**2.2 Classification**

264 Advances in Hematopoietic Stem Cell Research

The majority of normal HSCs are present among the CD34+/CD38− bone marrow cell fractions; some HSCs are also observed among CD34−/Lin− cells; CD34+/CD38+ cell fractions contain some HSCs but endowed with short-term repopulating activity. Other recognized marker is the tyrosine kinase receptor c-kit (CD117), concomitantly with the lack

Primitive HSCs populations show low fluorescence ratios after Hoechst 33342 and Rhodamine 123 staining; these cells are described as side population (SP). SP cells demonstrate high expression of ATP binding cassette (ABC) transporters as P-glycoprotein (P-gp/ABCB1), breast cancer resistance protein (BCRP/ABCG2) and lung resistance protein (LRP) (Huls et al., 2009). MDR1 has been implicated in the protection of cells against apoptotic cell death induced by a variety of methods including growth factor deprivation, UV irradiation, ionizing radiation, or tumor necrosis factor-α treatment. BCRP is a halftransporter and characterized as a novel stem cell transporter. Like MDR1, enforced overexpression of BCRP in human MCF-7 breast cancer cells confers a broad spectrum of drug resistance, and elevated levels of expression of BCRP have been reported to be

Since ABC transporter function is associated with both normal and aberrant hematopoiesis, it is important to fully characterize the function of this class of transporter proteins in

Fig. 3. HSCs main surface markers. HSCs express typical antigens as: CD34, CD117, CD164, CD202b, CD31, Flk-1, CD184, CD338 or ABCG2, Notch-1 concomitantly with the lack of

hematopoietic cell differentiation and to define the underlying mechanisms.

of terminal differentiation markers (as CD4 and CD8; Figure 3) (Rossi et al., 2011).

associated with acute myeloid leukemia.

terminal differentiation markers (CD4 and CD8).

According to its hematopoietic repopulation capacity, the hematopoietic stem cell pool can be subdivided into three main groups:

