Reconstructing Human Long-Term Hematopoietic Stem Cells

Generating bona fide human long-term hematopoietic stem cells (LT-HSCs) from pluripotent stem cells remains a major challenge in regenerative medicine. Although current protocols can reproduce mesoderm specification, hemogenic endothelium, endothelial-to-hematopoietic transition (EHT), and increasingly sophisticated HSC-like phenotypes, these milestones do not necessarily establish durable long-term self-renewal, multilineage reconstitution, or serial transplantation capacity. This Perspective proposes that LT-HSC identity may depend not only on endpoint phenotype, but also on the developmental history through which that state is acquired. We examine the possibility that a critical, incompletely reconstructed stage lies after EHT, during fetal-liver-associated HSC maturation. The fetal-liver niche is considered as a multidimensional developmental environment integrating cellular interactions, soluble signals, extracellular matrix, mechanical cues, metabolic regulation, immune feedback, and temporal control of developmental signals. We further distinguish four biologically separable problems: HSC generation, HSC maturation, fetal-to-adult transition, and LT-HSC maintenance/expansion. Based on this framework, we propose an isogenic “seed-and-soil” strategy in which PSC-derived nascent HSCs and fetal-liver-like niche components are generated separately and recombined within defined developmental windows. We also extend the concept to manufacturing, arguing that scalable LT-HSC production may require control not only of cell identity and yield, but also of developmental trajectory, signal timing, signal duration, signal withdrawal, niche exposure, and process-dependent trajectory drift. The central hypothesis is that successful LT-HSC engineering may require reconstruction not merely of the appearance of an HSC, but of the developmental sequence through which a stable, transplantable, self-renewing LT-HSC is formed. StemImmune Perspective | September 2026 Publication ID: SIP-2026-004