A pericyte is a cell that wraps around the smallest blood vessels in your body, sitting on the outer surface of capillaries and small venules like fingers gripping a garden hose. Endothelial cells form the inner lining of a blood vessel, and pericytes envelop the outside of that tube, making direct physical contact through gaps in the shared basement membrane that separates them.1PubMed Central. The role of pericytes in blood-vessel formation and maintenance That placement turns out to be enormously consequential. Pericytes regulate blood flow, maintain the barrier between blood and tissue, help build new vessels, participate in immune defense, and contribute to scar formation after injury. They also play central roles in some of the most challenging diseases in medicine, from diabetic retinopathy to Alzheimer’s.
Where Pericytes Sit and How to Identify Them
Pericytes belong to a broader family of cells called mural cells, a term that encompasses both pericytes on capillaries and the smooth muscle cells wrapped around larger arteries and veins. What sets pericytes apart is their location: they live along the tiniest vessels, the capillaries and postcapillary venules where oxygen and nutrients actually exchange with tissues. They extend long, finger-like projections that partially encircle the vessel wall, and they communicate with the endothelial cells beneath them through direct cell-to-cell contact points.
Pinning down a pericyte under a microscope is harder than it sounds, because no single molecular marker lights up pericytes and nothing else. Researchers rely on a combination of markers. PDGFRβ (a receptor for a growth factor called PDGF-B) and NG2 (a proteoglycan on the cell surface) are two of the most commonly used; in mouse brain tissue, PDGFRβ co-localizes specifically with established pericyte markers like NG2.2PubMed Central. Pericyte-specific expression of PDGF beta receptor in mouse models with normal and deficient PDGF beta receptor signaling In human brain tissue, capillary-associated pericytes express PDGFRβ, NG2, CD13, and CD146, while smooth muscle cells on larger vessels share some of those markers but also express others like α-smooth muscle actin and desmin more prominently.3PubMed. Markers for human brain pericytes and smooth muscle cells The overlap is part of what makes pericyte biology tricky: the boundary between a pericyte and a smooth muscle cell is not a sharp line, and cells along the vascular tree can share features of both.
Recent single-cell RNA sequencing has started to reveal that pericytes are not one uniform population. When researchers analyzed pericytes from mouse lung, heart, kidney, and bladder, they found organ-specific marker genes in each tissue, suggesting that pericytes in different organs have specialized molecular identities on top of their shared core features.4PubMed Central. Single Cell Transcriptomic Analysis Reveals Organ Specific Pericyte Markers and Identities This heterogeneity matters clinically, because a drug or therapy that targets pericytes in one organ might not work the same way in another.
Controlling Blood Flow at the Capillary Level
For a long time, textbooks attributed blood flow control almost entirely to smooth muscle cells on arteries and arterioles. Pericytes were treated as passive structural supports. That picture has changed dramatically. Pericytes sitting at capillary branch points actively constrict and relax, steering red blood cells into one branch or another depending on local tissue demand. In the brain, contractile pericytes at capillary junctions differentially squeeze their projections to set the direction of blood flow, and they respond to electrical signals propagating through the capillary network to dynamically channel red blood cells toward the area signaling for more oxygen.5PubMed Central. Contractile pericytes determine the direction of blood flow at capillary junctions
This capillary-level traffic control is not limited to the brain. In the pancreas, pericytes on islet capillaries regulate blood flow in ways that directly affect insulin secretion. When glucose levels rise, adenosine accumulates in the pancreatic islet and activates receptors on pericytes, causing them to relax and capillaries to widen by about 36%. Conversely, stress hormones like noradrenaline make those same pericytes clamp down, reducing capillary diameter by roughly 29%.6Cell Metabolism. Pericytes Regulate Vascular Tone in Pancreatic Islets and Are Defective in Type 2 Diabetes Only about 20% of islet capillaries responded to a given vasoactive stimulus, and that fraction matched the proportion of capillaries covered by pericytes expressing contractile proteins, reinforcing the idea that the pericyte itself is doing the squeezing.
