Chapter Summary

I. ESOPHAGUS

The esophagus is a muscular tube that delivers the bolus of food from the pharynx to the stomach. The esophagus, as well as the remainder of the digestive tract, is composed of four concentric layers: mucosa, submucosa, muscularis externa, and adventitia. The lumen of the esophagus is normally collapsed.

A. Mucosa

The mucosa has three regions: epithelium, lamina propria, and muscularis mucosae. It is thrown into longitudinal folds.

1. Epithelium

The epithelium is stratified squamous nonkeratinized.

2. Lamina Propria

The lamina propria is a loose connective tissue that contains mucus-producing esophageal cardiac glands in some regions of the esophagus.

3. Muscularis Mucosae

The muscularis mucosae is composed of a single layer of longitudinally oriented smooth muscle.

B. Submucosa

The submucosa, composed of fibroelastic connective tissue, is thrown into longitudinal folds. The esophageal glands proper of this layer produce a mucous secretion.

Meissner’s submucosal plexus houses postganglionic parasympathetic nerve cells.

C. Muscularis Externa

The muscularis externa is composed of inner circular

(tight helix) and outer longitudinal (loose helix) muscle layers. In the upper one-third of the esophagus, these consist of skeletal muscle; in the middle one-third, they consist of skeletal and smooth muscle; and in the lower one-third, they consist of smooth muscle. Auerbach’s myenteric plexus is located between the two layers of muscle.

D. Adventitia

The adventitia of the esophagus is composed of fibrous connective tissue. Inferior to the diaphragm, the esophagus is covered by a serosa.

II. STOMACH

The stomach is a sac-like structure that receives food from the esophagus and delivers its contents, known as chyme, into the duodenum. The stomach has three histologically recognizable regions: cardiac, fundic, and pyloric. The mucosa and submucosa of the empty stomach are thrown into folds, known as rugae, that disappear in the distended stomach.

A. Mucosa

The mucosa presents gastric pits, the bases of which accept the openings of gastric glands.

1. Epithelium

The simple columnar epithelium has no goblet cells. The cells composing this epithelium are known as surface lining cells and extend into the gastric pits.

2. Lamina Propria

The lamina propria houses numerous gastric glands, slender blood vessels, and various connective tissue and lymphoid cells.

a.Cells of Gastric Glands

Gastric glands are composed of the following cell types: parietal (oxyntic) cells, chief (zymogenic) cells, mucous neck cells, DNES (enteroendocrine) cells, and stem cells. Glands of the cardiac region have no chief and only a few parietal cells. Glands of the pyloric region are short and possess no chief cells and only a few parietal cells. Most of the cells are mucussecreting cells resembling mucous neck cells. Glands of the fundic region possess all five cell types.

3.Muscularis Mucosae

The muscularis mucosae is composed of an inner circular and an outer longitudinal smooth muscle layer. A third layer may be present in certain regions.

B. Submucosa

The submucosa contains no glands. It houses a vascular plexus as well as Meissner’s submucosal plexus.

C. Muscularis Externa

The muscularis externa is composed of three smooth muscle layers: the inner oblique, the middle circular, and

the outer longitudinal. The middle circular forms the pyloric sphincter. Auerbach’s myenteric plexus is located between the circular and longitudinal layers.

D. Serosa

The stomach is covered by a connective tissue coat enveloped in visceral peritoneum, the serosa.

III. SMALL INTESTINE

The small intestine is composed of three regions: duodenum, jejunum, and ileum. The mucosa of the small intestine presents folds, known as villi, that change their morphology and decrease in height from the duodenum to the ileum. The submucosa displays spiral folds, plicae circulares (valves of Kerckring).

A. Mucosa

The mucosa presents villi, evaginations of the epithelially covered lamina propria.

1. Epithelium

The simple columnar epithelium consists of goblet, surface absorptive, and DNES cells. The number of goblet cells increases from the duodenum to the ileum.

2. Lamina Propria

The lamina propria, composed of loose connective tissue, houses glands, known as the crypts of Lieberkühn, that extend to the muscularis mucosae. The cells composing these glands are goblet cells, columnar cells, and, especially at the base, Paneth’s cells, DNES cells, and stem cells. An occasional caveolated cell may also be noted. A central lacteal, a blindly ending lymphatic vessel, smooth muscle cells, blood vessels, solitary lymphatic nodules, and lymphoid cells are also present. Lymphatic nodules, with M cell epithelial caps, are especially abundant as

Peyer’s patches in the ileum.

3. Muscularis Mucosae

The muscularis mucosae consists of an inner circular and an outer longitudinal layer of smooth muscle.

B. Submucosa

The submucosa is not unusual except in the duodenum, where it contains Brunner’s glands.

C. Muscularis Externa

The muscularis externa is composed of the usual inner circular and outer longitudinal layers of smooth muscle, with Auerbach’s myenteric plexus intervening.

D. Serosa

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IV. LARGE INTESTINE

The large intestine is composed of the appendix, the cecum, the colon (ascending, transverse, and descending), the rectum, and the anal canal. The appendix and anal canal are described separately, although the remainder of the large intestine presents identical histologic features.

A. Colon

1. Mucosa

The mucosa presents no specialized folds. It is thicker than that of the small intestine.

a.Epithelium

The simple columnar epithelium has goblet cells and columnar cells.

b.Lamina Propria

The crypts of Lieberkühn of the lamina propria are longer than those of the small intestine.They are composed of numerous goblet cells, a few DNES cells, and stem cells. Lymphatic nodules are frequently present.

c. Muscularis Mucosae

The muscularis mucosae consists of inner circular and outer longitudinal smooth muscle layers.

2. Submucosa

The submucosa resembles that of the jejunum or ileum.

3. Muscularis Externa

The muscularis externa is composed of inner circular and outer longitudinal smooth muscle layers. The outer longitudinal muscle layer is modified into teniae coli, three flat ribbons of longitudinally arranged smooth muscles. These are responsible for the formation of haustra coli (sacculations). Auerbach’s plexus occupies its position between the two layers.

4. Serosa

The colon possesses both serosa and adventitia. The serosa presents small, fat-filled pouches, the appendices epiploicae.

B. Appendix

The lumen of the appendix is usually stellate-shaped, and it may be obliterated. The simple columnar epithelium covers a lamina propria rich in lymphatic nodules and some crypts of Lieberkühn. The muscularis mucosae, submucosa, and muscularis externa conform to the general plan of the digestive tract. It is covered by a serosa.

C. Anal Canal

The duodenum is covered by serosa and adventitia, whereas the jejunum and ileum are covered by a serosa.

The anal canal presents longitudinal folds, anal columns, which become joined at the orifice of the anus