The endocrine pancreas is composed of clusters of hormone-secreting cells, known as the Islets of Langerhans. These islets are scattered throughout the pancreas and play a crucial role in glucose homeostasis by secreting hormones directly into the bloodstream.
Embryologic Origin
- The islets arise from pancreatic buds (endodermal origin).
- They develop from the same epithelium that forms the exocrine pancreatic ducts.
Cellular Composition of Islets of Langerhans
| Cell Type | Location in Islet | Hormone Secreted | Function |
|---|---|---|---|
| α (Alpha) cells | Peripheral | Glucagon | Raises blood glucose by promoting glycogenolysis and gluconeogenesis in the liver. |
| β (Beta) cells | Central | Insulin | Lowers blood glucose by increasing glucose uptake (muscle/adipose) and promoting glycogen synthesis. |
| δ (Delta) cells | Interspersed throughout | Somatostatin | Inhibits the secretion of both insulin and glucagon; it decreases gastrointestinal motility and secretion. |
Functional Summary
| Hormone | Target Tissue | Physiologic Effect | Triggered by |
|---|---|---|---|
| Glucagon | Liver | ↑ Blood glucose | ↓ Blood glucose (hypoglycemia), fasting, stress |
| Insulin | Muscle, Adipose, Liver | ↓ Blood glucose | ↑ Blood glucose (hyperglycemia), nutrient intake |
| Somatostatin | Pancreas, GI tract | Inhibits insulin, glucagon, and GH | High glucose, amino acids, fatty acids |
Additional Notes
- Insulin and glucagon act in an antagonistic manner to maintain glucose balance.
- Somatostatin modulates both α and β cell activity, functioning as a local regulator.
- Blood flows from β → α → δ cells, allowing insulin to regulate glucagon secretion directly.
Clinical Correlation
| Disorder | Pathophysiology | Key Feature |
|---|---|---|
| Type 1 Diabetes Mellitus | Autoimmune destruction of β-cells → ↓ insulin | Hyperglycemia, ketoacidosis |
| Type 2 Diabetes Mellitus | Insulin resistance in target tissues | Hyperglycemia, obesity-related |
| Glucagonoma | Tumor of α-cells → ↑ glucagon | Necrolytic migratory erythema, diabetes |
| Somatostatinoma | Tumor of δ-cells → ↑ somatostatin | Diabetes, steatorrhea, gallstones |
Learning Objective
By the end of this topic, you should be able to:
- Identify the three major endocrine cell types in the pancreas and their locations.
- Describe the hormones they secrete and their physiologic functions.
- Understand how hormonal interactions maintain glucose homeostasis.
- Recognize clinical disorders associated with islet cell dysfunction.
