Recently, we identified in adult tissues a population of Oct4+SSEA-1+Sca-1+lin-CD45- very small embryonic-like stem cells (VSELs). First, to address recent controversies on Oct4 expression in cells isolated from adult organs we show here an evidence that Oct4 promoter in bone marrow (BM)-derived VSELs has an open chromatin structure and is actively transcribed. Next, to explain VSELs quiescence and lack of teratoma formation we demonstrate a unique DNA methylation pattern at some developmentally crucial, imprinted-genes, showing hypomethylation/erasure of imprints in paternally methylated and hypermethylation of imprints in maternally methylated ones. These epigenetic characteristics leading to upregulation in VSELs of H19 and p57KIP2 (also known as Cdkn1c) and repression of Igf2 and Rasgrf1 explain VSEL's quiescent status. Interestingly, this unique pattern in imprinted-genes methylation is reverted in co-cultures with a C2C12 supportive cell-line when VSELs are induced to form VSEL-derived spheres (VSEL-DSs) enriched for stem cells able to differentiate into all three germ layers. Therefore, we suggest that the proliferative/developmental potential of Oct4+ VSELs is epigenetically regulated by expression of Oct4 and some imprinted-genes, and postulate that restoring the proper methylation pattern of imprinted-genes will be crucial step for employing these cells in regenerative medicine.

Evidence has accumulated that adult hematopoietic tissues and other organs contain a population of dormant stem cells (SCs) that are more primitive than other, already restricted, monopotent tissue-committed stem cells (TCSCs). These observations raise several questions, such as the developmental origin of these cells, their true pluripotent or multipotent nature, which surface markers they express, how they can be efficiently isolated from adult tissues, and what role they play in the adult organism. The phenotype of these cells and expression of some genes characteristic of embryonic SCs (ESCs), epiblast SCs (EPiSCs), and primordial germ cells (PGCs) suggests their early-embryonic deposition in developing tissues as precursors of adult SCs. In this review we will critically discuss all these questions and the concept that small dormant stem cells related to migratory PGCs, described as very small embryonic-like stem cells (VSELs), are deposited during embryogenesis in bone marrow and other organs as a backup population for adult tissue committed stem cells (TCSCs) and are involved in several processes related to tissue or organ rejuvenation, aging, and cancerogenesis. The most recent results on successful ex vivo expansion of human VSELs in chemically defined media free from feeder-layer cells opens up new and exciting possibilities for their application in regenerative medicine.