Very Small Embryonic/Epiblast-Like Stem Cells (VSELs) – Novel Supporting Evidence for An Existence of Developmentally Distinct Mobile Pool of Oct-4 ⁺ Pluripotent Stem Cells in Embryonic and Adult Tissues: Emerging Concept for a Potential 4^th Migratory Germ Layer.

Abstract 1480

Poster Board I-503

We demonstrated the presence of very small (smaller than erythrocytes) Oct-4⁺ SSEA-1⁺Sca-1⁺Lin⁻CD45⁻ VSELs in bone marrow (BM) and several adult organs and tissues (Leukemia 2006:20;857, Cytometry 2009:73;1116). Furthermore, our recent promoter methylation studies and analysis of chromatin's structure revealed that the Oct-4 gene is truly expressed in these cells. The epigenetic changes in selected somatic-imprinted genes govern their quiescent status, thus preventing them from unleashed proliferation and the spontaneous growth of teratomas (Leukemia 2009: in press). In the current study, we addressed the developmental origin of VSELs and significance of their presence in adult tissues. We first evaluated a number of VSELs in developing murine embryos (7-16 dpc). These cells were easily identified at the early stages of embryonic development. Studies in heartbeat-deficient mice (Ncx1^−/−) revealed that VSELs are present in the embryo proper before initiation of circulation, which is in contrast to hematopoietic stem cells. Their number rapidly increases to ∼325k/embryo around 10-11 dpc during rapid vascularization of developing tissues/organs. These cells survive into adulthood; however, their number decreases with age. Molecular analysis revealed that VSELs express several primary epiblast- (Gbx2, Fgf5, Nodal) as well as epiblast-derived migratory primordial germ cell markers (Stella, Blimp1, Dnd1, and Nanos3). Furthermore, our recent BrdU staining studies revealed that, e.g., BM-residing VSELs are highly resistant to lethal irradiation and begin to proliferate even after lethal myeloablasia. This suggests that these cells could be a source of tissue-committed stem cells during tissue/organ damage and regeneration. Finally, in direct chemotactic studies we noticed that VSELs are highly mobile and respond robustly to SDF-1 and HGF gradients. This explains why they are mobilized into peripheral blood after heart infarct (Circulation 2004:110;3213, JACC 2009:53;1), stroke (Stroke 2009: 40,1237), and skeletal muscle and toxic liver damage (Stem Cells 2008:26;2083). Based on this, we propose a novel and challenging concept that during embryogenesis, a pool of highly migratory PSCs emerges (VSELs) in proximal epiblasts that contributes to organogenesis, tissue development, and specification of primordial germ cells. Furthermore, these cells survive into adulthood as a pool of PSCs that play some role in: i) steady-state conditions of tissue rejuvenation as a back up for tissue-committed stem cells; and ii) regeneration of damaged organs during emergencies. As such, we postulate that VSELs could somehow be a mammalian counterpart of blastema-like cells present in lower organisms. In mammalian tissues, they are kept in quiescent status due to epigenetic modification of selected imprinted genes. In addition, the age-related decrease of these cells in adult tissues and their primitive nature could explain the aging process. On the other hand, embryonic/epiblast-derived remnants in adult tissues could potentially initiate tumorogenesis (Virchow & Connheim). Finally, identification of VSELs supports the concept of a so-called ‘memoderm’, a putative 4^th, highly migratory, germ layer (Gaillard & Hernandez). Identification of VSELs in embryonic and adult tissues and our most recent data lend support to all these theories and provide novel view on regeneration, senescence and tumorigenesis.