Stroma is the supportive tissue framework that surrounds and sustains the functional cells of an organ or structure. While most people hear about the working cells of a tissue (the parenchyma, which does the organ’s signature job), stroma provides the physical scaffolding, chemical signals, and blood supply those working cells depend on. The term shows up across wildly different biological contexts, from the gel-like interior of a plant’s chloroplast to the connective tissue of a human ovary to the dense web of fibers surrounding a tumor. Understanding what stroma does in each setting reveals how much of biology depends not on the star players, but on the supporting cast.
The Structural Framework of Organs
In most animal tissues, stroma is made up of connective tissue cells (primarily fibroblasts), the extracellular matrix they secrete, blood vessels, nerves, and immune cells that together form a living scaffold. This framework is not passive. Stromal cells actively produce collagen, growth factors, and signaling molecules that guide how the parenchymal cells develop, function, and repair themselves. Researchers generating kidney organoids from stem cells, for example, found that without incorporating stromal progenitors alongside the functional kidney cell types, the organoids could not reproduce the complex branching architecture of a real kidney. Only when stromal cells were added did the organoid develop the properly organized structures seen in a living organ.1Nature Communications. Generation of the organotypic kidney structure by integrating pluripotent stem cell-derived renal stroma
A striking example of stroma’s structural importance is the cornea of the eye. The corneal stroma makes up the bulk of the cornea’s thickness and consists of collagen fibrils that are narrow, uniform in diameter, and arranged with precise regularity. That arrangement is what makes the cornea transparent: if the fibrils were randomly spaced or varied in size, light would scatter and vision would blur. The same collagen architecture also provides the mechanical strength the cornea needs to maintain its curved shape under the pressure of the fluid inside the eye.2PubMed Central. Corneal collagen-its role in maintaining corneal shape and transparency
The Bone Marrow Niche
Bone marrow stroma is one of the best-studied examples of how stromal tissue governs the behavior of other cells. The marrow contains mesenchymal stromal cells (MSCs) that sit in a perivascular position, meaning they wrap around or nestle against blood vessels within the bone. These stromal cells secrete signaling molecules, including CXCL12 and stem cell factor, that are critical for maintaining the pool of blood-forming stem cells and regulating when those stem cells divide, stay dormant, or leave the marrow.3PubMed Central. Mesenchymal Stromal Cells: Role in the BM Niche and in the Support of Hematopoietic Stem Cell Transplantation It is the more immature stromal cells in the bone marrow, rather than the fully mature bone-forming cells, that appear to be the key regulators of these blood stem cells.4Cell Stem Cell. Mesenchymal Stromal Cells in the Bone Marrow
This means that when the bone marrow stroma deteriorates or becomes abnormal, blood cell production can go haywire. Leukemia, myelodysplastic syndromes, and bone marrow failure conditions all involve disrupted stromal signaling. Transplant physicians have investigated co-infusing mesenchymal stromal cells alongside bone marrow transplants in hopes of rebuilding a healthy niche, though that work is still maturing.
