ANA 221: Histology of Alimentary Canal II

Stomach

• The wall of the stomach has four basic layers: mucous membrane, submucosa, muscularis externa, and serous layer.
• The mucous membrane shows numerous folds (rugae) that disappear when the stomach is distended.
• The mucous membrane is composed of:
  • A lining epithelium that is columnar and mucous-secreting.
  • A layer of connective tissue, the lamina propria, that supports the epithelium.
  • A thin layer of smooth muscle called the muscularis mucosae.
• The lining epithelium cells have apical parts filled with mucin that is usually removed during processing, making them appear empty or vacuolated.
• Mucous secreted by the lining epithelium cells protects the gastric mucosa against acid and enzymes produced by the mucosa itself.
• The lining epithelium dips into the lamina propria to form the walls of depressions called gastric pits, which extend for a variable distance into the thickness of the mucosa.
• Deep to the gastric pits, the mucous membrane is packed with numerous gastric glands of three types: main gastric, cardiac, and pyloric.

Main Gastric Glands

• The main gastric glands are present over most of the stomach, but not in the pyloric region and in a small area near the cardiac end.
• The main gastric glands are simple or branched tubular glands that lie at right angles to the mucosal surface.
• The glands open into gastric pits, each pit receiving the openings of several glands.
• The epithelium lining the glands is composed of:
  • Chief cells (peptic cells or zymogen cells) that are cuboidal or low columnar and contain prominent secretory granules in the apical parts of their cytoplasm.
  • Oxyntic or parietal cells that are large, ovoid or polyhedral, and contain a large central nucleus.

Small Intestine

• The small intestine is a tube about five meters long, divided into three parts.
• The wall of the small intestine is made up of the four basic layers: mucous membrane, submucosa, muscularis externa, and serous layer.
• The mucous membrane of the small intestine has a large surface area due to:
  • Its considerable length.
  • The presence of numerous circular folds (valves of Kerkring) in the mucosa.
  • The presence of numerous finger-like processes (villi) that project from the surface of the mucosa into the lumen.
  • The presence of numerous depressions (crypts) that invade the lamina propria.
  • The presence of microvilli on the luminal surfaces of the cells lining the mucosa.

Large Intestine

• The large intestine is composed of the colon, vermiform appendix, and rectum.
• The colon has a mucous membrane with a large surface area due to:
  • The presence of numerous circular folds (valves of Kerkring) in the mucosa.
  • The presence of numerous finger-like processes (villi) that project from the surface of the mucosa into the lumen.
  • The presence of numerous depressions (crypts) that invade the lamina propria.
  • The presence of microvilli on the luminal surfaces of the cells lining the mucosa.
• The mucous membrane of the colon is lined by an epithelium made up predominantly of columnar cells with a striated border.
• The main function of the columnar cells is to absorb excess water and electrolytes from intestinal contents.
• Many columnar cells secrete mucous and antibodies (IgA), providing protection against pathogenic organisms.

Vermiform Appendix

• The structure of the vermiform appendix resembles that of the colon with the following differences:
  • The appendix is the narrowest part of the gut.
  • The crypts are poorly formed.
  • The longitudinal muscle coat is complete and equally thick all around.
  • Taenia coli are not present.
  • The submucosa contains abundant lymphoid tissue that may completely fill the submucosa.

Rectum

• The structure of the rectum is similar to that of the colon except for the following:
  • A continuous coat of longitudinal muscle is present.
  • There are no taenia.
  • Peritoneum covers the front and sides of the upper one-third of the rectum, and only the front of the middle third.
  • There are no appendices epiploicae.

Anal Canal

• The anal canal is about 4 cm long, with the upper 3 cm lined by mucous membrane and the lower 1 cm lined by skin.
• The area lined by mucous membrane can be further divided into an upper part (15 mm) and a lower part (15 mm).
• The mucous membrane of the upper 15 mm of the canal is lined by columnar epithelium and shows six to twelve longitudinal folds (anal columns).
• The lower ends of the anal columns are united to each other by short transverse folds (anal valves) that form a transverse line (pectinate line).
• The mucous membrane of the next 15 mm of the rectum is lined by non-keratinized stratified squamous epithelium and does not have anal columns.