Enteroendocrine cells are specialized cells located within the lining of the gastrointestinal tract and pancreas. They act as chemical messengers for the digestive system, producing and releasing hormones in response to various internal and external stimuli. These cells orchestrate a wide array of gut processes, serving as a communication link within the body. They are important for maintaining the body’s internal balance.
Where Enteroendocrine Cells Reside
These specialized cells are found throughout the gastrointestinal tract, from the stomach to the colon. Unlike cells that form large glands, enteroendocrine cells are dispersed individually among other intestinal cells within the epithelial lining. They make up less than 1% of the total intestinal cell population, yet collectively form the body’s largest endocrine system.
Their scattered arrangement allows them to act as individual sensors across a vast surface area. This placement enables them to directly monitor the contents within the gut lumen, such as nutrients, chemicals, and signals from gut microbiota. This direct sensing capability helps initiate physiological responses to food intake and other luminal changes.
Diverse Types and Their Signals
Multiple distinct types of enteroendocrine cells exist, each producing and releasing specific hormones. These cells differentiate from common intestinal stem cells and respond to particular stimuli. For example, fats, proteins, or acids can trigger different enteroendocrine cell types.
Different enteroendocrine cell types and their hormones include:
- L-cells, found in the ileum and colon, produce glucagon-like peptide-1 (GLP-1) and peptide YY (PYY).
- K-cells, located in the small intestine, secrete gastric inhibitory peptide (GIP), also called glucose-dependent insulinotropic polypeptide.
- S-cells primarily release secretin.
- I-cells secrete cholecystokinin (CCK).
- Enterochromaffin (EC) cells, the most abundant type, produce serotonin (5-HT) and are widely distributed throughout the GI tract.
- G-cells in the stomach release gastrin.
- X/A-like cells produce ghrelin.
Master Regulators of Digestion and Beyond
Hormones released by enteroendocrine cells affect various organs and systems, orchestrating digestive processes and influencing broader physiological functions. For example, gastrin from G-cells in the stomach stimulates gastric acid secretion, important for protein digestion. Secretin from S-cells promotes bicarbonate release from the pancreas and bile ducts, neutralizing stomach acid as it enters the small intestine. Cholecystokinin (CCK) from I-cells stimulates gallbladder contraction, releasing bile for fat digestion, and triggers pancreatic enzyme release for nutrient breakdown.
These hormones also regulate appetite and metabolism. GLP-1 and GIP, known as incretins, enhance insulin secretion from the pancreas in a glucose-dependent manner, helping regulate blood sugar levels after a meal. GLP-1 also slows gastric emptying and contributes to feelings of fullness, influencing food intake. Conversely, ghrelin, produced by X/A-like cells, stimulates appetite and is known as the “hunger hormone.”
Serotonin, largely from EC cells, influences gut motility and sensation. The interplay of these hormones shows enteroendocrine cells’ role in the gut-brain axis, sending signals that impact local gut function and distant bodily systems.
Enteroendocrine Cells and Overall Well-being
The proper functioning of enteroendocrine cells is important for maintaining digestive harmony and systemic balance. When these cells or their hormonal signals become dysregulated, it can contribute to various health conditions. For instance, issues with GLP-1 and GIP secretion or action are linked to metabolic disorders such as type 2 diabetes, as these hormones regulate glucose. Impaired signaling from appetite-regulating hormones, like GLP-1, PYY, and ghrelin, can contribute to imbalances in energy intake and expenditure, potentially leading to obesity.
Disruptions in enteroendocrine cell function can also play a role in certain gastrointestinal disorders, affecting gut motility and sensation. Their ability to sense and respond to luminal contents, including gut microbiota, highlights their importance in immune responses and overall gut health. Understanding these cells helps in maintaining well-being and developing strategies for managing health challenges.