Stroma and the Immune System
Two immune organs where stroma plays an outsized role are the thymus and the lymph nodes. In the thymus, stromal cells called thymic epithelial cells are responsible for educating immature T cells. Cortical thymic epithelial cells use unique protein-processing systems to present fragments of the body’s own molecules to developing T cells, selecting those that can recognize threats. Medullary thymic epithelial cells then display proteins normally found only in specific tissues elsewhere in the body, weeding out any T cells that would mistakenly attack those tissues. This process is what prevents autoimmune disease in most people.5PubMed Central. Thymic stromal cell subsets for T cell development
In lymph nodes, stromal cells called fibroblastic reticular cells (FRCs) build and maintain the internal architecture. They create a network of channels and compartments that sort incoming immune cells into the right zones, bring antigens into contact with the cells that need to see them, and coordinate communication between different immune cell types. FRCs are not mere scaffolding; they actively shape the immune response by recruiting cells, presenting antigens, and modulating which signals get amplified.6PubMed Central. Lymph node fibroblastic reticular cells steer immune responses
How Tumor Stroma Helps Cancer Grow
Perhaps the most clinically urgent area of stroma research involves cancer. Tumors are not just masses of cancer cells; they are surrounded by a stroma that can make up the majority of the tumor’s bulk. The dominant stromal cell type in many solid tumors is the cancer-associated fibroblast (CAF), a cell that arises from normal fibroblasts or other stromal cells that have been reprogrammed by signals from the cancer. CAFs remodel the extracellular matrix around the tumor, create physical barriers, and secrete growth factors that help cancer cells migrate and invade new tissue.7Biochemical Society Transactions. Cancer-associated fibroblasts modulate growth factor signaling and extracellular matrix remodeling to regulate tumor metastasis
Lab experiments using three-dimensional tumor models have shown just how much this stromal remodeling matters. When patient-derived CAFs were mixed with cancer cell clusters, the cancer cells invaded significantly farther and over a greater area compared to models that used normal fibroblasts. CAF-containing tumors also showed elevated expression of genes linked to tissue invasion and matrix restructuring.8British Journal of Cancer. Cancer-associated fibroblasts mediate cancer progression and remodel the tumouroid stroma
Collagen deposited by CAFs can compose up to about 60% of total tumor mass in some cancers. This dense collagen meshwork does more than provide physical structure; it blocks immune cells from reaching the cancer. Research in bladder cancer patients found that when a growth factor called TGF-beta was highly expressed in the tumor stroma, cancer-killing T cells became trapped in the collagen-rich outer layers and could not penetrate to the tumor core. This rendered checkpoint immunotherapy ineffective. A similar pattern has been observed where high collagen levels exhaust the T cells themselves, further blunting the immune attack.9PubMed Central. Overcoming physical stromal barriers to cancer immunotherapy
Targeting Stroma as a Cancer Strategy
Because tumor stroma actively protects cancer, researchers are exploring ways to dismantle or repurpose it. One promising target is fibroblast activation protein (FAP), a molecule expressed at high levels on CAFs in many tumor types but largely absent from normal adult tissue. That selective expression makes FAP attractive for both imaging and therapy. Radioactive tracers that bind FAP are already being tested in clinical trials to light up tumors on PET scans, and some groups are developing FAP-targeted radioligand therapies that deliver radiation directly to the stroma.10PubMed Central. Fibroblast Activation Protein-Targeted Theranostics: Current Status in Clinical Development Engineered immune cells (CAR-T cells) designed to recognize FAP on stromal fibroblasts are another approach under investigation, aiming to strip away the tumor’s protective shield from the inside.11PubMed Central. Fibroblast Activation Protein (FAP)-Targeted CAR-T Cells: Launching an Attack on Tumor Stroma
These strategies represent a shift in oncology thinking. Rather than focusing exclusively on killing cancer cells, the field is beginning to treat the tumor ecosystem as a whole. If the stroma can be weakened or made more permeable, conventional chemotherapy and immunotherapy may work better against the cancer cells hiding inside.
Stroma in Wound Healing and Fibrosis
Stroma’s capacity for remodeling is vital for wound repair. When tissue is damaged, fibroblasts in the stroma transform into myofibroblasts, cells that produce large amounts of extracellular matrix proteins and contract the wound edges together. Myofibroblasts express alpha-smooth muscle actin, which gives them their contractile power, and they synthesize collagen at a much higher rate than resting fibroblasts.12PubMed Central. The myofibroblast, a key cell in normal and pathological tissue repair In a healthy wound, myofibroblasts do their job and then undergo programmed cell death once the tissue is repaired.
