**3. Fickleness of HSC surface markers**

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(AGM) area and only appears after day 11.5 of gestation on HSC in fetal liver (Matsubara et al., 2005; Our unpublished observation).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 & Orkin, 2006).

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 target cells even in the lineage depletion step.

Markers for Hematopoietic Stem Cells: Histories and Recent Achievements 81

Hematopoietic cells are thought to originate from the hemangioblast and/or the hemogenic endothelium, which can produce hematopoietic cells and endothelial cells. Therefore, it seems quite natural that HSC share some surface molecules with the endothelial lineage. CD34, PECAM-1/CD31, endoglin, Tie2 and VE-cadherin are well-known endothelial antigens that also mark HSC particularly at early developmental stages (Mikkola & Orkin 2006; Takakura et al., 1998; Yokota et al, 2006). In addition, recent studies have identified endomucin, endothelial protein-C receptor/CD201, and junctional adhesion molecule-A that are common to HSC and endothelial cells (Matsubara et al., 2005; Balazs et al., 2006; Sugano et al., 2008). Although, as discussed above, the expression level of some of these antigens declines or even diminishes at later stages of development (Mikkola & Orkin 2006), each of these advances offered the promise of learning more about how HSC arise de novo and function throughout life. It is crucial to define the means to identify the authentic HSC at all developmental stages so that we can ultimately understand the precise molecular

Table 2. Markers for human hematopoietic stem cells

**5. Endothelial-related markers** 

mechanisms of the HSC development.
