Immunophenotypic characterization and clinical relevance of passage number in umbilical cord mesenchymal stromal cells: A review

Umbilical cord mesenchymal stromal cells (UC-MSCs) are emerging as leading stem cells in regenerative medicine due to their high proliferative capacity, potent immunomodulatory effects, and non-invasive collection. However, the absence of standardized guidance on optimal passage number and tissue-specific characterization criteria limits clinical translation, raising concerns about variability in potency, genomic stability, and safety. This review synthesizes evidence on how <i>in vitro</i> passaging alters UC-MSC properties, including purity, proliferation, senescence, genomic integrity, differentiation, and immunomodulation. Preclinical data suggest that UC-MSCs tolerate extended passaging better than MSCs derived from other sources, with several functional attributes preserved up to later passages. Clinical evidence indicates that early to middle passages (P3-P5) achieve the best balance between scalability and therapeutic efficacy. We further propose an updated immunophenotypic framework incorporating tissue-specific positive and negative markers to enhance clinical-grade characterization. Establishing harmonized passage guidelines and potency assays is essential to maximize reproducibility, safety, and the translational potential of UC-MSC therapies.