**1. Introduction**

76 Advances in Hematopoietic Stem Cell Research

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Hematopoietic stem cells (HSC) are characterized with the capacity for self-renewal as well as multi-lineage differentiation, maintaining the immune system and blood cell formation throughout life. Although studies for the HSC biology have been in the forefront of the stem cell research field, many questions still remain with regard to the origin, development, and aging of HSC. Furthermore, needless to say, HSC are very useful for clinical medicine, particularly in the transplantation and/or regeneration therapy for hematological malignancies. Success of those therapies depends on how effectively HSC are purified and transplanted to the patients. In order to address those important issues in both basic and clinical science, information of cell surface molecules that selectively mark HSC is essential.

Since the frequency of HSC in bone marrow or peripheral blood is extremely low, many studies have attempted to identify unique markers associated with those rare cells. As a result, it is now possible to purify long-term reconstituting HSC from mouse bone marrow with very high efficiency. However, many of these parameters change dramatically during ontogeny or inflammation, and what is worse still, they differ between mouse and man. Efficient HSC-based therapies and the emerging field of tissueregenerative medicine will benefit from more precise information about what defines HSC.

In this chapter, we summarize a large body of information with respect to the HSCrelated markers and introduce Endothelial cell-selective adhesion molecule (ESAM) as a novel marker for HSC (Yokota et al., 2009). Indeed, ESAM is expressed throughout the ontogeny in mouse and can be used as a gating parameter for sorting long-term repopulating HSC. In addition, the marker appears to be useful for the purification of human HSC.

Markers for Hematopoietic Stem Cells: Histories and Recent Achievements 79

\*Tip-SP: The highest Hoechst-efflux fraction in the Side Population

**3. Fickleness of HSC surface markers** 

target cells even in the lineage depletion step.

Orkin, 2006).

Table 1. Markers for hematopoietic stem cells in adult mouse bone marrow

It is important to stress here that none of the surface markers shown above is entirely specific to the long-term HSC. In addition, many of these parameters differ between strains of mice and change dramatically during developmental age. For example, Sca1, which has been a center in the canonical HSC marker "LSK", is not detectable on the emerging HSC in the aorta-gonad-mesonephros (AGM) area and only appears after day 11.5 of gestation on HSC in fetal liver (Matsubara et al., 2005; Our unpublished observation). Furthermore, the expression level of Sca1 on HSC differs between strains and is not very effective to enrich HSC from Balb/c mice (Spangrude & Brooks, 1993). Likewise, the SLAM family CD150 is not useful for the emerging and developing HSC in embryos (McKinney-Freeman et al., 2009). On the contrary, CD41, CD11b/Mac1, vascular endothelial (VE)-cadherin, and CD34 are known to mark the emerging and developing HSC during the fetal period, but gradually disappear along the ontogeny (Mikkola &

It is also a well-recognized fact that cell surface markers on HSC in adult bone marrow do fluctuate according to the cell-cycle status and the differentiating behavior, which change depending on the physiological requirement. Bone marrow suppression by irradiation and/or chemotherapy revives several disappeared markers including CD11b/Mac1 and CD34 whereas it significantly down-regulates the expression level of c-kit on long-term HSC (Randall & Weissman, 1997; Ogawa 2002). Molecular crosstalk between HSC and bone marrow microenvironment is thought to control the status of HSC and influence their surface phenotypes, but precise mechanisms remain largely unknown. Therefore, researchers in the HSC field need to carefully choose an appropriate marker set and a sorting gate depending on the HSC characteristics, otherwise they would miss important
