Species: rabbit. (Oryctolagus cuniculus. Mammal).
Technique: paraffin embedding, Mallory's trichome.
The esophagus a is rather straight tube communicating the pharynx and the stomach. It consists of the following tissue layers: mucosa, submucosa, muscularis propria and adventitia.
The mucosa is composed of a stratified squamous epithelium (Figure 1), usually non-keratinized. The lamina propria is found under the epithelium. It is a thin layer of connective tissue with scarce cells, scattered lymphocytes, and some lymphatic nodules located close to the excretory ducts of exocrine glands (see below). Compared to other sections of the digestive tract, the muscularis mucosae is quite thick in the upper part of the esophagus, and most of the muscle cells are arranged longitudinally (parallel to the long axis of the esophagus).
The submucosa is dense connective tissue with many elastic and collagen fibers, which allow the large expansion of the esophagus wall when the food, or alimentary bolus, goes down the lumen to the stomach. The submucosa contains many nerves and sensory ganglion cells, altogether forming the Meissner plexus. There are also lymphatic nodules in the submucosa that may extend into the mucosa. The mucosa and submucosa are folded in several longitudinal ridges that shape the luminal surface (Figure 2). These folds are distended by the alimentary bolus on its way toward the stomach.
The muscularis propria layer is composed of muscle cells. There are striated muscle cells in the upper one-third of the esophagus, continuous with the muscle of the pharynx. As we move downward, smooth muscle cells are observed intermingled with the striated muscle cells. The smooth muscle cell proportion increases progressively, and there are no striated muscle cells in regions near the stomach. In the lower part of the esophagus, the organization of muscle cells in two layers is clearly visible: an inner layer with cells arranged circularly and an outer layer with cells oriented longitudinally. Between the two layers lies a plexus of nerves and ganglionic cells known as the Auerbach plexus or myenteric plexus. This nerve network is responsible for the peristaltic movement of the esophagus wall that pushes the alimentary bolus from the oral cavity down to the stomach.
The outer layer of the esophagus is the adventitia, a connective tissue sheet linking the esophagus to other body structures. Adventitia is replaced by a serous layer when the esophagus enters the abdominal cavity.
Esophagus
Virtual microscopy
The esophagus regulates the entrance and exit of the alimentary bolus by means of muscles organized as sphincters. The upper one, known as the pharyngoesophageal sphincter, controls the entrance of the alimentary bolus from the pharynx, whereas the lower one, the gastroesophageal sphincter, restricts the entrance into the stomach and prevents the stomach reflux.
The esophagus contains glands that release mucous substances. These secretions lubricate the walls of the esophagus to protect them from food movement and from stomach reflux.