Upper GI Structure & Function - Battle

Question 1

What happens to food in the following parts of the digestive tract?

1. Mouth
2. Stomach
3. Duodenum
4. Small Intestine
5. Large Intestine

Answer 1

1. Mouth- mechanical and chemical breakdown of food, lubrication of bolus with saliva
2. Stomach- bolus is broken down via mechanical, chemical means to form liquid chyme
3. Duodenum- pancreatic enzymes and bile are added to chyme
4. Small Intestine- nutrient absorption
5. Large Intestine- fluid absorption

Question 2

1. Where is voluntary muscle seen in the GI tract?
2. Where is involuntary muscle seen in the GI tract?

Answer 2

1. oral cavity, upper third of the esophagus, and presumably the rectum
2. essentially the entire length of the GI tract (peristalsis)

Question 3

What are the four tunic layers in the GI tract?

Answer 3

(from apical to basal)

• Mucosa
• Submucosa
• Muscularis externa
• Adventitia/Serosa

Question 4

What are the three layers of the mucosa?

Answer 4

(from apical to basal)

Epithelium

Lamina propria

Muscularis mucosae

Question 5

1. What is the main purpose of the epithelium?
2. What structural variants can be seen?

Answer 5

1. main site of secretion, absorption
2. stratified squamous, simple columnar

Question 6

1. What is the purpose of the lamina propria?
2. What makes up the lamina propria?

Answer 6

1. Supports the epithelium
2. connective tissue, with extremely dense lymphatics and capillaries (this is likely the source of the mucosal immunity)

Question 7

What is the muscularis mucosae?

Answer 7

Thin layer of smooth muscle required for mucosal folding and movement

Question 8

1. What is the purpose of the submucosa?
2. What does the submucosa contain?

Answer 8

1. Support the mucosa
2. collagenous and adipose tissue (physical support); blood, lymphatic vessels (nutritional support); submucosal/Meissner’s plexus (neural regulation)

Question 9

What is seen in the submucosa of the duodenum and esophagus?

Answer 9

Mucous secreting glands

Question 10

1. Where is the muscularis externa?
2. What does the muscularis externa do?

Answer 10

1. The ME is basal to the submucosa
2. The ME is responsible for movement of the GI tract

Question 11

1. What muscle layers make up the muscularis externa?
2. What controls the muscularis externa?

Answer 11

1. An inner, circular layer and an outer, longitudinal layer
2. controlled by the myenteric/auerbach’s plexus

Question 12

1. What is the adventitia/serosa?
2. What does it do?

Answer 12

1. Outermost covering of the GI tract
2. protects the GI tract

Question 13

What is the difference between adventitia and serosa?

Answer 13

Adventitia is a loose connective tissue that is outside of the peritoneal cavity

Serosa is connective tissue with a simple squamous epithelium. The epithelium lubricates the peritoneal interfaces and prevents friction damage of the GI tract.

Question 14

What is the enteric nervous system?

Answer 14

The neurons that control movement of the GI tract AKA the submucosal and myenteric plexuses

Question 15

How is the enteric nervous system controlled?

Answer 15

• neurotransmitters and neuromodulators
• hormones

INDEPENDENT OF BRAIN AND SPINAL CORD

Question 16

1. What does the parasympathetic system do in the GI tract?
2. What does the sympathetic system do in the GI tract?

Answer 16

1. stimulates secretion and peristalsis
2. represses peristalsis, activates sphincters

Question 17

1. What does the submucosal plexus do?
2. What does the myenteric plexus do?

Answer 17

1. controls mucosal movement, secretion, and blood flow
2. controls peristalsis and gut movements

Question 18

1. What does protective epithelium look like on histology?
2. Where is protective epithelium found in the GI tract?

Answer 18

1. stratified squamous epithelium
2. found in the upper GI and the anus

Question 19

1. What does secretory epithelium look like on histology?
2. Where is secretory epithelium found in the GI tract?

Answer 19

1. Fan-shaped, with lots of tubular glands
2. found in the stomach

Question 20

1. What does absorptive epithelium look like on histology?
2. Where is absorptive epithelium found in the GI tract?

Answer 20

1. Villi with empty space in between the villi
2. found throughout the small intestine

Question 21

1. What does the hybrid absorptive/protective epithelium look like on histology?
2. Where is absoprtive/protective epithelium found in the GI tract?

Answer 21

1. villi closely packed together, lots of mucous granules
2. lines the large intestine

Question 22

What is seen in the mucosal layer of the esophagus?

Answer 22

Epithelium: stratified squamous epithelium with langerhans cells

Lamina propria: esophageal cardiac glands

Muscularis mucosae: very thin, hard to identify

Question 23

What is seen in the submucosa layer of the esophagus?

Answer 23

mucous and serous glands

remember, only the duodenum and the esophagus have glands in the submucosa

Question 24

What is seen in the muscularis externa of the esophagus?

Answer 24

• inner layer of circular muscle
• an outer layer of longitudinal muscle skeletal muscle is seen in the upper third

Question 25

What is seen in the adventitia/serosa of the esophagus?

