PIONEERS OF OSTEOBIOLOGICS · PART ONE

The Cell

How Friedenstein, Owen, and Caplan Discovered — and Redefined — the Cells Behind Bone Regeneration

Bone marrow is not merely a factory for blood. It harbors, in quiet reserve, the precursor cells capable of rebuilding the entire skeleton.

Introduction

Every clinical advance in osteobiologics — from demineralized bone matrix to bone marrow aspirate concentrate to today's skeletal stem cell therapies — rests on a single insight: bone marrow contains a rare population of non-hematopoietic cells capable of forming bone, cartilage, and the supportive architecture of the skeleton itself.

That insight was built over roughly four decades by three scientists working across different continents, disciplines, and eras — Alexander Friedenstein, Maureen Owen, and Arnold I. Caplan.

This is the story of how they found the cell, what they called it, and why the argument over its name still matters to every surgeon, researcher, and product developer working in osteobiologics today.

I.  Alexander Friedenstein (1924–1998)

The Man Who Found the Osteogenic Precursor

A Scientist in the Shadow of the Iron Curtain

Alexander Yakov Friedenstein was born on June 24, 1924, in Kyiv, Ukraine. When he was four years old, his family moved to Moscow, where he would spend the rest of his life. He trained as a physician and immunologist at the Gamaleya Institute for Epidemiology and Microbiology within the Soviet Academy of Medical Sciences — an institution that, despite geographic isolation from the West, produced scientific work of lasting significance.[1,2]

At the time, the idea that bone marrow contained dedicated skeletal progenitors was radical. Bone formation was largely viewed as a property of tissue itself rather than the product of a rare, identifiable cell population. While many contemporaries were focused on lymphocytes and transplant immunology, Friedenstein directed his attention toward bone — and toward a question that had no satisfying answer: where do the cells that build bone actually come from in the adult organism?

The Diffusion Chamber Experiments

Beginning in the early 1960s, Friedenstein pursued that question using an elegant experimental system: the diffusion chamber. By placing fragments of bone marrow into sealed chambers that allowed nutrients and soluble molecules to pass while preventing cellular escape, then implanting those chambers into recipient animals, he could observe the intrinsic capabilities of the transplanted marrow cells without interference from host tissue.[3]

The marrow fragments generated bone and cartilage at ectopic sites entirely removed from native skeletal tissue — something within the marrow itself could initiate osteogenesis from scratch.[3] In 1968, Friedenstein published what would become a defining paper in bone biology, demonstrating that bone marrow contains dedicated osteogenic precursor cells separable from hematopoietic tissues.[4]

Colony-Forming Units–Fibroblastic (CFU-F)

Friedenstein's next major advance came in 1970. By plating bone marrow cell suspensions at low density on plastic tissue culture dishes, he and his colleagues observed the formation of discrete adherent colonies composed of fibroblast-like cells, each arising from a single clonogenic progenitor.[4] Friedenstein initially termed these cells fibroblast colony-forming cells (FCFCs) — later standardized as colony-forming units–fibroblastic (CFU-F) — terminology that remains clinically relevant today when assessing progenitor content in bone marrow aspirates.[5]

Over subsequent years, he demonstrated that these cells were not merely adherent fibroblasts. They were multipotent stromal progenitors capable of generating bone, cartilage, adipose tissue, and hematopoietic-supportive stroma when transplanted in vivo.[6] He also calculated, based on the cells' extraordinary proliferative capacity, that very small quantities of marrow-derived progenitors could theoretically yield remarkable amounts of skeletal tissue following extensive expansion in culture.[8] It was the first compelling evidence that genuine osteogenic stem-like cells existed within postnatal bone marrow.

The high proliferative potential of bone marrow [fibroblast colony-forming cells] and their ability to serve as common precursors of bone and cartilage-forming cells makes them probable candidates for the role of osteogenic stem cells.

— Friedenstein, Chailakhyan & Gerasimov — Cell and Tissue Kinetics, 1987

Why Friedenstein's Work Was Initially Overlooked

Despite its significance, Friedenstein's work went largely unrecognized in Western science for nearly two decades.[2] Many early papers appeared in Russian journals or Soviet-era English translations, scientific exchange across the Iron Curtain was limited, and bone biology at the time remained focused on matrix proteins, calcium metabolism, and endocrinology rather than cellular progenitor systems. It would take the work of a British scientist — and an Oxford laboratory housed in a converted hospital laundry — to bring his discoveries into broader international view.

II.  Maureen Owen, PhD

The Oxford Bridge Builder

From Nuclear Physics to Bone Biology

Maureen Owen's path to osteogenic stem cells was genuinely unusual. After graduating with honors from Queen's University Belfast in 1948, she earned a doctorate in nuclear physics from Oxford in 1952 — an unlikely foundation for someone who would become one of the most influential figures in skeletal biology.[7]Following postdoctoral work at the Donner Laboratory for Biophysics at the University of California, Berkeley, Owen joined the Medical Research Council's Bone Seeking Isotopes Research Unit at Churchill Hospital, Oxford, in 1958.[7]

Her physics and biophysics training turned out to be an unexpected advantage. Owen brought a quantitative, cell-kinetics perspective to bone biology, publishing important work on osteoblast turnover and lineage tracking as early as 1963.[7] The analytical precision of that approach was uncommon in skeletal biology at the time.

