No single person discovered stem cells. The concept emerged gradually over more than a century, shaped by German embryologists in the 1860s, Russian histologists in the early 1900s, Canadian radiation biologists in the 1960s, and molecular biologists working across multiple continents from the 1980s onward. Each wave of discovery redefined what “stem cell” meant, expanding from a theoretical label for the fertilized egg to a concrete, isolatable biological entity with enormous medical implications. The story is less a eureka moment than a relay race, and the baton is still being passed.
Ernst Haeckel and the Birth of a Term
The phrase “stem cell” first appeared in the scientific literature in 1868, coined by the German biologist Ernst Haeckel. A vocal champion of Darwin’s theory of evolution, Haeckel drew elaborate phylogenetic trees he called “Stammbäume” (German for stem trees or family trees). He used the word “Stammzelle” (stem cell) to describe the hypothetical ancestral single-celled organism from which all multicellular life evolved. A few years later, in a revised edition of his book on human development, Haeckel made a conceptual leap: he proposed that the fertilized egg should also be called a stem cell, since it gives rise to every cell type in the body. So from the very beginning, the term carried two meanings: the evolutionary ancestor of complex life and the developmental origin of an individual organism.1Cell Stem Cell. On the Origin of the Term “Stem Cell”
The idea was abstract at this stage. Haeckel and his contemporaries, including the embryologist August Weismann, who drew the first cellular “stem tree” diagram showing how cell lineages branch during development, were working from microscopes and theory. They had no way to isolate a stem cell or prove its properties experimentally. Yet the basic framework they established, that undifferentiated cells sit at the root of increasingly specialized cell types, would persist for more than a century before anyone could test it rigorously.2History and Philosophy of the Life Sciences. Images of cell trees, cell lines, and cell fates: the legacy of Ernst Haeckel and August Weismann in stem cell research
Maximow and the Idea of a Common Blood Cell
The next conceptual leap came from a different direction: blood. In 1909, the Russian scientist Alexander Maximow proposed that all the varied cells of the blood, red cells, white cells, platelets, originate from a single common precursor cell within the bone marrow. He also suggested that these precursor cells depended on a specific local environment, or niche, to function properly.3ScienceDirect. On the origin of hematopoietic stem cells: Progress and controversy – Section: The concept of hematopoietic stem cells: More than a century ago This was a remarkably prescient idea. The notion that stem cells need a supportive microenvironment, now called the “stem cell niche,” remains central to modern stem cell biology. But like Haeckel’s work, Maximow’s theory ran well ahead of the technology available to prove it. Demonstrating that a single cell could generate the entire blood system would take another half century.
Radiation, Bone Marrow, and the First Functional Proof
The breakthrough that turned stem cells from a theoretical concept into an experimentally verified reality came in the early 1960s, thanks to two Canadian researchers: Ernest McCulloch and James Till, working at the Ontario Cancer Institute in Toronto. Their work grew out of a very practical concern. Scientists studying the effects of radiation on animals had noticed that lethal doses of radiation destroyed the blood-forming system, but that mice could be rescued by transplanting bone marrow from healthy donors. The question was why. What was in the bone marrow that rebuilt the blood?
In 1961, Till and McCulloch injected bone marrow cells into irradiated mice and noticed that visible nodules, or colonies, formed on the spleens of the recipient animals. Each nodule turned out to contain a mixture of different blood cell types. Crucially, in a follow-up study published in 1963, they demonstrated that individual colonies were clonal, meaning each colony arose from a single transplanted cell that had multiplied and diversified into multiple blood cell lineages.4PubMed Central. Cytological demonstration of the clonal nature of spleen colonies derived from transplanted mouse marrow cells This was the first hard experimental evidence for a cell that could both renew itself and produce different specialized offspring, the two defining properties of what we now call a stem cell. The hematopoietic (blood-forming) stem cell had been found.
Tumors That Taught Scientists About Embryos
While the hematopoietic stem cell was being pinned down in blood, a parallel and initially unrelated line of research was unfolding in cancer biology. Scientists had long been fascinated by teratocarcinomas, bizarre tumors that contain a jumble of tissue types including teeth, hair, bone, and gut lining, all growing chaotically within a single mass. By the 1960s and 1970s, researchers realized that these tumors were driven by a particular kind of cancer cell called an embryonal carcinoma (EC) cell. EC cells could divide indefinitely and give rise to many different tissue types, much like the cells of an early embryo.5PubMed Central. From teratocarcinomas to embryonic stem cells
Work on a specific strain of mice (strain 129) that was especially prone to germ cell tumors provided a crucial bridge. Researchers traced the origin of these tumors to normal embryonic cells that had gone awry, demonstrating that the malignancy arose from developmentally totipotent stem cells, normal cells that still had the potential to generate all tissue types.6PubMed Central. Somatic cell origin of teratocarcinomas The recognition that EC cells were essentially the cancerous counterparts of normal embryonic stem cells was pivotal.7PubMed Central. The origins of human pluripotent stem cells: the road from a cancer to regenerative medicine If the malignant version of these cells could be grown in a lab, perhaps the normal version could too. That insight set the stage for one of the most consequential experiments in modern biology.