Answer 25

* Above the diaphragm, the esophagus is covered with adventitia; * below the diaphragm, it turns into serosa.

Question 26

What histological change is noted at the gastoesophageal junction?

Answer 26

Sharp change from **stratified squamous** (**protective**) epithelium to a **simple columnar **(**secretory**) epithelium.

Question 27

1. Name two physiological sphincters in the upper GI tract that serve to prevent reflux. 2. Why are they considered "physiological" spincters instead of "anatomical" sphincters?

Answer 27

1. Pharyngoesophageal & Gastoesophageal sphincters 2. They are not "anatomical" because there is no obvious thickening of muscle seen with other sphincters. But they still serve a clear purpose as sphincters: thus, "physiological".

Question 28

1. When the stomach is empty, what macroscopic anatomical feature is seen? 2. About how much can a fully distended stomach hold?

Answer 28

1. Rugae - prominent folds of mucosa and submucosa. These dissapear as the stomach distends to accomadate food. 2. About 1.5L

Question 29

1. Describe the makeup of **gastic juice**. 2. Where is gastric juice secreted from?

Answer 29

1. Composition: * HCl * Digestive Enzymes * Pepsin * Rennin * Gastric lipase * Mucus 2. Gastric Glands

Question 30

Name and describe the four anatomical regions of the stomach.

Answer 30

From proximal to distal: 1. **Cardia**: Small area at gastroesophageal junction. 2. **Fundus**: Upper left to cardia; secrete acid and enzymes 3. **Body (corpus)**: Central area; secrete acid and enzymes 4. **Pylorus**: Constricted region at base; regulates **release of chyme** into duodenum (via pyloric sphincter); secretes **gastrin**

Question 31

1. Describe the histological appearance of *stomach mucosa* **epithelium**. 2. What is the function of stomach mucosa epithelium?

Answer 31

1. Simple columnar 2. These cells generate a **thick** mucous covering that **traps bicarbonate ions**, protecting the mucosa from the stomach's low pH. It also produces a **soluble **mucous that acts as a **lubricant**.

Question 32

1. Describe the makeup of the stomach mucosa's **lamina propria**. 2. What special secretory cell, whose purpose is to increase acid production, is found in this layer? 3. What product of these cells mediates this function?

Answer 32

1. Loose connective tissue layer, densely packed with **gastric glands**. 2. **Enterochromaffin-type cells** 3. **Histamine**

Question 33

1. Describe the makeup and function of the stomach mucosa's **muscularis mucosae**. 2. What component of the lamina propria layer extend into this muscular layer?

Answer 33

1. Smooth muscle; influences gastric gland outward flow 2. Gastric glands

Question 34

What are gastric pits?

Answer 34

Invaginations of the stomach mucosa's **epithelium** into the **lamina propria**; denote entrances to the tubular-shaped gastric glands.

Question 35

Describe the makeup of the stomach's submucosa.

Answer 35

* Thick layer of dense connective tissue * Numerous **wandering immune cells** * Vascular & lymphatic structures * Submucosal (Meissner's) plexus

Question 36

1. Name the layers of the stomach's **muscularis externa **from **internal** **to external**. 2. Contraction of these muscular layers is mediated by what?

Answer 36

1. Internal to external: 1. Oblique (if present) 2. Circular 3. Longitudinal 2. Myenteric (**Auerbach's**) Plexus

Question 37

1. How many gastric glands open into a single gastric pit? 2. What are the three regions of each tubular-shaped gastric gland?

Answer 37

1. One to seven 2. Regions: * Base (deepest) * Neck * Isthmus (empties into gastric pit)

Question 38

1. Name the six cell types found within gastric glands and the region of the pit/gland that each is found in (pit, isthmus, neck, or base). 2. What type of epithelium is every one of these cell types?

Answer 38

1. Surface mucus / lining cells * Pit 2. Parietal (oxyntic) cells * Isthmus, neck, and base 3. Neck mucous cells * Neck and base 4. Chief (zymogenic, peptic) cells * Base 5. Enteroendocrine cells * Base 6. Stem cells * Neck

Question 39

What two gastric gland cells types produce mucous? How are the two types of mucous different? How would you tell the cells types apart using EM?

Answer 39

1. **Surface lining cells** produce a **thick, dense** mucous that protects against acid digestion (binds bicarbonate ions). * EM shows **practically pitch-black secretory glands (dense!)** 2. **Mucous neck cells** produce a **soluble** mucous that **lubricates **the stomach contents to aid digestion. * EM shows varyingly gray secretory granules

Question 40

1. What are enteroendocrine cells? 2. In what fashion do their products act on other cells?

Answer 40

1. Small ovoid or pyramidal cells between the base of neighboring epithelial cells. Generaly secrete a **single characteristic bioactive agent**. 2. Either **paracrine** of **endocrine**

Question 41

How many types of enteroendocrine cells are there? List their shorthand names and their products.