The Friedenstein–Owen Connection

In the 1970s, Owen encountered Friedenstein's work and recognized its importance immediately.[2] In 1977, Friedenstein visited her newly established MRC Bone Research Laboratory at the Nuffield Orthopaedic Centre in Oxford — a modest operation at the time, comprising Owen, James Triffitt, two technicians, and a part-time histologist, all working out of a space that had formerly been the hospital laundry.[2]

Owen's group set out to confirm Friedenstein's findings, refine CFU-F methodologies, characterize the distribution of osteogenic progenitors within the marrow, and study the effects of systemic hormones on differentiation.[2] Oxford's disciplined replication of the stromal progenitor system brought Friedenstein's work into the mainstream of skeletal biology in a way it had never managed on its own.

Formalizing the Stromal Stem Cell Concept

Owen's most lasting contribution came in 1988 with the publication of "Stromal Stem Cells: Marrow-Derived Osteogenic Precursors," co-authored with Friedenstein in the Ciba Foundation Symposium proceedings.[9]The paper proposed that the CFU-F population contained authentic stromal stem cells capable of self-renewal, hierarchical differentiation, and long-term maintenance of the marrow microenvironment.

That same year, Owen published "Marrow Stromal Stem Cells" in the Journal of Cell Science Supplement, outlining a differentiation hierarchy spanning fibroblastic, reticular, adipocytic, and osteogenic lineages.[10]The hierarchical model — a stem cell giving rise to multiple specialized skeletal-supportive tissues — anticipated much of the conceptual framework that now governs regenerative medicine. Owen also argued that bone and marrow function as an integrated biological system sharing a common progenitor pool, a reframing that would later become central to understanding why bone marrow aspiration itself carries osteobiologic meaning.

III.  Arnold I. Caplan, PhD

The Namer and the Reframer

From Embryonic Chick Limbs to Human Bone Marrow

Arnold Caplan arrived at the osteogenic progenitor problem from an entirely different direction. Working at Case Western Reserve University in Cleveland, Ohio, he spent the late 1960s and 1970s tracing the developmental lineage of cartilage, bone, and muscle using embryonic chick limb models.[11] The work gave him an unusually precise picture of how skeletal tissues arise from undifferentiated mesodermal precursors.

In the late 1970s, Caplan attended a Gordon Conference presentation by Marshall R. Urist and came away convinced that the bone-inducing molecules Urist described must act on a defined population of responsive progenitor cells.[11] The two questions — what signals drive bone formation, and which cells respond to those signals — had been running in parallel. Caplan saw that they were asking about the same biology.

Naming the Mesenchymal Stem Cell

By the late 1980s, Caplan and colleagues had developed monoclonal antibody techniques capable of characterizing marrow-derived progenitor populations and tracking their differentiation in culture.[11]Drawing on his developmental biology background, Caplan formally coined the term mesenchymal stem cell (MSC) in his 1991 paper published in the Journal of Orthopaedic Research[12] — a paper that became one of the most cited in regenerative medicine.[11]

The paper proposed a "Mesengenic Process" — a differentiation hierarchy in which a single MSC could give rise to bone, cartilage, tendon, ligament, marrow stroma, adipose tissue, and muscle. The term "mesenchymal" reflected the prevailing embryologic framework of the time, which traced these connective tissues to mesodermal developmental lineages.

Bone and cartilage formation in the embryo and repair and turnover in the adult involve the progeny of a small number of cells called mesenchymal stem cells.

— Arnold I. Caplan — Journal of Orthopaedic Research, 1991

From MSC to "Medicinal Signaling Cell"

As clinical MSC research expanded through the 1990s and 2000s, it became clear that these cells often did not function primarily through direct tissue replacement after transplantation. Much of their therapeutic activity appeared to arise instead from paracrine signaling, immunomodulation, progenitor recruitment, vascular support, and niche regulation.[14]

In 2017, Caplan proposed a deliberate conceptual shift: retaining the MSC acronym while redefining it as "medicinal signaling cell."[13] The proposal generated debate, but the underlying argument — that the therapeutic value of these cells lies as much in what they secrete as in what they become — has since taken hold. It now informs the rationale behind exosome therapies, secretome-based products, immunomodulatory orthobiologics, and current interpretations of how bone marrow aspirate works clinically.

IV.  The Naming Debate — and Why It Matters

The three pioneers did not agree on what to call the cell they spent their careers studying. Friedenstein favored "osteogenic precursor." Owen formalized "stromal stem cell." Caplan popularized "mesenchymal stem cell." Later, scientists Paolo Bianco and Pamela G. Robey made the case for "skeletal stem cell," arguing that "MSC" implied a developmental potency the experimental evidence did not support.[15]

The argument is not merely semantic. The name attached to a cell shapes research priorities, regulatory classification, commercial positioning, and patient expectations. "Mesenchymal stem cell" proved enormously powerful as a commercial term — it captured imagination and funding in equal measure — but it also contributed to an era in which MSC therapies were sometimes promoted as universal regenerative solutions rather than targeted osteobiologic tools.