Trouble starts when myofibroblasts persist. Chronic myofibroblast activity drives fibrosis in the liver (cirrhosis), lungs (pulmonary fibrosis), kidneys, and skin (keloids and hypertrophic scars). These same cells also play a coordinating role in new blood vessel formation. During both wound healing and tumor growth, fibroblasts and myofibroblasts help orchestrate the growth of new capillaries by degrading the basement membrane around existing vessels, directing the sprouting of new vessel tips, and regulating the attachment of the cells that stabilize new vessels.13PubMed Central. Endothelial-stromal interactions in angiogenesis
Stromal Roles in Reproduction
The uterine lining contains a population of endometrial stromal cells that undergo one of the most dramatic transformations in human biology. Each menstrual cycle, in response to progesterone, these stromal cells swell, change shape, and begin secreting a new set of molecules in a process called decidualization. Decidualized stromal cells remodel the uterine tissue, promote local blood vessel growth, suppress oxidative damage, and create an immune-tolerant environment so the body does not reject an implanting embryo.14Reproductive and Developmental Medicine. Stromal cell decidualization and embryo implantation: a vulnerable step leading to successful pregnancy When decidualization is defective, implantation fails or pregnancy complications such as recurrent miscarriage can follow.15PubMed Central. The Regulators of Human Endometrial Stromal Cell Decidualization
In the ovary, stromal tissue has a different but equally important endocrine function. Ovarian stroma produces steroid hormones, including progesterone, androgens, and estrogens. Contrary to older models that assigned hormone production to just one compartment of the ovary, all three compartments (granulosa cells, thecal tissue, and stroma) have the biosynthetic capacity to produce the same set of hormones, at every stage of the menstrual cycle and follicle development.16The Journal of Clinical Endocrinology & Metabolism. The Production of Progesterone, Androgens, and Estrogens by Granulosa Cells, Thecal Tissue, and Stromal Tissue from Human Ovaries in Vitro
The Prostate and Other Hormone-Sensitive Stroma
Prostate stroma offers a case study in how stromal-immune interactions drive common disease. In benign prostatic hyperplasia (BPH), the prostate gland enlarges partly because its stromal cells are proliferating. Androgen receptor signaling in prostate stromal cells promotes the recruitment of macrophages into the tissue, and those macrophages in turn secrete factors that stimulate further stromal cell growth, creating a self-reinforcing loop.17Journal of Biological Chemistry. Androgen receptor promotes prostate stromal cell proliferation and macrophage infiltration in benign prostatic hyperplasia via stromal AR/CCL3 signaling This crosstalk between stromal cells and immune cells helps explain why BPH often features both tissue overgrowth and chronic inflammation, and why treatments that block androgen signaling can reduce prostate size.
The Chloroplast Stroma in Plants
The word “stroma” has a separate and much older usage in plant biology. Inside every chloroplast, the stroma is the gel-like fluid that fills the space around the thylakoid membranes where light reactions occur. The stroma is where carbon fixation happens: the enzyme Rubisco and the rest of the Calvin cycle machinery reside here, converting carbon dioxide into sugars. The pH of this compartment is critical. When light hits the chloroplast, protons are pumped into the thylakoid interior, causing the stromal pH to rise by about one unit to a peak around 8.1. Carbon fixation virtually stops if stromal pH drops below about 7.3, making this light-driven pH shift essential for photosynthesis.18PubMed. The role of pH in the regulation of carbon fixation in the chloroplast stroma. Studies on CO2 fixation in the light and dark
In some algae, the stroma houses a structure called the pyrenoid, where Rubisco is concentrated into liquid droplets through phase separation. Purified Rubisco and the linker protein EPYC1 spontaneously form liquid droplets when mixed, and the Rubisco inside these droplets remains fully active, fixing carbon at the same rate as dissolved Rubisco. This natural concentration mechanism is thought to supercharge carbon fixation by keeping Rubisco near a local supply of carbon dioxide, and plant scientists are trying to engineer a similar system into crop plants to boost yields.19Nature Communications. The phase separation underlying the pyrenoid-based microalgal Rubisco supercharger
