Stomach

I. MUCOSA is lined by surface mucous cells that are attached by juxtaluminal tight junctions and that secrete mucus to protect the mucosa from the acid pH and hydrolytic enzymes contained in the gastric juice.

II. GASTRIC GLANDS contain the following cell types:

A. Stem cells, which play a role in regeneration

B. Mucous neck cells

C. Parietal cells (Figure 13-1), which secrete the following:

1. Hydrochloric acid (HC1) is secreted into the gastric lumen. HC1 is produced through the action of carbonic anhydrase and H+-K+-adenosine triphosphatase (ATPase) (a H+ pump). CI" are secreted along with H+, so the secretion product of parietal cells is HC1.

2. Bicarbonate (HCOj) secreted into the bloodstream causes an increase in the pH called the alkaline tide.

3. Intrinsic factor is necessary for vitamin B1Z absorption. Pernicious anemia may result due to vitamin B, , deficiency caused by atrophic gastritis with decreased intrinsic factor production (see Chapter 9; Figure 9-4).

D. Chief cells secrete pepsinogen (inactive), which is converted to pepsin (active) upon contact with the acid pH of the gastric juice.

E. Enteroendocrine cells

1. G cells a. These are stimulated in response to a meal.

b. They secrete gastrin, which stimulates HC1 secretion from parietal cells. C. They are found predominately in the antrum of the stomach.

d. If ulcers occur, the antrum may be resected to reduce the amount of HCI secretion.

2. EC cells secrete serotonin, which increases gut motility.

3. D cells secrete somatostatin, which inhibits secretion of nearby enteroendocrine cells.

4. A cells secrete glucagon, which stimulates hepatic glycogen degradation, thereby increasing blood glucose levels.

Figure 13-1. (A) Light micrograph of parieral cells. (B) Electron micrograph of a parietal cell. Note the centrally located nucleus, numerous mitochondria, and intracellular canaliculi. (Courtesy of Dr. S. Ito.)

III. GASTRIC EMPTYING describes the process whereby the lower portion of the stomach contracts to move food into the duodenum.

A. Isotonic stomach contents increase gastric emptying.

B. Fat inhibits gastric emptying.

C. H+ in the duodenal lumen inhibits gastric emptying.

IV. REPAIR (REGENERATION). Stem cells have a high rate of mitosis. They migrate upward to replace surface mucous cells every 4-7 days. Stem cells also may migrate downward to replace mucous neck, parietal, chief, and enteroendocrine cells.

V. CLINICAL CONSIDERATIONS

A. Gastric ulcers

1. An ulccr is a brcach in the mucosa that extends into the submucosa or deeper.

2. Ulcers occur where exposure to the aggressive action of the gastric juice is high (e.g., stomach, duodenum, esophagus).

3. The bacteria Helicobacter pylori can contribute to ulcers, so antibiotics may be effective in treatment.

4. Other treatments for ulcers include ways to reduce HC1 secretion, such as:

a. Surgical resection of the pyloric antrum b. Omeprazole (an H+-K+-ATPase inhibitor)

C. Atropine [a muscarinic ACh receptor (mAChR) antagonist], which blocks the stimulatory effects of ACh released from postganglionic parasympathetic neurons on HC1 secretion d. Cimetidine [a histamine (H2) receptor antagonist], which blocks the stimulatory effects of histamine released from mast cells on HC1 secretion

B. Zollinger-Ellison syndrome is due to a gastrin-secreting tumor of the pancreas that causes increased H+ secretion from parietal cells, which continues unabated because the tumor cells arc not subject to feedback inhibition.

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