Guarding the Blood-Brain Barrier
The blood-brain barrier is one of the body’s most selective checkpoints, preventing toxins, pathogens, and most large molecules in the bloodstream from entering brain tissue. Pericytes are essential to maintaining it. They do so not primarily by sealing the gaps between endothelial cells (those gaps are held shut by tight junction proteins like Claudin-5), but by suppressing transcytosis, the process by which tiny vesicles shuttle material straight through an endothelial cell from the blood side to the brain side.
In experiments with mice that lack certain pericyte-derived matrix proteins, endothelial cells ramp up transcytosis dramatically, filling with vesicles that ferry material across the barrier, while tight junctions remain intact.7Neuron. Matrix proteins plug a hole: How pericytes suppress blood brain barrier transcytosis The same pattern appears in mice bred to have fewer pericytes: the barrier leaks because vesicular trafficking goes unchecked, not because the seals between cells come apart. Understanding exactly how pericytes suppress transcytosis at the molecular level is considered a potential route to designing better drug delivery systems for the brain, where getting a therapeutic molecule past the barrier is one of the biggest challenges in neurology.8PubMed. Pericytes and the blood-brain barrier: recent advances and implications for the delivery of CNS therapy
Building and Stabilizing New Blood Vessels
When new blood vessels sprout during development, wound healing, or tumor growth, endothelial cells push outward to form a fresh tube. But that tube is fragile and leaky without pericyte coverage. Pericytes are recruited to new vessels through a signaling loop: endothelial cells release PDGF-B, and pericytes expressing the matching receptor (PDGFRβ) migrate toward the signal and wrap around the nascent vessel. In tumors, both endothelial and non-endothelial sources of PDGF-B drive this recruitment, and disrupting the PDGF-B/PDGFRβ system leaves tumor vessels poorly covered by pericytes.9JCI Insight. Endothelial and nonendothelial sources of PDGF-B regulate pericyte recruitment and influence vascular pattern formation in tumors
Tumor vessels are notoriously abnormal, with chaotic branching, uneven blood flow, and high permeability. Part of what makes them that way is abnormal pericyte coverage. Pericytes on tumor vessels tend to be loosely attached and fewer in number, which contributes to leaky vessels, high fluid pressure inside the tumor, and easier routes for cancer cells to escape into the bloodstream.10PubMed Central. Pericytes on the tumor vasculature: jekyll or hyde? This observation has led researchers to explore “vessel normalization” strategies: instead of trying to destroy tumor blood vessels entirely, the idea is to improve their pericyte coverage so they function more like normal vessels, which could improve drug delivery to the tumor and reduce metastasis.
Pericytes as Immune Participants
Pericytes are not part of the classical immune system, but they behave in surprisingly immune-like ways. Brain pericytes respond to inflammatory signals, produce a range of cytokines and chemokines, can present antigens to immune cells, and even display phagocytic ability, meaning they can engulf and digest cellular debris.11PubMed. Brain Pericytes As Mediators of Neuroinflammation When tissue is inflamed, pericytes respond to signaling molecules like TNF-α and CCL2 and in turn secrete their own CCL2, nitric oxide, and other cytokines, effectively amplifying and shaping the local immune response.12PubMed. Beyond barrier functions: Roles of pericytes in homeostasis and regulation of neuroinflammation
This immune activity cuts both ways. In healthy tissue, pericyte-mediated inflammation helps clear infections and remove damaged cells. But in chronic neuroinflammatory conditions, overactive pericytes can contribute to a self-reinforcing cycle of inflammation and barrier breakdown. Whether pericyte immune activity is helpful or harmful depends heavily on context, timing, and which signals dominate.
