Adhesion Molecules in Stem Cells - Adhesion Molecules - page 122

clearly dei ned; there have been some reports indicating that CD34-knockout
mice did not show evident decrease in absolute number of mature cells (Cheng
et al. 1996), and that HSCs with a long-term marrow reconstitution capacity were
found almost exclusively in a CD34-negative population in mice (Wang et al. 2003).
Fackler et al. (1995) reported that CD34 played a role in maintaining a murine
myeloid cell line (M1 cells) in an immature state, since enforced surface expression
of CD34 on the M1 cells inhibited their dif erentiation into macrophages. h is
observation is consistent with the well-known phenomenon that immature HSCs
and HPCs are highly positive for CD34 but the expression of CD34 molecules
is downregulated according to their dif erentiation. CD34 has also been shown
to be associated with regulating the adhesive and migratory properties of HSCs
(Gangenahalli et al. 2006) through interaction with L-selectin (CD62L) expressed
by endothelial cells and niche cells. However, more extensive studies are necessary
to investigate the exact role of the CD34 molecule in hematopoiesis.
MESENCHYMAL STEM CELLS
Mesenchymal stem cells (MSCs) were i rst identii ed in mouse bone marrow by
Friedenstein et al. in 1976. h ey were originally isolated from bone marrow by
subcultures of a heterogeneous adherent population obtained from the primary
culture of bone marrow cells. Nowadays, there are many reports showing that MSCs
can be isolated from various adult and fetal tissues (e.g., adipose tissue, scalp tissue,
dermal tissue, peripheral blood, umbilical cord blood, amnion, placenta). MSCs
were originally described as stem cells capable of dif erentiating into adipocytes,
osteoblasts and chondrocytes. Recently, the cells were further shown to be able
to dif erentiate into mesodermal, endodermal and ectodermal lineages: e.g.,
myocytes, cardiac cells, hepatocytes and neurons. In addition to this pluripotency,
MSCs have an outstanding expansion capacity; the cells can continue to proliferate
at er 20 or more passages. Recently, MSCs have been attracting considerable
attention because they facilitate the treatment of osteogenesis imperfecta, grat -
versus-host disease and myocardial infarction. Indeed, it has been shown that
MSCs secrete some immunosuppressive cytokines and hormones, such as
TGFβ, HGF and prostaglandin-E. h us, the pluripotency, expansion capacity
and immunosuppressive ef ect of MSCs might provide important clues for the
treatment of intractable diseases, based on the assumption that the mobilization
of MSCs into damaged tissues and their subsequent dif erentiation into functional
mature tissue cells could be induced.
Table 1 shows the cell surface markers expressed by MSCs. Hematolymphoid
markers (CD4, CD8 and CD45, etc.) are not expressed, but the expression of many
adhesion molecules has been reported. Recent analyses indicate the expression
of NCAM in mouse and monkey MSCs (Wang et al. 2006, Kato et al. 2008).