Lecture 23: Digestive system continued

Functions of the stomach

Digestion of food

• chemical: acids and enzymes

• Mechanical

Storage of food

Stomach’s four regions

• Cardiac stomach

• Funding stomach

• Body

• Pyloric stomach

Mucosa, submucosa, muscularis, and serosa of stomach

Mucosa:

– Epithelium is simple columnar

– Mucosa includes extensive

secretory glands for chemical digestion of food

• Submucosa:

– Dense irregular CT

– Forms structure of rugae to allow

stomach to expand and contract as needed

• Muscularis:

– Smooth muscle

– Thick and layered to churn food

• Serosa:

– Simple squamous epithelium,

attached to mesenteries

Organization of the stomach mucosa

• Lined with a simple columnar epithelium comprised of multiple types of secretory cells

• Epithelium is flat on top, invaginates to form gastric pits and gastric glands

– Pits and glands are continuous straight glands; apical “pit” region has larger lumen and includes different types of secretory cells than the more basal “gland” regions

• Glands are supported by underlying loose CT and muscularis mucosa

Mucosal variations throughout stomach

All regions contain gastric glands, but they are much less extensive in pyloric and cardiac stomachs, which only produce mucous and do not contribute to chemical digestion

Epithelium/secretory cells of the cardiac and pyloric stomachs

• Contain only mucus-secreting cells (faintly staining cytoplasm); do not contribute to chemical digestion of food

• Mucus provides protective coating over epithelial surface

Epithelium/secretory cells of the fundus and body

• The gastric pits and glands of the fundic stomach contain multiple types of secretory cells

• The gastric glands are particularly extensive, with lots of secretory cells whose products contribute to chemical digestion of food in the stomach

• Secretory cells differ in appearance depending on their function (i.e. their secretory product)

The secretory cells of the fundic stomach epithelium: 1. Surface mucous cells

• Line the gastric

• Pronounced columnar shape

• Mucous secretions will result in lightly staining cytoplasm

• Mucus protects epithelium from acid and enzymes in stomach lumen

The secretory cells of the fundic stomach epithelium: Mucous neck cells

– Located in isthmus/neck, and less columnar than surface cells (often squished by neighbors)

– Again, light-staining cytoplasm due to mucous secretions

– Function: protection of neighboring cells from enzymes and acid

– Don’t worry too much about finding/identifying these cells

The secretory cells of the fundic stomach epithelium: Parietal cells

– Located in neck and gastric glands

– Secrete HCl into lumen for chemical digestion, and bicarbonate ions into submucosa to neutralize pH

– Lots of mitochondria

• Ion pumps require lots of ATP

Parietal cells

• Large, non-polarized cells with central nuclei

– Non-polarized since they are secreting into both lumen and underlying tissue

• Acidophilic cytoplasm due to abundance of mitochondria

The secretory cells of the fundic stomach epithelium

– Located in gastric glands

– Secrete digestive secretory granules

• Highly polarized

• Basal region is basophilic due to enrichment of RER enzymes (pepsins and lipases) into lumen for chemical digestion

– Abundant RER and apical

Stomach submucosa

• Dense irregular CT with embedded blood vessels and nerves

• Includes folds called rugae that can stretch to increase size of stomach lumen when full

Stomach muscularis extern

• Muscularis externa is comprised of multiple layers of smooth muscle – involuntary churning of food

• Layers are thick but not as clearly organized as in intestines