Histology of Gastric Glands and G-Protein Coupled Receptor Mechanisms
In the study of medicine, understanding the stomach's histology is essential. Pyloric and Oxyntic glands secrete gastric juices, which are vital for physiological function. Chemical digestion in the stomach is achieved by secretions from oxyntic and pyloric glands that break down proteins.
The Histology of Gastric Glands
Gastric glands are categorized based on their location and cellular composition. Oxyntic glands are found in the body of the stomach, and contain chief and parietal cells. In contrast, Pyloric glands are found in the stomach antrum; they contain mucous cells and G-cells.
Cellular Secretions and Functions
- Chief cells secrete pepsinogen.
- Parietal cells secrete HCL & Intrinsic factor.
- Mucous cells secrete bicarbonate-rich mucus, which protects stomach lining from acids.
- G-cells secrete gastrin, which stimulates parietal cell secretion of HCL.
| Gland Type | Location | Cell Types | Primary Secretions |
|---|---|---|---|
| Oxyntic Glands | Body of the stomach | Chief and Parietal cells | Pepsinogen, HCL, Intrinsic factor |
| Pyloric Glands | Stomach antrum | Mucous cells and G-cells | Bicarbonate-rich mucus, Gastrin |
Cell Biology: G-Protein Coupled Receptors (GPCR)
Modern medicine also focuses on molecular signaling. All eukaryotes have G-protein coupled receptors, which act indirectly on a separate protein (enzyme or ion channel). In this system, a trimeric GTP-binding protein (G-protein) mediates this interaction. Interestingly, the same signal molecule can activate multiple different receptors.
Structural Characteristics
The receptor itself has seven transmembrane segments (hallmark). The G-protein is trimeric (alpha, beta, gamma subunits). Specifically, the Alpha and Gamma subunits have small lipid molecules that anchor them to inner layer of plasma membrane. These proteins also feature a GTPase binding domain.
The Activation Mechanism
The activity of these receptors depends on the state of the G-protein: GDP - Inactive state and GTP - Active state. The activation steps are as follows:
- Signal molecule binds to receptor
- Receptor undergoes conformational change
- Alpha subunit kicks out GDP and binds GTP
- Alpha subunit becomes activated as does beta/gamma complex
- Activated units activate other proteins in the signaling pathway
- Eventually GTP hydrolyzed into GDP and the G-protein becomes inactive
Pharmacologic Correlation
Understanding these pathways is critical for treatment development. About half of all known drugs target G-protein coupled receptors or some part of a G-protein coupled receptor pathway.