Hematopoiesis - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 27

inside the disk assigned for each progenitor and precursor with overlaps (as a final
cell fate can be reached via several types of intermediate progenitors) or involved
transcription factors in hematopoietic stem cells.

2.3 Stem Cell Niches

The embryonic origin of adult hematopoietic stem cells (multipotent progenitors)
is the aorta-gonad-mesonephros region. Hematopoietic stem cells subsequently
colonize fetal and adult hematopoietic organs, i.e., fetal liver and spleen on the
one hand and, in adults, the bone marrow as well as spleen and liver during
hematopoietic stress on the other. Colony-forming units in the spleen reconstitute
transiently multipotent progenitors.

2.3.1 Types of Bone-Marrow Niches

Different niche types for hematopoietic stem cells exist: (1) osteoblastic and
(2) vascular bone marrow HSC niches. In adults, hematopoietic stem cells have
their own bone marrow microenvironment, 13 close to the endosteal 14 surface of
bone marrow cavities in trabecular regions of long bones, whereas more differ-
entiated hematopoietic progenitors are mainly located in the central bone marrow
region [ 56 ]. The endosteal niche is characterized by a sinusoidal endothelium.
Changes in the HSC niche do not allow appropriate HSC maintenance in vivo.
In addition to osteoblastic niches, the vascular bone marrow HSC niche de-
marcated by bone marrow sinusoidal endothelial cells is a second specialized
HSC microenvironment in the bone marrow, with a large quantity of SLAMF1

+
hematopoietic stem cells attached to the fenestrated endothelium of bone marrow
sinusoids. Endothelial cells of bone marrow sinusoids yield a cellular platform
for the differentiation of lineage-committed progenitors such as megakaryocytic
progenitor cells (CFU megakaryocyte).
Bone marrow vascular and endosteal niches cooperate to control HSC quiescence
and self-renewal. A small number of hematopoietic stem cells is constantly released
into the blood circulation. Endothelial cells of bone marrow sinusoids differ from
endothelial cells of the microvasculature of any other organ. These endothelial cells

13 Extramedullary transient hematopoiesis, in the liver or spleen, occurs after bone marrow stress;
but after repair, bone marrow hematopoiesis follows.
14 The endosteum is a bone remodeling site characterized by high concentrations of calcium ions.
Extracellular Ca 2 + concentration is assessed by calcium-sensing receptors.