**2.3 HSC sources**

HSCs can be harvested from healthy donors either by bone marrow aspiration, peripheral stem cell mobilization or from umbilical cord blood (Dick, 2003). HSCs located in the bone marrow present an estimated frequency of 0.01% of total nucleated cells and can be collected by iliac crest puncture and then separated from the other blood cells by magnetic beads or cell sorting.

Umbilical cord blood (UCB) is a source of the rare but precious primitive HSCs and progenitor cells that can reconstitute the hematopoietic system in patients with malignant and nonmalignant disorders treated with myeloablative therapy. UCB cells possess an enhanced progenitor cell proliferation capacity and self-renewal *in vitro*. UCB is usually discarded and it exists in almost limitless supply. The blood remaining in the delivered placenta is safely and easily collected and stored. The predominant collection procedure currently involves a relatively simple venipuncture, followed by gravity drainage into a standard sterile anti-coagulant-filled blood bag, using a closed system, similar to that utilized for whole blood collection (Bojanic & Golubic Cepulic, 2006).

Peripheral blood hematopoietic stem cells (PBSCs) have numerous advantages in comparison with traditionally used bone marrow. PBSCs collection by leukapheresis procedure is simple and better tolerated than bone marrow harvest. PBCSs are mobilized by myelosupressive chemotherapy or/and hematopoietic growth factors. Leukapheresis product contains PBSCs along with committed lineage of progenitors and precursors which contribute to faster hematopoietic recovery.

Unfortunately, the expansion of HSCs *in vitro* is difficult to achieve because the proliferation is accompanied by differentiation. This is presumably caused by a lack of appropriate cues that are provided *in vivo* by the microenvironment. The most excellent defined culture medium for HSCs expansion is supplemented with cytokines such as fetal liver tyrosine kinase-3 ligand (FLT3-L), stem cell factor (SCF), interleukin-3 (IL-3) and thrombopoietin (TPO). Interestingly, mesenchymal stem cells (MSCs), which are characterized by multi-differentiation potential, are important players of the bone marrow HSCs niche. In recent years, MSCs have been shown to support HSCs maintenance and engraftment (Jing et al., 2010).
