**4. Difference between mouse and man**

Another very critical issue on the topic of the HSC markers is their diversity between species. Although essential difference has not been observed between mouse and man regarding either the organs producing HSC or the transcription factors regulating their differentiation, completely different markers have been used to sort HSC in the two species. Human HSC do not express Sca1 or the SLAM family CD150 (Larochelle et al., 2011). While the CD34+ CD38 phenotype has been regarded as the canonical marker set for human HSC, it has been repeatedly reported that murine adult HSC locate in the CD34- CD38+ fraction (Randall et al., 1996; Matsuoka et al., 2001; Tajima et al., 2001). There is no reasonable explanation so far for the change along evolution, and such phenotypic differences between murine and human HSC have been an obstacle to apply achievement in mouse studies to human.

Early studies by Berenson et al demonstrated that autologous CD34+ cells enriched from bone marrow effectively radioprotected baboons and promoted hematopoietic recovery in human patients after marrow ablative therapy (Berenson et al, 1988, 1991). Over the past 2 decades, the use of CD34 as a marker for hematopoietic stem/progenitor cells has been a strong tool in the field of clinical hematology. Since the CD34+ fraction of human bone marrow contains lineage-committed progenitors as well as long-term multi-lineage HSC, many laboratories have sought additional markers to further enrich the CD34+ population for long-term HSC. CD90/Thy1, Tie, CD117/c-kit, and CD133/AC133 have been found as positive markers to enrich long-term-HSC whereas several negative markers including CD38 have been reported (Baum et al., 1992; Hasiyama et al., 1996; Gunji et al., 1993; Yin et al., 1997; Terstappen et al., 1991).

Recent advances of xenotransplantation models and techniques have enabled the assessment of pluripotency as well as self-renewal of human hematopoietic progenitors in vivo (Shultz et al., 2007). A series of studies by John Dick's laboratory have successfully enriched human long-term HSC within the Lin- CD34+ CD38- population (McKenzie et al., 2007; Doulatov et al., 2010). In a very recent report, they have purified human HSC from cord blood with a maker set of Lin- CD34+ CD38- CD45RA-CD90/Thy1+ Rhodamin123Low CD49f+. Indeed, those cells were capable of long-term multilineage engraftment in NOD/SCID/IL2 receptor common- chain null mice at a single-cell level (Notta et al., 2011). The information regarding human HSC markers is summarized in Table 2.

While CD34 has been playing an important role as a reliable marker for human hematopoietic stem/progenitor cells in the practical medicine, several studies have demonstrated that long-term reconstituting activity is also detectable in the CD34- Linpopulation (Bhatia et al., 1998; Gallacher et al., 2000; Wang et al., 2003). A prior study using Hoechst 33342 by Goodell et al also identified CD34 cells in the side-population of human and rhesus bone marrow, and actually rhesus CD34- side-population cells acquired the ability to form hematopoietic colonies after long-term cultivation on bone marrow stromal cells (Goodell et al., 1997). It should be interesting to examine molecular signatures associated with those CD34- HSC in primates, and compare their features with murine CD34- LSK cells.

Another very critical issue on the topic of the HSC markers is their diversity between species. Although essential difference has not been observed between mouse and man regarding either the organs producing HSC or the transcription factors regulating their differentiation, completely different markers have been used to sort HSC in the two species. Human HSC do not express Sca1 or the SLAM family CD150 (Larochelle et al., 2011). While

(Randall et al., 1996; Matsuoka et al., 2001; Tajima et al., 2001). There is no reasonable explanation so far for the change along evolution, and such phenotypic differences between murine and human HSC have been an obstacle to apply achievement in mouse studies to

Early studies by Berenson et al demonstrated that autologous CD34+ cells enriched from bone marrow effectively radioprotected baboons and promoted hematopoietic recovery in human patients after marrow ablative therapy (Berenson et al, 1988, 1991). Over the past 2 decades, the use of CD34 as a marker for hematopoietic stem/progenitor cells has been a strong tool in the field of clinical hematology. Since the CD34+ fraction of human bone marrow contains lineage-committed progenitors as well as long-term multi-lineage HSC, many laboratories have sought additional markers to further enrich the CD34+ population for long-term HSC. CD90/Thy1, Tie, CD117/c-kit, and CD133/AC133 have been found as positive markers to enrich long-term-HSC whereas several negative markers including CD38 have been reported (Baum et al., 1992; Hasiyama et al., 1996; Gunji et al., 1993; Yin et

Recent advances of xenotransplantation models and techniques have enabled the assessment of pluripotency as well as self-renewal of human hematopoietic progenitors in vivo (Shultz et al., 2007). A series of studies by John Dick's laboratory have successfully enriched human long-term HSC within the Lin- CD34+ CD38- population (McKenzie et al., 2007; Doulatov et al., 2010). In a very recent report, they have purified human HSC from cord blood with a maker set of Lin- CD34+ CD38- CD45RA-CD90/Thy1+ Rhodamin123Low CD49f+. Indeed, those cells were capable of long-term multilineage engraftment in NOD/SCID/IL2 receptor common- chain null mice at a single-cell level (Notta et al., 2011). The information regarding human HSC markers is

While CD34 has been playing an important role as a reliable marker for human hematopoietic stem/progenitor cells in the practical medicine, several studies have demonstrated that long-term reconstituting activity is also detectable in the CD34- Linpopulation (Bhatia et al., 1998; Gallacher et al., 2000; Wang et al., 2003). A prior study using Hoechst 33342 by Goodell et al also identified CD34- cells in the side-population of human and rhesus bone marrow, and actually rhesus CD34- side-population cells acquired the ability to form hematopoietic colonies after long-term cultivation on bone marrow stromal cells (Goodell et al., 1997). It should be interesting to examine molecular signatures associated with those CD34- HSC in primates, and compare their features with murine

it has been repeatedly reported that murine adult HSC locate in the CD34-

phenotype has been regarded as the canonical marker set for human HSC,

CD38+ fraction

**4. Difference between mouse and man** 

the CD34+ CD38-

al., 1997; Terstappen et al., 1991).

summarized in Table 2.

CD34- LSK cells.

human.


Table 2. Markers for human hematopoietic stem cells
