The esophagus is a muscular tube that conducts food from the oral cavity to the stomach. Review the general organization of the gastrointestinal tract, noting especially the four major layers, which are clearly seen in the esophagus:

• Mucosa (M) - epithelia, lamina propria, and muscularis mucosa
• Submucosa (SM) - dense irregular connective tissue containing blood vessels, lymphatics, and nerves
• Muscularis externa (ME) - two thick layers of smooth muscle for peristalsis
• Adventitia or serosa (A or S) - outer connective tissue or epithelial covering

Examine the images below and transverse sections of the esophagus (sample 1, sample 2). Identify the various layers of the wall, including:

• Stratified squamous epithelium (SSE)
• Lamina propria (LP)
• Muscularis mucosa (MM)
• Submucosa (SM) CT with small mucous glands and lymphoid nodules
• Myenteric plexus (MP) with autonomic ganglia and nerves
• Muscularis externa (ME) (striated muscle of upper 1/3, mixed striated and smooth muscle of middle 1/3, and smooth muscle of lower 1/3)
• Adventitia or serosa

Examine these two sections of the esophagogastric junction (sample 1, sample 2). Note the abrupt, change in the lining epithelium from stratified squamous to simple columnar epithelium.

Clinical note: If the muscularis of the lower esophagus fails to maintain the state of partial contraction that normally prevents reflux of the gastric contents upward, heartburn results. Since the esophageal mucosa lacks a thick blanket of protective mucus, acidic gastric juices irritate it, causing painful esophagitis. If chronic, this is called gastro-esophageal reflux disorder (GERD) and can lead to metaplasia of stratified squamous epithelium into a simple columnar, mucous epithelium. (Image courtesy of WebMD)

Now to the stomach.