Wikipedia Deep Dive

Pancreatic islets

14 min read

The Million Islands Inside You

Scattered throughout your pancreas right now are roughly one million tiny clusters of cells, each about the width of two human hairs. These clusters—called the islets of Langerhans—make up barely two percent of your pancreas by volume, yet they receive up to fifteen percent of the organ's blood supply. That wildly disproportionate blood flow hints at their importance: these microscopic islands are the body's glucose command centers, and when they fail, the consequences can be fatal.

The story of their discovery belongs to a twenty-two-year-old medical student named Paul Langerhans. In 1869, while examining thin slices of pancreatic tissue under his microscope in Berlin, Langerhans noticed something peculiar. Amid the dense tissue that produces digestive enzymes, he spotted small, scattered clusters of cells that looked fundamentally different. He didn't know what they did—he simply described them as "small cells of almost perfect homogeneous content and of a polygonal form." It would take another two decades before scientists connected these islands to the hormone that keeps us alive: insulin.

Five Cell Types, One Mission

Each islet is a self-contained factory, wrapped in a thin capsule of connective tissue that separates it from the surrounding pancreas. Inside that capsule, at least five distinct types of cells work together to regulate your blood sugar through an intricate chemical conversation.

The beta cells are the stars of the show. Comprising roughly seventy percent of each islet in rodents—though only about forty to fifty percent in humans—these cells produce insulin, the hormone that tells your cells to absorb glucose from your bloodstream. When you eat a meal and your blood sugar rises, beta cells spring into action, releasing insulin to bring those levels back down.

But insulin needs a counterpart. That's where alpha cells come in. Making up about twenty percent of the islet, alpha cells produce glucagon, a hormone with the opposite effect of insulin. When your blood sugar drops too low—say, between meals or during exercise—glucagon signals your liver to release stored glucose back into your bloodstream. Without this balance, you'd either slip into a diabetic coma from high blood sugar or lose consciousness from low blood sugar.

The remaining cell types play supporting roles that scientists are still working to fully understand. Delta cells produce somatostatin, a hormone that acts as a brake on both insulin and glucagon secretion. It's like having a referee who can call a timeout when the insulin-glucagon tug-of-war gets too intense. PP cells—also called gamma cells or F cells—secrete pancreatic polypeptide, which helps regulate appetite and digestion. And epsilon cells, the rarest of the bunch at less than one percent of the islet, produce ghrelin, the famous "hunger hormone" that tells your brain it's time to eat.

An Architecture That Varies by Species

Here's something fascinating: the way these cells arrange themselves inside an islet depends on what species you belong to.

In mice and rats, the islet has a clear architectural plan. Beta cells cluster in the center, forming a dense core, while alpha, delta, and PP cells huddle around the periphery like a protective shell. It's a neat, organized arrangement that made rodent islets relatively easy to study.

Human islets are messier. Alpha and beta cells intermingle throughout the cluster, sitting right next to each other with no clear separation. For decades, scientists assumed human islets worked like rodent islets, and they built their understanding of diabetes on that assumption. The discovery that human islet architecture is fundamentally different forced researchers to reconsider some of what they thought they knew.

This matters because the cells in an islet don't work in isolation. They talk to each other constantly through chemical signals—a process called paracrine communication. When glucose floods the bloodstream, beta cells release insulin, but that insulin also tells neighboring alpha cells to stop producing glucagon. Meanwhile, the glucagon from alpha cells actually stimulates both beta and delta cells. Delta cells, in turn, pump out somatostatin that tells everyone to calm down. It's a feedback loop of extraordinary complexity, all happening in a space smaller than a pinhead.

Beta cells have another trick: they're electrically connected to each other. Each beta cell forms electrical connections with six or seven of its neighbors, allowing them to coordinate their insulin release like a chorus singing in unison. Interestingly, they don't form these electrical connections with other cell types—only with fellow beta cells. This selective wiring allows beta cells to respond as a unified team to changes in blood glucose.

A Blood Supply Like No Other

The blood flow through pancreatic islets borders on the absurd. Each gram of islet tissue receives five to six milliliters of blood per minute—up to fifteen times more blood flow than the surrounding pancreatic tissue. To put this in perspective, your islets make up less than two percent of your pancreas but demand ten to fifteen percent of its blood supply.

Why such extravagance? The answer lies in what islets do. To regulate blood sugar effectively, the cells in an islet need to constantly sample the glucose in your bloodstream and respond within seconds. The dense network of blood vessels running through each islet—which researchers describe as resembling tiny kidney structures called glomeruli—ensures that every endocrine cell sits mere micrometers from a blood vessel. The vessels themselves are highly fenestrated, meaning they're riddled with tiny pores that allow hormones to diffuse quickly into the bloodstream.

