Very small embryonic-like stem cells (VSELs) are possibly lost during cord blood banking and bone marrow (BM) processing for autologus stem cell therapy mainly because of their small size.
The present study was conducted on human umbilical cord blood (UCB, n=6) and discarded red blood cells (RBC) fraction obtained after separation of mononuclear cells from human BM (n=6), to test this hypothesis.
The results show that VSELs, which are pluripotent stem cells with maximum regenerative potential, settle along with the RBCs during Ficoll-Hypaque density separation.
VSELs are very small in size (3-5 μm), have high nucleo-cytoplasmic ratio, and express nuclear Oct-4, cell surface protein SSEA-4, and other pluripotent markers such as Nanog, Sox-2, Rex-1, and Tert as indicated by immunolocalization and quantitative polymerase chain reaction (Q-PCR) studies.
Interestingly, a distinct population of slightly larger, round hematopoietic stem cells (HSCs) with cytoplasmic Oct-4 were detected in the "buffy" coat, which usually gets banked or used during autologus stem cell therapy.
Immunohistochemical studies on the umbilical cord tissue (UCT) sections (n=3) showed the presence of nuclear Oct-4-positive VSELs and many fibroblast-like mesenchymal stem cells (MSCs) with cytoplasmic Oct-4.
These VSELs with nuclear Oct-4, detected in UCB, UCT, and discarded RBC fraction obtained after BM processing, may persist throughout life, maintain tissue homeostasis, and undergo asymmetric cell division to self-renew as well as produce larger progenitor stem cells, viz. HSCs or MSCs, which follow differentiation trajectories depending on the somatic niche.
Hence, it can be concluded that the true stem cells in adult body tissues are the VSELs, whereas the HSCs and MSCs are actually progenitor stem cells that arise by asymmetric cell division of VSELs.
The results of the present study may help explain low efficacy reported during adult autologous stem cell trials, wherein unknowingly progenitor stem cells are injected rather than the pluripotent stem cells with maximum regenerative potential.