhao et al. 2014 ). These cells function
by restricting HSC proliferation via CXCL4, TGFβ and Thrombopoietin (Bruns
et al. 2014 ; Nakamura-Ishizu et al. 2014 , 2015 ; Zhao et al. 2014 ). Megakaryocyte
ablation or loss of megakaryocyte-derived signals does not cause loss of HSC. Instead
it causes HSC proliferation and relocation away from sinusoids, loss of quiescence
and eventual exhaustion (Bruns et al. 2014 ; Nakamura-Ishizu et al. 2014 , 2015 ;
Zhao et al. 2014 ). Because megakaryocytes are hematopoietic cells this regulation
suggest the existence of a feedback loop through which HSC are informed of their
cellular output via regulation by their own progeny. In line with these results it has
been shown that HSC and multipotent hematopoietic progenitors can also directly
promote HSC proliferation via production of ESL which functions by limiting
TGFβ availability (Leiva et al. 2016 ).
Niche cells not associated with the vasculature: The endosteal (inner) surface
of the bone and trabecular areas are enriched in HSC in the steady-state and during
regeneration suggesting that bone-lining cells might be niche components. Imaging
analyses of HSC after transplantation revealed that many HSC where in contact
with osteolineage (bone-forming) cells in the endosteal surface of the bone
(Silberstein et al. 2016 ). Purification of these cells revealed that they were enriched
in mRNAs for embigin and angiogenin. Deletion of these molecules in osteolineage
cells led to loss of HSC quiescence and proliferation thus indicating that embigin
and angiogenin were HSC-regulatory molecules and that bone-lining osteolineage
cells were components of the HSC niche (Goncalves et al. 2016 ; Silberstein et al.
2 The Bone Marrow Microenvironment for Hematopoietic Stem Cells