All this blood flow comes with a bonus: oxygen. The oxygen levels inside pancreatic islets are significantly higher than in the surrounding tissue. This makes sense when you consider that the cells are working constantly, monitoring blood sugar and pumping out hormones around the clock.

Here's another species difference worth noting: human islets have about five times fewer blood vessels than rodent islets. Scientists aren't entirely sure what to make of this. It might mean human islets operate differently, or it might simply reflect the challenges of studying human tissue compared to the readily available rodent pancreases in laboratories.

When the Immune System Turns Traitor

Type 1 diabetes is, at its core, a story of betrayal.

For reasons scientists still don't fully understand, the immune system sometimes decides that beta cells are foreign invaders. It launches an attack, methodically destroying the very cells responsible for producing insulin. Without beta cells, insulin production grinds to a halt. Without insulin, glucose accumulates in the bloodstream while cells starve for energy. Before the discovery of injectable insulin in 1921, a diagnosis of type 1 diabetes was essentially a death sentence.

Today, people with type 1 diabetes survive through daily insulin injections—sometimes multiple injections per day—or insulin pumps that deliver the hormone continuously. These treatments are remarkably effective, but they're not a cure. Even with careful management, maintaining blood sugar in a healthy range requires constant vigilance. The body's natural glucose regulation system, honed over hundreds of millions of years of evolution, is simply more sophisticated than any artificial substitute.

This is why researchers have spent decades trying to figure out how to replace destroyed beta cells. If you could somehow restore beta cell function, you might be able to cure type 1 diabetes rather than merely managing it.

The Promise of Islet Transplantation

The idea is elegantly simple: take islets from a deceased donor, purify them, and transplant them into a person whose own islets have been destroyed. The transplanted beta cells would then take over insulin production, potentially freeing the recipient from daily injections.

Researchers began pursuing this approach in earnest in the early 1970s, and progress over the following decades was steady if unspectacular. The breakthrough came in 2000, when a team at the University of Alberta in Edmonton, Canada, reported that they had achieved insulin independence in seven consecutive patients using a new protocol. The "Edmonton Protocol" used a combination of immunosuppressive drugs that were gentler on islet cells than previous approaches, and the results were dramatic.

By 2008, clinical trials had confirmed that islet transplantation could reproducibly restore insulin independence in patients with difficult-to-control type 1 diabetes. The procedure was particularly valuable for patients with "hypoglycemic unawareness"—people who couldn't sense when their blood sugar was dropping to dangerous levels, putting them at risk of sudden unconsciousness.

The transplant itself is surprisingly straightforward. Surgeons don't need to open the abdomen. Instead, they insert a catheter through the skin into the portal vein, a major blood vessel that carries blood from the intestines to the liver. The isolated islets—suspended in a liquid solution—flow through the catheter and lodge in the small blood vessels of the liver. There, if all goes well, they begin sensing glucose and secreting insulin just as they did in the donor's pancreas.

This represents a significant advantage over whole pancreas transplantation, which requires major surgery, general anesthesia, and carries risks of complications like pancreatitis. Islet transplantation patients can often return home the same day.

The Challenges Remaining

If islet transplantation sounds too good to be true, that's because the reality is more complicated.

The first problem is immunosuppression. Remember, the recipient's immune system destroyed their own beta cells. Those same immune responses will happily attack donor islets too, recognizing them as foreign tissue. To prevent rejection, transplant recipients must take powerful immunosuppressive drugs for the rest of their lives. These drugs have serious side effects, including increased susceptibility to infections and certain cancers. For many patients, the risks of lifelong immunosuppression outweigh the benefits of insulin independence.

The second problem is supply. There are far more people with type 1 diabetes than there are donor pancreases available for islet isolation. The United States alone has roughly 1.6 million people with type 1 diabetes, and each islet transplant typically requires islets from two or more donors. The math simply doesn't work.

There's also the issue of islet survival. When islets are infused into the portal vein, they enter an environment very different from their original home in the pancreas. The blood vessels in the liver are denser and flow differently than those in the pancreas. Many transplanted islets die within minutes of infusion. Those that survive face months of reduced blood supply while new blood vessels slowly grow around them—a process called neovascularization. During this vulnerable period, islets depend on a protein called Vascular Endothelial Growth Factor, or VEGF, to encourage new vessel formation.

Some transplanted islets cause blood clots in the portal vein, a dangerous complication. And the liver, while convenient for transplantation, may not be the ideal long-term home for islets. Researchers are actively investigating alternative implantation sites that might offer a more hospitable environment.

The Search for Alternative Beta Cell Sources

Given the shortage of donor organs, scientists have been working on ways to create beta cells in the laboratory.

One approach uses stem cells—cells that haven't yet committed to becoming a specific tissue type. By exposing stem cells to the right combination of chemical signals, researchers can coax them to differentiate into insulin-producing cells. Progress in this field has accelerated dramatically in recent years, with several companies now conducting clinical trials of stem cell-derived islet cells.

