Islets of Langerhans: Alpha and Beta Cell Functions

Scattered throughout the pancreas are small, dense clusters of endocrine tissue known as the Islets of Langerhans. First described in 1869 by Paul Langerhans, these micro-organs are a collection of specialized cells that secrete hormones directly into the bloodstream. Although the pancreas is largely an exocrine gland producing digestive enzymes, the islets provide its endocrine function, regulating the body’s metabolism. The hormones they release manage how energy from food is stored and used by cells. The human pancreas contains approximately one million of these islets, which, despite making up only 1-2% of the organ’s volume, receive a disproportionately high amount of its blood flow.

Cellular Composition of the Islets

The Islets of Langerhans are composed of several cell types, each with a specific hormonal product. The two principal types are alpha and beta cells, which are the most numerous for metabolic regulation. Beta cells are the most abundant, making up 70-75% of the islet cell population, and produce a hormone that lowers blood glucose. Alpha cells are the second most common, at about 20%, and secrete a hormone that raises blood glucose.

Islets also contain a smaller number of other cells. Delta cells account for less than 10% of islet cells and produce somatostatin, a hormone that inhibits other hormone releases. PP cells and epsilon cells are rarer, producing pancreatic polypeptide and ghrelin, respectively.

The Function of Alpha Cells and Glucagon

The primary role of alpha cells is to synthesize and secrete the hormone glucagon, which counteracts hypoglycemia (low blood sugar) by raising glucose concentrations in the bloodstream. When blood glucose levels drop, such as during periods of fasting or intense exercise, alpha cells are stimulated to release glucagon. This release signals the body to access its stored energy reserves.

Once released, glucagon travels through the bloodstream to the liver, its main target organ. There, it binds to specific receptors on liver cells, which initiates two processes. The first is glycogenolysis, the breakdown of glycogen—the stored form of glucose—into glucose molecules that are then released into the blood. The second process is gluconeogenesis, where the liver creates new glucose from other substances, such as amino acids.

The Function of Beta Cells and Insulin

Beta cells produce and secrete insulin, the counterpart to glucagon. Insulin’s function is to manage hyperglycemia (high blood sugar) by promoting the uptake and storage of glucose from the blood. After a meal, carbohydrates break down into glucose, causing blood glucose levels to rise. This increase is the primary trigger for beta cells to release insulin.

Insulin circulates throughout the body and acts on cells in muscle, fat, and the liver. It functions like a key, binding to insulin receptors on the cell surface to unlock channels that allow glucose to move from the bloodstream into the cell’s interior. Once inside, the glucose can be immediately used for energy. If energy needs are met, insulin facilitates the conversion of excess glucose into glycogen for storage or into fat in adipose tissue.

Coordinated Regulation of Blood Glucose

Stable blood glucose levels (glucose homeostasis) are maintained by the coordinated, opposing actions of insulin and glucagon. This system operates as a negative feedback loop. The balance between these hormones ensures cells have a constant energy supply while preventing the damage from prolonged high or low blood sugar.

When blood glucose is high, beta cells release insulin to promote glucose uptake by cells, lowering blood sugar. High glucose and insulin also inhibit alpha cells from secreting glucagon. Conversely, when blood glucose is low, alpha cells release glucagon, triggering the liver to release stored glucose. Low glucose and the presence of glucagon inhibit insulin secretion from beta cells, keeping blood glucose in a healthy range.

Dysfunction and Clinical Relevance

Disruptions in the function of the Islets of Langerhans can lead to metabolic disorders like diabetes mellitus. These conditions arise when the balance between insulin and glucagon is lost. In Type 1 diabetes, the body’s own immune system mistakenly attacks and destroys the beta cells. This autoimmune destruction results in an absolute deficiency of insulin, meaning the body can no longer effectively lower blood glucose levels.

Type 2 diabetes is a more complex condition characterized by a dual defect. Initially, the body’s cells become resistant to the effects of insulin. To compensate for this insulin resistance, the beta cells work harder to produce more insulin. Over time, these beta cells can become exhausted and fail to produce enough insulin to overcome the resistance, leading to elevated blood glucose.

Dysfunction of the alpha cells also contributes to the high blood sugar levels seen in diabetes. In a healthy state, high glucose levels suppress glucagon secretion. In individuals with Type 2 diabetes, alpha cells often become dysregulated and continue to secrete glucagon even when blood sugar is high. This excess glucagon exacerbates hyperglycemia by stimulating the liver to produce and release more glucose, compounding the problem of insulin resistance.