Mouse Embryonic Stem Cells, 1981
In 1981, two independent groups accomplished what the teratocarcinoma research had been pointing toward. Martin Evans and Matthew Kaufman in Cambridge, England, reported that they had established pluripotent cell lines directly from mouse blastocysts, the hollow ball of cells that forms a few days after fertilization.8Nature. Establishment in culture of pluripotential cells from mouse embryos Simultaneously, Gail Martin at the University of California, San Francisco, isolated similar pluripotent cells from the inner cell mass of mouse blastocysts using medium that had been conditioned by teratocarcinoma stem cells, a direct debt to the earlier cancer research. Martin is also credited with coining the term “embryonic stem cells” to describe these lines.9PubMed Central. Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells
These mouse embryonic stem (ES) cells could be maintained in culture indefinitely while retaining the ability to differentiate into virtually any cell type. Their isolation opened the door to gene-targeting experiments (the creation of “knockout mice” to study gene function), which would eventually earn Evans a share of the 2007 Nobel Prize in Physiology or Medicine. But the bigger question, at least for medicine, was whether the same feat could be achieved with human cells.
Human Embryonic Stem Cells, 1998
It took seventeen more years. In 1998, James Thomson and colleagues at the University of Wisconsin–Madison reported the derivation of stem cell lines from human blastocysts. These cells had normal chromosomes, expressed markers characteristic of primate embryonic stem cells, and after months of growth in culture still retained the ability to form tissues from all three embryonic germ layers: gut lining (endoderm), bone and muscle (mesoderm), and nerve tissue (ectoderm).10PubMed. Embryonic stem cell lines derived from human blastocysts Thomson’s paper immediately recognized the potential applications in developmental biology, drug testing, and transplantation medicine.
It also ignited fierce ethical and political debate. Because deriving these cells required destroying human embryos at the blastocyst stage, the research became entangled with disputes over the moral status of early embryos and the onset of personhood.11PubMed Central. Ethical issues in stem cell research Different countries responded with strikingly different regulatory frameworks. In the United States, federal funding for embryonic stem cell research was restricted under the George W. Bush administration to a small number of pre-existing cell lines, a policy that was relaxed under subsequent administrations. China’s stance evolved as well, shifting over two decades from strict moral standards toward what has been described as “ethical neutrality,” gradually recognizing certain types of embryonic stem cell research as legitimate.12PubMed Central. From strict moral standards to ethical neutrality: a policy-guided shift in the patentability of human embryonic stem cells in China These policy differences have had lasting effects on which countries lead in specific areas of stem cell research.
Yamanaka’s Reprogramming Revolution
In 2006, Shinya Yamanaka and Kazutoshi Takahashi at Kyoto University published a paper that reshaped the entire field. They showed that ordinary mouse skin cells (fibroblasts) could be reprogrammed into a state closely resembling embryonic stem cells by introducing just four genes. These reprogrammed cells, which they called induced pluripotent stem (iPS) cells, looked like embryonic stem cells, grew like them, and expressed the same molecular markers.13PubMed. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors The team later repeated the achievement with human cells.14Cell. Shinya Yamanaka
The implications were enormous. iPS cells offered a path to patient-specific pluripotent cells without destroying embryos, potentially sidestepping the central ethical objection to embryonic stem cell research.11PubMed Central. Ethical issues in stem cell research Yamanaka shared the 2012 Nobel Prize in Physiology or Medicine with John Gurdon, whose earlier work on nuclear transfer in frogs had established that cell specialization could be reversed. The iPS technology has since become a workhorse of disease modeling, drug screening, and early-stage regenerative medicine research.
Adult Stem Cells Found Hiding in Plain Sight
Parallel to the dramatic story of embryonic and induced pluripotent stem cells, researchers were discovering that many adult tissues harbor their own resident stem cell populations, quietly replenishing worn-out cells throughout a person’s lifetime.
One of the earliest clues came from the work of Alexander Friedenstein and colleagues, who in the 1960s identified a population of cells in bone marrow that could form bone. These cells, now called mesenchymal stem cells, are distinct from the blood-forming stem cells that Till and McCulloch had found in the same tissue.15PubMed Central. Alexander Friedenstein, Mesenchymal Stem Cells, Shifting Paradigms and Euphemisms Mesenchymal stem cells can give rise to bone, cartilage, and fat cells, and they have become one of the most widely studied cell types in regenerative medicine, though their true potency and therapeutic value remain hotly debated.
The adult brain posed an even bigger surprise. For most of the twentieth century, neuroscience textbooks stated flatly that the adult brain could not produce new neurons. That dogma crumbled in the 1990s and 2000s as researchers demonstrated that neural stem cells persist in specific regions of the adult mammalian brain, continuously generating functional neurons throughout life.16PubMed Central. Adult neural stem cells in the mammalian central nervous system The degree to which this process, called adult neurogenesis, occurs in the human brain remains a subject of active investigation, but the existence of the cells themselves is well established.