Phase separation also plays a role in sorting proteins within the stroma. Newly made proteins destined for the thylakoid membranes are captured by a stromal sorting complex that forms liquid droplets upon binding its cargo. These droplets effectively gather the right proteins together before handing them off for insertion into the membrane.20PubMed. Liquid-Liquid Phase Transition Drives Intra-chloroplast Cargo Sorting Chloroplasts can also extend tubular projections of their stroma, called stromules, out into the surrounding cell. During a pathogen attack, stromules reach toward the cell’s nucleus and appear to shuttle defense signals, including hydrogen peroxide and defense proteins, from the chloroplast to the nucleus, amplifying the plant’s immune response.21PubMed Central. Chloroplast Stromules Function during Innate Immunity
Stromal Aging and Senescence
As the body ages, stromal cells accumulate damage and enter a state called senescence, where they stop dividing but remain metabolically active and begin secreting a cocktail of inflammatory cytokines, growth factors, and tissue-degrading enzymes. This secretory profile reshapes the tissue environment and can push neighboring healthy cells toward senescence as well, creating a self-propagating cycle.22PubMed. Senescent cells in systemic aging: SASP heterogeneity, immune escape, and endocrine modulation
In bone marrow, aging mesenchymal stromal cells secrete inflammatory signals that activate a damaging inflammatory program in nearby blood-forming stem cells, potentially impairing blood cell production in older adults.23Stem Cells Translational Medicine. Mesenchymal Stem/Stromal Cell Senescence: Hallmarks, Mechanisms, and Combating Strategies In the heart, aged cardiac mesenchymal stromal cells recruit inflammatory immune cells called monocytes. Those monocytes mature into macrophages that produce the inflammatory signal IL-1 beta, which feeds back on the stromal cells to deepen their senescent state and reduce their ability to differentiate into vascular cells. The result is a loop of inflammation and declining tissue repair capacity.24PubMed Central. Aging induces cardiac mesenchymal stromal cell senescence and promotes endothelial cell fate of the CD90 + subset This is one reason why wound healing slows with age and why fibrotic diseases become more common later in life: the stromal cells that should be orchestrating repair are instead broadcasting inflammatory signals that make things worse.
Engineering Stroma for Regenerative Medicine
Regenerative medicine is increasingly interested in recreating stromal environments outside the body. One approach strips donor tissues of all their cells using detergents, leaving behind the bare extracellular matrix, a kind of stromal ghost. These decellularized scaffolds retain the original tissue’s architecture, growth factor reservoirs, and mechanical properties, and when seeded with new cells, they can guide those cells to rebuild functional tissue. Decellularized extracellular matrix scaffolds have moved from the bench into preclinical and some clinical applications for reconstructing skin, cartilage, heart valves, and more, though the mechanisms by which they promote functional tissue restoration are still being worked out.25PubMed Central. Bioactive scaffolds for tissue engineering: A review of decellularized extracellular matrix applications and innovations These matrix substitutes can also drive stem cell proliferation and differentiation for building three-dimensional organoids in a dish, opening a path toward growing replacement tissues from a patient’s own cells.26PubMed Central. Decellularized extracellular matrix mediates tissue construction and regeneration
The intestinal stroma shows why getting the details right matters. The lining of the gut is constantly renewing itself, and that renewal depends on fibroblast subsets in the lamina propria, the thin stromal layer just beneath the intestinal surface cells. Different fibroblast populations sit in distinct anatomical niches along the crypt-villus axis and express unique sets of genes that support epithelial cell maintenance. Both the small and large intestine contain similar stromal subsets, but their precise signaling differs, which means an engineered intestinal graft would need to match the stromal signals appropriate to the specific gut region being repaired.27bioRxiv. The small and large intestine contain transcriptionally related mesenchymal stromal cell subsets that derive from embryonic Gli1+ mesothelial cells