Another intriguing approach leverages a natural phenomenon: alpha cells can sometimes spontaneously transform into beta cells. Scientists have observed this "transdifferentiation" in both mouse and human islets, in healthy pancreases and diabetic ones. If researchers could figure out how to trigger and control this transformation, they might be able to replenish a patient's beta cells using their own alpha cells—no donor required.

The developmental biology here is fascinating. During embryonic development, the cells that will become islets migrate together in cohesive groups, forming bud-like structures that researchers have poetically named "peninsulas." In these early islet precursors, alpha cells form the outer layer, and beta cells develop beneath them. Understanding this natural developmental sequence might provide clues for regenerating beta cells in adults.

One practical innovation has been cryopreservation—freezing isolated islets for later use. This allows transplant centers to bank islets from multiple donors, accumulating enough for a transplant even when individual donations are small. It also enables better matching between donors and recipients, potentially improving outcomes.

An Evolutionary Journey

Pancreatic islets as we know them are a vertebrate invention. You won't find islet organs in any invertebrate or in the most primitive chordates like tunicates (sea squirts) and lancelets.

But the hormones produced by islets are ancient. In tunicates, cells that secrete an insulin-like peptide are found in the brain. Lancelets have brain cells producing relatives of all four major islet hormones: insulin, somatostatin, glucagon, and pancreatic polypeptide. Over hundreds of millions of years of vertebrate evolution, these hormone-producing cells migrated from the brain to the gut lining, and eventually clustered together to form the islets we see today.

The sequence of this evolutionary journey is remarkably clear. Jawless fish like lampreys and hagfish have primitive islets containing cells that make insulin and somatostatin. Sharks and rays—jawed cartilaginous fish—added glucagon-producing cells to the mix. Lobe-finned fish, the ancestors of all land vertebrates, have few or no PP cells in their islets, while certain ray-finned fish developed abundant PP cells. Ghrelin-producing cells have been detected in catfish islets. And in birds, islets have expanded to include additional hormone-producing cells secreting peptides like Insulin-like Growth Factor 1, peptide YY, and adrenomedullin.

This evolutionary perspective has practical implications. American anglerfish have an unusual pancreas: unlike mammalian pancreases, which mix endocrine islets among exocrine digestive tissue, anglerfish pancreases are dominated by endocrine tissue with very little exocrine contamination. This made anglerfish pancreases ideal for research. Scientists used them to isolate the genetic sequence for proglucagon, which revealed not only the code for glucagon itself but also two related peptides: Glucagon-Like Peptide 1 and Glucagon-Like Peptide 2, usually abbreviated GLP-1 and GLP-2.

GLP-1 turned out to be enormously important. It stimulates insulin secretion and has become the basis for an entire class of diabetes drugs. If you've heard of medications like Ozempic, Wegovy, or Mounjaro, you've encountered the legacy of those anglerfish studies. These drugs mimic or enhance GLP-1 signaling, helping patients with type 2 diabetes control their blood sugar—and, as a side effect that surprised everyone, causing significant weight loss.

The Cannabinoid Connection

In recent years, researchers made a discovery that would have seemed bizarre just a few decades earlier: pancreatic islets are covered in cannabinoid receptors.

Your body produces its own cannabis-like compounds called endocannabinoids. These molecules play roles throughout the body, from pain perception to appetite to immune function. The two main receptors they bind to are called CB1 and CB2, and both are widely expressed in the islets of Langerhans.

Studies have found that endocannabinoids help regulate beta cell function, affecting insulin production, secretion, and even cellular survival and proliferation. This discovery opened new questions about how cannabis use might affect diabetes risk and blood sugar control—questions that researchers are still working to answer.

The presence of cannabinoid receptors in islets is another reminder of how interconnected the body's systems are. Your pancreatic islets don't operate in isolation. They're woven into a vast network of hormonal, neural, and immunological signaling systems, all working together to maintain the delicate balance we call health.

A Frontier of Medicine

The islets of Langerhans represent one of the most promising frontiers in regenerative medicine. Cell therapies derived from stem cells are moving from laboratory curiosities to clinical reality. Researchers are developing new techniques for encapsulating transplanted islets—surrounding them with protective materials that shield them from immune attack while allowing nutrients and hormones to pass through. Novel immunosuppression strategies aim to protect transplanted cells without the harsh side effects of current drugs.

For the roughly 1.6 million Americans with type 1 diabetes—and the millions more worldwide—these advances carry profound hope. The dream is no longer merely to manage diabetes but to cure it: to restore the body's ability to regulate its own blood sugar, freeing patients from the constant calculations and needle sticks that define life with the disease.

It all comes back to those million tiny islands, first glimpsed by a German medical student over 150 years ago, each one a marvel of biological engineering packed into a space smaller than a grain of sand.