The gut provided another landmark. In 2007, Hans Clevers and colleagues identified a molecular marker called LGR5 that labels actively dividing stem cells at the base of intestinal crypts, the tiny pocket-like structures lining the intestine.17The American Journal of Pathology. Wnt Signaling, Lgr5, and Stem Cells in the Intestine and Skin – Section: Lgr5 Marks Adult Intestinal SCs These LGR5-positive cells divide roughly once a day and are responsible for the constant renewal of the gut lining, which replaces itself approximately every five days.18PubMed. Stem cells marked by the R-spondin receptor LGR5 The discovery of LGR5 as a definitive stem cell marker was transformative because it gave researchers a way to prospectively identify, isolate, and study individual stem cells rather than inferring their existence indirectly.
Cancer Stem Cells and Why They Matter
The stem cell concept also reshaped thinking about cancer. In the mid-1990s, John Dick and colleagues at the University of Toronto demonstrated that acute myeloid leukemia is organized as a hierarchy, similar to the normal blood system. Only a rare subpopulation of leukemia cells, those with a specific set of surface markers, could re-initiate the disease when transplanted into immunodeficient mice. These leukemic stem cells sat at the top of the hierarchy, generating the bulk of cancer cells that had limited ability to sustain the disease on their own.19Haematologica. Leukemic stem cells and therapy resistance in acute myeloid leukemia – Section: Leukemic stem cells re-initiate leukemia
This was the first functional proof of a cancer stem cell, and the concept was later extended to solid tumors by Irving Weissman and others.20Cell. Who Discovered Stem Cells and How Did They Do It? – Section: Stem cell hierarchies in leukemias The cancer stem cell idea has practical consequences for treatment. If conventional chemotherapy kills the bulk of a tumor but spares the rare stem cells driving it, the cancer can regrow. Developing therapies that specifically target cancer stem cells has become a major goal in oncology, though translating the concept into effective drugs has proven difficult.
Stem Cells in Flatworms and Plants
Stem cell biology is not limited to mammals. Some of the most dramatic examples of stem cell power come from organisms that can regenerate entire body parts, or even entire bodies, from tiny fragments.
Planarian flatworms are the standout example. These small freshwater animals contain a population of adult stem cells called neoblasts. Some neoblasts are genuinely pluripotent: a single transplanted neoblast can restore tissue turnover and full regenerative ability to a worm whose own stem cells have been destroyed by radiation. Researchers demonstrated this by transplanting individual neoblasts into lethally irradiated planarians and watching the recipients rebuild their entire bodies, brains, guts, sensory organs, and all.21PubMed Central. Specialized progenitors and regeneration – Section: The specialized neoblast model Complete, perfectly proportioned animals can regenerate even from tiny tissue remnants, thanks to these cells.22PubMed Central. Stem cell systems and regeneration in planaria
Plants have their own version of stem cells, though they work in a very different architectural context. Plant stem cells reside in structures called apical meristems at the tips of shoots and roots. These cells remain in a pluripotent state throughout the plant’s lifespan, continuously producing daughter cells that differentiate into leaves, flowers, bark, and roots.23PubMed Central. Stem cells within the shoot apical meristem: identity, arrangement and communication A tree hundreds of years old is still generating new tissue from the same stem cell populations that were present in its seedling stage.24PubMed Central. Initiation and maintenance of plant stem cells in root and shoot apical meristems The parallels with animal stem cells are striking: both require specialized niches, both balance self-renewal against differentiation, and both use conserved signaling pathways to maintain that balance. Yet the plant and animal systems evolved independently, suggesting that the stem cell strategy is so useful that evolution converged on it more than once.
Organoids and Where the Field Is Heading
One of the most visible recent outgrowths of stem cell discoveries is the development of organoids, miniature three-dimensional structures grown from stem cells that mimic the architecture and function of real organs. By supplying the right combination of growth signals, researchers can coax stem cells from tissues such as the stomach, intestine, colon, pancreas, and liver into forming self-organized structures that recapitulate key features of the original tissue.25PubMed. SnapShot: Growing Organoids from Stem Cells Brain organoids, sometimes called “mini-brains,” have attracted particular attention and controversy.
Organoid technology has been described as a revolution in biomedical research tools, creating powerful models for studying disease, screening drugs, and eventually, researchers hope, growing replacement tissues for transplantation.26PubMed Central. A brief history of organoids The technology traces a direct line back through every stage of the discovery story outlined here. The LGR5 marker identified in the gut gave researchers the ability to isolate the precise stem cells needed to grow intestinal organoids. The culture techniques descended from those developed for embryonic stem cells in the 1980s. And the conceptual foundation, that a single cell type can self-organize into complex tissue if given the right environment, reaches all the way back to Maximow’s 1909 insight about stem cell niches and Haeckel’s tree diagrams of cellular differentiation. What began as an abstract metaphor about ancestral cells now has a physical form: a cluster of cells in a dish, building itself into something that looks remarkably like a piece of a human organ.