Very Small Embryonic-like Stem Cells in the Human Cord Blood for Fetal–Neonatal Regenerative Medicine: A Systematic Review and Reproducibility-focused Evidence Map of Identity, Isolation, and Clinical Translational Signals

Very small embryonic-like stem cells (VSELs) have been proposed as a rare, primitive cell population residing in human umbilical cord blood, with potential implications for fetal and neonatal regenerative medicine. Since their initial description, VSELs have occupied an uneasy position in stem cell biology – at once intriguing and deeply contested. Reports alternately describe pluripotent-like properties, lineage contribution, or, conversely, technical artifacts lacking functional relevance. Despite more than a decade of investigation, no synthesis has yet reconciled these conflicting narratives within a perinatal-specific translational framework. We conducted a systematic review in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines to critically evaluate the identity, isolation, biological behavior, and clinical relevance of VSELs, with particular emphasis on human cord blood and fetal–neonatal contexts. Electronic database searches identified 312 records, of which 34 studies met predefined inclusion criteria after rigorous screening and eligibility assessment. Data were synthesized across experimental, methodological, and clinical domains, with a structured appraisal of risk of bias and reproducibility. Across studies, VSEL-like cells were consistently described as extremely rare, phenotypically heterogeneous, and frequently epigenetically quiescent. Evidence supporting stable pluripotency or self-renewal was limited and method-dependent, while multiple high-stringency investigations failed to confirm key stem cell properties. Methodological rigor emerged as a central determinant of divergent findings, with increasing specificity accompanied by progressive signal loss and reduced yield. Translational claims, particularly in perinatal medicine, were largely extrapolative and unsupported by definitive in vivo functional validation. Taken together, the available evidence suggests that VSELs are best understood not as a discrete, clinically actionable stem cell entity, but as a biologically ambiguous population shaped by technical resolution and context. Future progress will depend on standardized isolation protocols, single-cell multiomics approaches, and lineage-tracing strategies capable of distinguishing true developmental potential from methodological artifact.