Adhesion Molecules in Stem Cells - Adhesion Molecules - page 120

h e concept of a niche for HSCs was proposed by Schoi eld in the 1970s,
but the niche itself was poorly characterized until recently, when, as a result of
developments in molecular biological techniques, huge advances have been made.
It has been shown that osteoblasts contacting the endosteal surface of the bones
constitute the niche (Calvi et al. 2003, Zhang et al. 2003). h ese cells, which were
named spindle-shaped N-cadherin-positive cells (SNO cells) by Zhang et al. , are
closely associated with HSCs expressing N-cadherin. More recently, sinusoidal
endothelium has been proposed as another niche (Sugiyama et al. 2006). h is
concept is supported by the observation that osteoblasts do not exist in spleen
and liver but that extramedullary hematopoiesis can be induced in the case of
hypoplasia in the bone marrow. At present, it is speculated that the endosteum/
osteoblast niche contributes mainly to the maintenance of P-HSCs, whereas the
sinusoidal endothelium niche contributes to the proliferation/dif erentiation
of HSCs and their mobilization into the periphery as well as their maintenance
(Fig. 1B) .
Fig. 1B Regulation of self-renewal and dif erentiation of HSCs in their niche: Endosteum/
osteoblast niche and sinusoidal endothelium niche. Osteoblasts in contact with the endosteal
surface of the bones constitute endosteum/osteoblast niche. Sinusoidal endothelium is also
proposed as another niche.