Because of their small size and density, umbilical cord blood (UCB)-derived very small embryonic/epiblast-like stem cells (VSELs) are usually lost at various steps of UCB preparation. Accordingly, we noticed that a significant number of these cells, which are smaller than erythrocytes, are lost during gradient centrifugation over Ficoll-Paque as well as during routine volume depletion of UCB units before freezing. To preserve these cells in final UCB preparations, we propose a relatively short and economical three-step isolation protocol that allows recovery of approximately 60% of the initial number of Lin(-)/CD45(-)/CD133(+) UCB-VSELs present in freshly harvested UCB units. In this novel approach (i) UCB is lysed in a hypotonic ammonium chloride solution to deplete erythrocytes; (ii) CD133(+) including VSELs cells are enriched by employing immunomagnetic beads; and subsequently (iii) Lin(-)/CD45(-)/CD133(+) cells are sorted by fluorescence-activated cell sorting. The whole isolation procedure takes approximately 2-3 h per UCB unit and isolated cells are highly enriched for an Oct-4(+) and SSEA-4(+) population of small Lin(-)/CD45(-)/CD133(+) cells.

A population of CD133+lin−CD45− very small embryonic-like stem cells (VSELs) has been purified by multiparameter sorting from umbilical cord blood (UCB). In order to speed up isolation of these cells, we employed anti-CD133-conjugated paramagnetic beads followed by staining with Aldefluor to detect aldehyde dehydrogenase (ALDH) activity; we subsequently sorted CD45−/GlyA−/CD133+/ALDHhigh and CD45−/GlyA−/CD133+/ALDHlow cells, which are enriched for VSELs, and CD45+/GlyA−/CD133+/ALDHhigh and CD45+/GlyA−/CD133+/ALDHlow cells, which are enriched for hematopoietic stem/progenitor cells (HSPCs). While freshly isolated CD45− VSELs did not grow hematopoietic colonies, the same cells, when activated/expanded over OP9 stromal support, acquired hematopoietic potential and grew colonies composed of CD45+ hematopoietic cells in methylcellulose cultures. We also observed that CD45−/GlyA−/CD133+/ALDHhigh VSELs grew colonies earlier than CD45−/GlyA−/CD133+/ALDHlow VSELs, which suggests that the latter cells need more time to acquire hematopoietic commitment. In support of this possibility, real-time PCR analysis confirmed that, while freshly isolated CD45−/GlyA−/CD133+/ALDHhigh VSELs express more hematopoietic transcripts (e.g., c-myb), CD45−/GlyA−/CD133+/ALDHlow VSELs exhibit higher levels of pluripotent stem cell markers (e.g., Oct-4). More importantly, hematopoietic cells derived from VSELs that were co-cultured over OP9 support were able to establish human lympho-hematopoietic chimerism in lethally irradiated NOD/SCID mice 4–6 weeks after transplantation. Overall, our data suggest that UCB-VSELs correspond to the most primitive population of HSPCs in UCB.