Notes
Protection in the Digestive Tract
Sections
Protection
Digestive tract utilizes various mechanisms to protect the body (internal environment) from the external environment.
It specifically protects against:
- Chemical damage
- Exposure to toxins
- Microorganism entry
KEY protective features
(found throughout the entire GI tract)
1. Tight junctions
- Dense network of claudins and other proteins just below the apical surface of the GI epithelium.
- Intrinsic barrier of the digestive tract. They form a nearly impermeable barrier that prevents GI tract luminal contents from freely leaking through the mucosal layer.
Tight junctions prevent entry of microorganisms and other potentially harmful substances or toxins (such as HCl produced in the stomach) into the digestive tract wall.
2. Mucus lining
- Feature of all mucous membranes (mucosa).
- Mucus = alkaline (bicarbonate) secretion that protects against shear stress and chemical damage throughout the digestive tract.
Secreted by:
1. Mucous cells in oral cavity
- Forms mucus lining
- Secreted as a component of saliva to primarily aid food bolus formation.
- Minor protective role in the oral cavity (protects the mouth from acidic food and pathogens)
3. Goblet cells in small intestine.
- Mucus lining continues as a chemical barrier between the acidic chyme present in the intestines and the intestinal mucosa.
Unique protective features in these three GI organs.
oral cavity
Secretes saliva, which contains:
- Lysozymes, which lyse bacteria.
- IgA antibodies, which maintain mucosal immunity, and
- Defensins, which are host defense antimicrobial peptides of innate immunity.
Stomach
Parietal cells
(in the epithelium of the mucosal layer, which lines the lumen of the stomach)
- Secrete HCl, which primarily digests protein; however,
- HCl creates a harsh environment, which kills many microorganisms. The gastric mucosa is largely protected from this harsh environment by the alkaline mucus lining.
Clinical Correlate: Gastric Ulcers
- Breaks in the mucosal barrier exposes the GI wall to corrosive HCl and proteases
- Causes gastric wall erosion and inflammation
- Common cause: H. pylori (bacteria) is a common cause of gastric ulcers, which erodes the epithelial barrier.
- Ulcers can also occur in lower esophagus and in the small intestine (specifically, the duodenum).
High Cellular Turnover Rate
- GI epithelium sheds frequently as it becomes damaged from continued exposure to its lumen's harsh chemical environment and continuous shear stress gastric motility.
- Regeneration: epithelial stem cells replenish damaged or dead epithelial cells.
- The GI epithelium divides, or turns over, almost constantly to replace damaged cells with new, mature ones – unlike the heart, which almost never replaces its cardiac cells.
Stem cells
- Located at the top of gastric glands
- Replenish gastric secretory and mucous cells that protect the stomach surface.
small intestine.
Paneth cells
- Found in its villi crypts
- Contain secretory granules filled antimicrobial peptides, which are secreted into the GI lumen.
- Provide another layer of host defense in the small intestine.
Stem cells
- Replenish damaged, or dead, absorptive and goblet cells that are shed from villi; they reside adjacent to paneth cells in villi crypts.
Paneth cells = "protectors of stem cells"
- Secrete factors to maintain these stem cells to promote cellular renewal.
Note: Although the digestive tract relies on high rates of cellular division to replace old, damaged cells – this also corresponds to higher rates of mutations and, thus, increased incidence of cancer in GI epithelium (carcinoma).
Full-Length Text
- Here we will learn about protection in the digestive tract.
- First start a table.
- Begin with the Key Functions of the digestive tract.
- Denote the following 5 major functions:
- Motility
- Secretion
- Digestion
- Absorption
- Protection
- And the minor function: Elimination.
- In this tutorial we will address protection throughout the entire digestive tract as well as specific protective mechanisms unique to each organ of the digestive tract to promote this key function.
- Denote that the digestive tract utilizes various mechanisms to protect the body (internal environment) from the external environment.
- Denote that it specifically protects against:
- Chemical damage
- Exposure to toxins
- Microorganism entry
Let's first discuss key protective features throughout the entire digestive tract. We will then show how they contribute to protection in the oral cavity, stomach, and small intestine.
- First, draw a couple of mucosal epithelial cells, which are a sub-layer of the mucosa and are in direct contact with the lumen, to illustrate two key protective features.