Answer 41

Five types: 1. G cells - gastrin 2. EC cells - serotonin 3. D cells - somatostatin 4. A cells - enteroglucagon 5. ECL cells - histamine

Question 42

1. Parietal cells generate what product(s)? 2. How are they distinctive by H&E staining? 3. How are they distinctive morphologically?

Answer 42

1. HCl & Intrinsic Factor 2. Cytoplasm is heavily eosinophillic 3. Presence of a secretory canaliculus

Question 43

Describe the appearance and function of a parietal cell's **secretory canaliculus**.

Answer 43

**Structure**: An invagination of the apical surface which can almost encircle the nucleus, filled with a dense network of microvilli. **Function**: Hugely increases the secretory surface area of the cell.

Question 44

1. What is the **tubulovesicular network** of parietal cells? 2. Explain its relationship with the **secretory canaliculus**. 3. Explain why its abundance is inversely correlated with HCl secretion.

Answer 44

As per Wikipedia... 1. The tubulovesicular network is a collection of **canalicular precursors**. 2. The tubulovesicles can be **fused** with the cell membrane to form **additional** secretory canaliculus, or parts of the canaliculus can be **endocytosed** to **reform** the tubulovesicles. In this way, the parietal cell can **up- or downregulate HCl secretion** as needed. 3. Because the canaliculus represents **active** secretory surface area and the tubulovesicles are **inactive**, a **more abundant** tubulovesicular network would indicate less active secretory canaliculus and thus **decreased** HCl secretion.

Question 45

What cell organelle are parietal cells particularly rich in? Why?

Answer 45

Mitochrondria! They provide energy for HCl secretion.

Question 46

1. What is the principal cell type at the **base** of the gastric glands? 2. Again, what do these cells secrete?

Answer 46

1. Chief cells 2. Pepsinogen, rennin, and gastric lipase

Question 47

Where in the chief cell are the secretory granules located? The nucleus? What cell organelles are rich in chief cells? Why?

Answer 47

Granules: **apical** Nucleus: **basal** Rich in **rER** and **golgi** to accomplish high volume protein production.

Question 48

Describe two mechanisms by which chief cells can be stimulated to release their secretory granules.

Answer 48

1. Binding of the hormone **secretin** to receptors on chief cells leads to rapid exocytosis of the granules. 2. Neural stimulation via CNX (vagus) also causes enzyme release * The "more significant" release method

Question 49

1. Again, where are gastric stem (regenerative) cells found? 2. How can gastric stem cells be idenified in stained sections?

Answer 49

1. The **neck** of the gastirc glands. * Daughter cells differentiate and migrate upward or downward along the gland to replace all epithelial cell types. 2. Presence of **mitotic figures**

Question 50

1. What is the demand on the gastric stem cells? 2. How can the surface epithelium of the stomach be quickly repaired following injury?

Answer 50

1. Complete renewal of the mucosa every **three days** 2. Rapid migration of epithelial cells that were lying deep in the protected environment of the gastric pits can replace the damaged epithelium (**mucosal restitution**)

Question 51

Describe the differences in the appearance of gastric pits/glands in the different areas of the stomach.

Answer 51

**Fundus & Body** (indistinguishable from each other): **straight** tubular glands **Pylorus**: **Branched** glands, predominance of **mucous neck cells** *(extra lubrication as the chyme is squeezed though the relatively smaller pylorus?)* **Cardiac **region: **Shallower** gastric pits, **highly coiled** glands. Abundance of **enteroendocrine cells** *(want to send signals to the rest of the stomach at the start of digestion?)*

Question 52

Name and broadly describe the three phases of the stomach's response to a meal.

Answer 52

1. Cephalic phase: brain anticipates food & sends signals to prepare the stomach 2. Gastric phase: food enters the stomach; chemical & mechanical digestion 3. Intestinal phase: Chyme release into duodenum, suppression of gastric activity

Question 53

Describe how the brain readies the stomach for digestion during the cephalic phase of digestion?

Answer 53

* Smells & visual cues cause brain to send parasympathetic signals via CNX. * Vagus nerve causes ACh release, leading to gastrin secretion by G cells * Gastrin, ACh, and histamine stimulate **~30%** of total gastric acid release from parietal cells.

Question 54

About how much of total secreted gastric acid is released during the gastric phase of digestion? When is the remainder secreted? How much is the remainder?

Answer 54

**~60%** of the total to be secreted. 30% cephalic + 60% gastric = 90% of total has been released up to this point The remaining **10%** is released during the **intestinal** phase.

Question 55

Describe the enterogastric reflex.

Answer 55

Inhibition of gastric digestion and emptying by distension of the small intestine, mediated by the ENS.

Question 56

Name two enzymes that suppress gastric activity (during the intestinal phase of digestion.)

Answer 56

1. Cholecystokinin 2. Secretin * *N.B.* Although secretin **increases **secretion of **digestive enzymes** from chief cells, it also **reduces acid secretion** from the stomach by inhibiting **gastrin** release from G cells.