The stomach is the site where food is converted to a thick fluid called chyme that allows for efficient enzymatic digestion of macromolecules. The wall of the stomach contains gastric folds or rugae that expand when the stomach is full. As seen in the image at the right, the stomach contains the typical layers seen in the GI tract, including the mucosa (M, lining epithelium and glands, lamina propria, and muscularis mucosa), submucosa (SM), muscularis externa (ME, containing three layers, oblique, circular, and longitudinal layers of smooth muscle), and a serosa.

Histologically, three regions are distinguishable in the stomach: the cardia, fundus/body, and pylorus. The cardia is the transitional region between the esophagus and body of the stomach. The pylorus is the narrow region that connects the stomach to the duodenum. The cardia and pylorus contain branched coiled tubular glands that secrete mucus. In contrast, the mucosa of the fundus/body contains long, branched tubular gastric glands with several cell types that secrete substances that aid in digestion. See the diagram and table below to review the cells and secretions of the gastric glands.

• Examine the images below and this section of the stomach fundus and body. Identify the layers and sublayers, noting that the simple columnar epithelium invaginates into gastric pits, which are tightly packed together. These lead into the gastric glands, which extend down to the thin muscularis mucosa. Cells of the lamina propria are seen scattered loosely around the gastric pits.
• On the same slide of stomach, find a region of mucosa in which the gastric glands are cut longitudinally. Starting at the surface, identify the mucus-secreting cells (called surface and neck cells depending on their proximity to the surface lining the stomach cavity), the large eosinophilic parietal cells, and the smaller, more basophilic peptic chief cells. Enteroendocrine cells are also present (e.g. G cells that secrete gastrin), but these cells are especially difficult to identify without special stains.

• Examine the images below and these slides of the mucosa of the cardia and mucosa of the pyloric stomach. Note that the pyloric glands, unlike the gastric glands found in the fundus and body of the stomach, contain mainly mucus-secreting cells with few parietal and gastrin secreting cells.

• Finally, examine this section of pyloric-duodenal junction, and note the abrupt change in the mucosa.
   

On to the small intestine.