- Indicate the digestive tract lumen, above the apical surface of the cells.
- First, join the adjacent sides of the epithelial cells with tight junctions, which are a dense network of claudins and other proteins just below the apical surface of the GI epithelium.
- Denote that tight junctions are an intrinsic barrier of the digestive tract.
- They form a nearly impermeable barrier that prevents GI tract luminal contents from freely leaking through the mucosal layer.
- Specifically, show that tight junctions prevent entry of microorganisms and other potentially harmful substances or toxins (such as HCl produced in the stomach) into the digestive tract wall.
- Next, coat the GI epithelium with a mucus lining, a feature of all mucous membranes (mucosa).
- Denote that mucus is an alkaline (bicarbonate) secretion that protects against shear stress and chemical damage throughout the digestive tract.
Let's now discuss the specific role of mucus in three key areas of the digestive tract.
- Draw a series of mucosal epithelial cells that correspond to those of the:
- Oral cavity
- Stomach, and
- Small intestines.
- Remember to join the cells with tight junctions.
- Leave spaces between these representative cells as we introduce more specialized of the mucosal epithelium later.
- Draw a mucous cell.
- Show that it secretes mucus in the oral cavity to form the mucus lining.
- Write that it is secreted as a component of saliva to primarily aid food bolus formation.
- It has a minor protective role in the oral cavity. It protects the mouth from acidic food and pathogens.
- Now, show a mucous neck cell.
- And indicate that they secrete mucus in the stomach.
- Write that the mucus lining provides a chemical barrier between the stomach lumen and its epithelium. Specifically, write that it:
- Neutralizes acidic secretions during a meal (with its bicarbonate component), and
- Prevents autodigestion of the mucosa by proteases.
- Next, draw a goblet cell.
- Show that they secrete mucus in the small intestine.
- Write that the mucus lining continues as a chemical barrier between the acidic chyme present in the intestines and the intestinal mucosa.
Let's now highlight unique protective features in these three GI organs.
- Show that the oral cavity secretes saliva.
- Indicate that, among other key components, it also contains:
- Lysozymes, which lyse bacteria.
- IgA antibodies, which maintain mucosal immunity, and
- Defensins, which are host defense antimicrobial peptides of the innate immunity.
- For the stomach, show that it also has parietal cells in the epithelium of the mucosal layer, which lines the lumen of the stomach.
- Indicate they secrete HCl, which primarily digests protein; however,
- Write that HCl creates a harsh environment, which kills many microorganisms. The gastric mucosa is largely protected from this harsh environment by the alkaline mucus lining.
- As a clinical correlate, denote that breaks in the mucosal barrier exposes the GI wall to corrosive HCl and proteases causes gastric wall erosion and inflammation which may potentially cause gastric ulcers.
- Write that that H. pylori (bacteria) is a common cause of gastric ulcers, which erodes the epithelial barrier.
- Ulcers can also occur in lower esophagus and in the small intestine (specifically, the duodenum).
- Denote that the digestive tract has a high turnover rate of its epithelium because the GI epithelium sheds frequently as it becomes damaged from continued exposure to its lumen's harsh chemical environment and continuous shear stress gastric motility.
- Denote that epithelial stem cells replenish damaged or dead epithelial cells through the process of regeneration.
- The GI epithelium divides, or turns over, almost constantly to replace damaged cells with new, mature ones – unlike the heart, which almost never replaces its cardiac cells.
- Thus, indicate the stomach contains stem cells, located at the top of gastric glands to replenish gastric secretory and mucous cells that protect the stomach surface.
Lastly, let's transition to the small intestine.
- Show that it also has paneth cells, which are found in its villi crypts
- And show they contain secretory granules filled antimicrobial peptides.
- Show they are secreted into the GI lumen.
- These provide another layer of host defense in the small intestine.
- Indicate the small intestine also has epithelial stem cells throughout its mucosal layer, which replenish damaged, or dead, absorptive and goblet cells that are shed from villi
- They reside adjacent to paneth cells in villi crypts.
- Write that paneth cells are "protectors of stem cells" by secreting factors to maintain these stem cells to promote cellular renewal.
- Although the digestive tract relies on high rates of cellular division to replace old, damaged cells – this also corresponds to higher rates of mutations and, thus, increased incidence of cancer in GI epithelium (carcinoma).