The naming debate reflects a tension that still defines orthobiologics: the gap between what the science demonstrates and what the market promotes. Knowing how carefully these pioneers originally defined their work is essential context for evaluating claims made in its name today.

V.  What "The Cell" Means for Clinical Practice

The work of Friedenstein, Owen, and Caplan established four principles that remain central to osteobiologic practice.

1

Bone marrow — not peripheral blood — is the right source. Bone marrow contains a rare but potent population of osteogenic progenitor cells distinct from hematopoietic lineage cells. This is why bone marrow aspiration became the basis of cell-based orthobiologic therapies rather than peripheral blood draw.[3,6]

2	These cells can be expanded. They are clonogenic, multipotent, and capable of substantial expansion in culture — the scientific foundation for modern ex vivo cell expansion strategies.[4,5,6]

3

Differentiation is not the whole story. Paracrine signaling, immunomodulation, and microenvironment regulation are equally important mechanisms — concepts now shaping how clinicians think about bone marrow aspirate concentrate, exosome therapies, growth factor delivery, and biologic augmentation.[13,14]

4

Quality and quantity vary — and technique matters. Progenitor concentration and quality depend on patient age, aspiration site, aspiration technique, biologic dilution, and patient-specific biology. Every bone marrow-derived orthobiologic protocol is, in practice, an attempt to optimize delivery of the progenitor system Friedenstein first identified more than half a century ago.[16]

NEXT IN THE SERIES: PART TWO — "THE SIGNAL"

Part Two traces how Marshall R. Urist, A. Hari Reddi, and John Wozney identified the molecular signals capable of instructing these progenitor cells to form bone — culminating in the cloning of BMP-2 and the development of the first FDA-approved osteoinductive biologics.

Closing Thought

Modern orthobiologics are discussed as products — BMP kits, BMAC systems, stem cell therapies, synthetic grafts. But every one of those technologies traces back to a simpler and more consequential discovery: bone marrow contains cells capable of rebuilding the skeleton. Everything that followed was an attempt to understand, harness, and direct them.

References

1.  Phinney DG. Alexander Friedenstein, mesenchymal stem cells, shifting paradigms and euphemisms. Bioengineering (Basel). 2024;11(6):534.

2.  Triffitt JT. The collaborative spark that ignited the field of stromal stem cell biology. JBMR Plus. 2024;8(8):ziae079.

3.  Friedenstein AJ, Petrakova KV, Kurolesova AI, Frolova GP. Heterotopic of bone marrow: analysis of precursor cells for osteogenic and hematopoietic tissues. Transplantation. 1968;6(2):230–247.

4.  Friedenstein AJ, Chailakhjan RK, Lalykina KS. The development of fibroblast colonies in monolayer cultures of guinea-pig bone marrow and spleen cells. Cell Tissue Kinet. 1970;3(4):393–403.

5.  Friedenstein AJ, Chailakhyan RK, Gerasimov UV. Bone marrow osteogenic stem cells: in vitro cultivation and transplantation in diffusion chambers. Cell Tissue Kinet. 1987;20(3):263–272.

6.  Bianco P, Riminucci M, Gronthos S, Robey PG. Bone marrow stromal stem cells: nature, biology, and potential applications. Stem Cells. 2001;19(3):180–192.

7.  Owen M. Cell population kinetics of an osteogenic tissue. J Cell Biol. 1963;19(1):19–32.

8.  Triffitt JT. A brief history of the development of stromal stem cells (stem cells of the skeleton). J Bone Miner Res. 2022;37(8):1597–1609.

9.  Owen M, Friedenstein AJ. Stromal stem cells: marrow-derived osteogenic precursors. Ciba Found Symp. 1988;136:42–60.

10.  Owen M. Marrow stromal stem cells. J Cell Sci Suppl. 1988;10:63–76.

11.  Caplan AI. An interview with cell therapy pioneer, Arnold Caplan. Stem Cell Res Ther. 2022;13:182.

12.  Caplan AI. Mesenchymal stem cells. J Orthop Res. 1991;9(5):641–650.

13.  Caplan AI. Mesenchymal stem cells: time to change the name! STEM CELLS Transl Med. 2017;6(6):1445–1451.

14.  Caplan AI, Dennis JE. Mesenchymal stem cells as trophic mediators. J Cell Biochem. 2006;98(5):1076–1084.

15.  Bianco P, Robey PG. Skeletal stem cells. Development. 2015;142(6):1023–1027.

16.  Muschler GF, Boehm C, Easley K. Aspiration to obtain osteoblast progenitor cells from human bone marrow: the influence of aspiration volume. J Bone Joint Surg Am. 1997;79(11):1699–1709.