From Vessel Wall to Scar Tissue
One of the more surprising discoveries about pericytes is that they are a major source of the scar-forming cells that drive fibrosis in multiple organs. When tissue is injured, pericytes can detach from their capillary, migrate into the surrounding space, and transform into myofibroblasts, the cells that deposit the dense collagen matrix of a scar. In the kidney, pericytes lining the microvasculature have been identified as the precursor cells that become scar-producing myofibroblasts during kidney injury.13PubMed. Novel insights into pericyte-myofibroblast transition and therapeutic targets in renal fibrosis
In the central nervous system, a specific subset called type A pericytes has been identified as the primary source of fibrotic scar tissue after brain and spinal cord injuries. Using lineage tracing in mice, researchers found that after spinal cord injuries, nearly all of the scar-forming stromal cells derived from type A pericytes. A similar pattern held after brain stab wounds, where about 80% of scar-forming cells traced back to this pericyte subset.14Nature Communications. Pericyte-derived fibrotic scarring is conserved across diverse central nervous system lesions This finding is consistent with broader evidence placing pericytes at the center of fibrosis in many organs.15PubMed Central. Origins of fibrosis: pericytes take centre stage
The scar itself is a double-edged sword. In the short term after a spinal cord injury, the fibrotic scar walls off the damaged area and prevents the lesion from expanding. But in the long term, that same scar acts as a physical and chemical barrier to nerve regeneration. Researchers are exploring whether modulating the pericyte-to-myofibroblast transition could reduce scarring enough to improve nerve repair without losing the initial protective benefit.
Pericyte Dysfunction in Disease
Because pericytes are involved in so many vascular functions, their loss or malfunction shows up in a wide range of diseases. Three stand out for the depth of evidence linking pericyte failure to disease progression.
Diabetic Retinopathy
The earliest visible change in diabetic retinopathy is the selective loss of pericytes from the tiny blood vessels of the retina. This loss precedes and enables the cascade of damage that follows: basement membrane thickening, breakdown of tight junctions between endothelial cells, microaneurysms, capillary blockages, and eventually endothelial cell death.16PubMed. Pericyte loss in diabetic retinopathy: mechanisms and consequences The retina’s pericyte-to-endothelial-cell ratio is one of the highest in the body, which may be why the retina is so vulnerable when high blood sugar starts killing pericytes. This early and selective pericyte loss is a hallmark that pathologists look for when grading the severity of diabetic eye disease.
Alzheimer’s Disease
Pericytes degenerate in Alzheimer’s disease, and the rate of degeneration tracks with how much the blood-brain barrier leaks. People carrying the APOE4 gene variant, the strongest known genetic risk factor for late-onset Alzheimer’s, show accelerated pericyte loss compared to APOE3 carriers, which correlates with greater barrier breakdown.17PubMed Central. Accelerated pericyte degeneration and blood-brain barrier breakdown in apolipoprotein E4 carriers with Alzheimer’s disease In mouse models, reducing pericyte numbers elevated brain levels of amyloid-beta by roughly 2.4- to 2.7-fold, not because more amyloid was being produced but because the brain’s ability to clear it was impaired.18Nature Communications. Pericyte loss influences Alzheimer-like neurodegeneration in mice This clearance failure preceded visible amyloid plaque deposits, suggesting pericyte degeneration may be an upstream event that accelerates the disease rather than just a bystander consequence.19PubMed Central. The pericyte: a forgotten cell type with important implications for Alzheimer’s disease?
Stroke and No-Reflow
When a clot blocks a brain artery and then blood flow is restored (through a clot-busting drug or a mechanical thrombectomy), you would expect the capillaries downstream to start flowing again. Often they do not, a phenomenon called no-reflow. Pericytes are heavily implicated. Using two-photon microscopy to watch individual pericytes in living mice over seven days, researchers found that 87% of pericytes constricted during the stroke and stayed constricted even after the upstream blockage was removed. Half of all pericytes were acutely damaged, and their continued dysfunction limited and arrested capillary blood flow in the first 24 hours after the stroke.20PubMed. Continued dysfunction of capillary pericytes promotes no-reflow after experimental stroke in vivo No-reflow is one reason why restoring blood flow after a stroke does not always restore brain function, and targeting pericyte relaxation is now being explored as a potential therapeutic strategy.
