A function-first legacy in hematopoietic stem cell biology: the scientific impact of Hal E. Broxmeyer

Hal E. Broxmeyer profoundly shaped modern hematopoietic stem cell biology through a rigorously functional approach that defined stem and progenitor cells by what they do rather than how they appear. Across 5 decades, his work established a unifying principle: biological mechanisms matter most when they preserve or enhance durable, multilineage hematopoietic reconstitution, particularly in transplantation. This function‑first philosophy guided seminal contributions spanning cytokine regulation of hematopoiesis, umbilical cord blood transplantation, stem cell mobilization, and the biology of hypoxia. Broxmeyer helped define hematopoietic regulation as emerging from a complex, context‑dependent "sea of cytokines," challenging reductionist models that assigned fixed roles to individual factors. This conceptual framework informed translational advances, including the identification of dipeptidyl peptidase‑4 (DPP4/CD26) as a key regulator of chemokine activity, stem cell homing, mobilization, and engraftment, ultimately influencing clinical mobilization strategies and cord blood transplantation outcomes. His pioneering demonstration that human umbilical cord blood contains functionally competent hematopoietic stem cells transformed discarded biological material into a globally used graft source. Equally transformative was his recognition of oxygen tension as a critical, often overlooked determinant of stem cell integrity. By defining extraphysiological oxygen shock/stress (EPHOSS), Broxmeyer revealed how routine handling conditions compromise stem cell function and identified mechanistic strategies to preserve engraftment capacity. Together, these contributions reshaped experimental standards, aligned basic discovery with clinical reality, and trained generations of scientists to prioritize functional validation. Broxmeyer's legacy endures not only in clinical practice worldwide, but in a way of thinking that anchors discovery to biological and therapeutic relevance.