Esophagus

The esophagus passes from the hypopharynx to the stomach, it is about 25-30 cm long and 2-3 cm wide. The esophagus is part of the digestive system but it does not have any absorptive or intrinsic digestive function. It is a muscular tube that transports food and water through the neck region and chest cavity to the stomach. The regular layers of the tubular organs of the GI-tract can be recognized. The mucosal layer is covered by a stratified squamous nonkeratinized epithelium. The basal layer of the epithelium consists of columnar cells with spherical cell nuclei. The basal layer is the location for cell renewal. As new cells are produced they gradually lose contact with the basement membrane and migrate upwards as they differentiate and change shape. In the overlying prickle cell layer, the cell nuclei appear polygonal and not as densely packed. In the superficial layer, the cell nuclei are flattened and condensed. The lamina propria underlies the epithelial cells and consists of loose connective tissue. Focal lymphocytes are also present in this layer. Lamina muscularis mucosae separate the mucosa from the submucosa. Between the lamina propria and the muscle layer is the submucosa, composed of loose connective tissue containing small blood vessels and lymphocytes.

An inner circular layer and an external longitudinal smooth muscle layer form tunica muscularis. In its proximal third the external layer is composed of skeletal muscle, the middle third contains a mixture of smooth and skeletal muscle, and the distal third contains only smooth muscle. Since the esophagus is located outside of the abdominal cavity it has no mesothelial covering, the outermost layer is an adventitia.

General histology of gastrointestinal tract (GI-tract)

The gastrointestinal canal consists of the esophagus, stomach, duodenum, jejunum, ileum, colon, rectum and anal canal. It is best viewed as a long tube passing from the oral to the anal opening. The main function is to supply the body with water, electrolytes and nutrients from ingested food. Our main sources of nutrients are carbohydrates, proteins and fats, which in general cannot be absorbed in the form they are ingested. First, they have to be broken down into small enough compounds. The process of digestion and absorption is carried out in a stepwise fashion as the food passes down the different parts of the gastrointestinal tract.

The general structure of all parts of the GI-tract is

1. tunica serosa /adventitia - Loose connective tissue with elastic and collagen fibers, nerves and vessels, covered by a single layer of flat mesothelial cells. Where there is no mesothelial cover the outermost layer is called the adventitia.

2. tela subserosa - a thin layer of loose connective tissue separating the serosa and muscle layer.

3. tunica muscularis - which for most parts is composed of an inner circular and outer longitudinal smooth muscle layer. Between the muscle fibers the myenteric plexus of Auerbach can be identified.

4. tela submucosa - a thick layer of loose connective tissue with numerous blood and lymphatic vessels. Here is where the ganglion cells of the submucosal plexus of Meissner might be seen.

5. tunica mucosa - the innermost layer that comes in contact with the gastrointestinal content. It has a secretory and absorptive function. The mucosa consists of the innermost epithelium that forms surface cells and glands, embedded in the lamina propria containing mainly loose connective tissue with small blood vessels and immune cells. A thin layer of smooth muscle, lamina muscularis mucosae, demarcates the division of the mucosa and submucosa.