Beyond Passage Numbers: How Culture Conditions and Population-Doubling Metrics Reporting Shape the Quality of Umbilical Cord-Derived MSCs and Extracellular Vesicles.

Mesenchymal stromal cells (MSCs) are central to regenerative medicine and advanced therapies. However, the absence of consensus on reporting kinetic parameters, such as population doubling level (PDL), population doubling time (PDT), and the reliance on passage number alone obscures biological age and manufacturing history, and limits correlation of potency with expansion dynamics. Here, we clarify the distinctions among passages, PDL, PDT, and replication rate; we synthesize evidence that identical passage numbers can conceal multifold differences in cumulative doublings, with downstream effects on transcriptomic stability, and immunomodulatory performance. We further highlight culture determinants, oxygen tension, seeding density, media formulation, surface/bioreactor systems, and early niche mimetic stimuli, that shape proliferative kinetics and cellular aging trajectories in WJ-MSCs. Critically, we propose extracellular vesicles (EVs) as sensitive functional readouts of bioprocess stress and expansion history: EV quantity can increase while functional bioactivity declines, and EV miRNA cargo captures cell state programs not evident from minimal identity markers. To address these gaps, we recommend a reporting framework that incorporates: (1) culture conditions, (2) passage number and PDL at harvest, and (3) functional consequences of expansion. Adopting kinetic metrics beyond passage number will harmonize data capture and enable pooled analyses, accelerating clinical translation while safeguarding patient outcomes.