Organ-Specific Pericyte Identities
Pericytes go by different names depending on the organ, which has sometimes obscured the fact that they are all variations of the same basic cell type. The most prominent example is in the liver, where pericytes are called hepatic stellate cells. They sit in the narrow space between hepatocytes and the sinusoidal endothelial cells that line liver capillaries, and in a healthy liver they store the majority of the body’s vitamin A.21PubMed Central. Pericytes in the Liver When the liver is injured, whether by alcohol, viral hepatitis, or fatty liver disease, these stellate cells activate, lose their vitamin A stores, and begin producing the collagen that leads to liver fibrosis and eventually cirrhosis.22PubMed. Hepatic stellate cells–the pericytes in the liver This is the same pericyte-to-myofibroblast transition seen in the kidney and central nervous system, playing out in an organ-specific context.
In the kidney, pericytes regulate blood flow through the renal medulla and cortex and participate in the formation of new blood vessels. During kidney injury, they detach and become the main producers of scar tissue, just as in the liver. But in a healthy kidney, pericytes may serve as a local stem-like population that replaces vascular and interstitial cells lost through normal aging.23PubMed Central. Kidney pericytes: roles in regeneration and fibrosis The recurring theme across organs is the same: pericytes stabilize vessels and maintain tissue health, but when provoked by injury, they shift into a wound-healing mode that can become pathological if it does not resolve.
The Stem Cell Debate
For years, a compelling idea gained traction: that pericytes might be the body’s resident mesenchymal stem cells, capable of turning into bone, fat, cartilage, or muscle when needed. Evidence for this came from multiple angles. Perivascular cells, including pericytes, express the same surface markers as mesenchymal stem cells and, when grown in culture, produce long-lasting progeny that can differentiate into multiple cell types.24PubMed. Perivascular ancestors of adult multipotent stem cells Perivascular stem cells isolated from the adult human brain showed multipotency, immunomodulatory properties, and the ability to secrete growth factors.25PubMed Central. Perivascular mesenchymal stem cells in the adult human brain: a future target for neuroregeneration?
Then came a study that threw cold water on the idea, at least in its strongest form. Using genetic lineage tracing in mice, researchers tagged pericytes and smooth muscle cells and followed their fate over time in aging and various injuries. In living tissue, pericytes maintained their identity and did not significantly transform into other cell lineages.26PubMed Central. Pericytes of Multiple Organs Do Not Behave as Mesenchymal Stem Cells In Vivo The same cells that demonstrated impressive plasticity in a dish stayed pericytes in the body. The researchers concluded that the multipotency seen in culture likely arises from artificial manipulation during the isolation and growth process, not from an inherent property of pericytes inside an intact tissue.
The field has not fully settled the question. It is possible that pericytes possess a latent plasticity that only manifests under specific injury conditions not captured in the lineage-tracing experiments, or that subsets of pericytes in certain organs have more stem-like properties than the general population. But the evidence so far suggests that calling pericytes “stem cells” overstates what they normally do. Their primary job is being pericytes, and they are very good at it.
A Brief History of the “Rouget Cell”
Pericytes were first described nearly 140 years ago by the French scientist Charles-Marie Benjamin Rouget, who observed contractile cells on the outside of capillaries and named them after himself. The name “Rouget cell” persisted for decades before being replaced by “pericyte,” a term derived from the Greek for “around the cell,” reflecting their anatomical position wrapped around the endothelium.27PubMed. Pericytes: pluripotent cells of the blood brain barrier Despite being noticed so early, pericytes were largely neglected for most of the twentieth century, overshadowed by endothelial cells and smooth muscle cells. Their renaissance began in the 1990s and 2000s with the development of genetic tools in mice and imaging technologies like two-photon microscopy, which allowed researchers to watch individual pericytes in living brain tissue for the first time.28PubMed. Two-photon imaging of brain pericytes in vivo using dextran-conjugated dyes That ability to observe pericytes in real time in a living animal transformed the field, making it possible to ask and answer questions about blood flow regulation and injury response that were previously inaccessible.
Today pericytes are among the most actively studied cells in vascular biology. Their involvement in conditions from cancer to neurodegeneration to organ fibrosis, combined with the realization that they can be both protective and destructive depending on the context, has placed them at the center of multiple therapeutic strategies. The cell Rouget noticed in the nineteenth century turned out to be far more than a structural bystander on a capillary wall.