The serosa is a serous membrane consisting of a layer of simple squamous epithelium, called the mesothelium, and a small amount of underlying connective tissue. It is equivalent to the visceral peritoneum described in gross anatomy. The serosa is the most superficial layer of those parts of the digestive tract that are suspended in the peritoneal cavity. As such, the serosa is continuous with both the mesentery and the lining of the abdominal cavity.
Large blood and lymphatic vessels and nerve trunks travel through the serosa (from and to the mesentery) to reach the wall of the digestive tract. Large amounts of adipose tissue can develop in the connective tissue of the serosa (and in the mesentery).
Parts of the digestive tract do not possess a serosa. These include the thoracic part of the esophagus and portions of structures in the abdominal and pelvic cavities that are fixed to the cavity wall—the duodenum, ascending and descending colon, rectum, and anal canal. These structures are attached to the abdominal and pelvic wall by connec tive tissue, the adventitia, which blends with the connective tissue of the wall.
The esophagus is a fixed muscular tube that delivers food and liquid from the pharynx to the stomach
The esophagus courses through the neck and mediastinum where it is fixed to adjacent structures by connective tissue. As it enters the abdominal cavity, it is free for a short distance, approximately 1 to 2 cm. The overall length of the esophagus is about 25 cm. On cross section (Fig. 16.2), the lumen in its normally collapsed state has a branched appearance due to longitudinal folds. When a bolus of food passes through the esophagus the lumen expands without mucosal injury.
The mucosa that lines the length of the esophagus has a nonkeratinized stratified squamous epithelium (Fig. 16.3). In many animals, however, the epithelium is keratinized, reflecting a coarse food diet. In humans, the surface cells may exhibit some keratohyalin grannies, but keratinization does not normally occur. The underlying lamina propria is similar to the lamina propria throughout the alimentary tract; diffuse lymphatic tissue is scattered throughout, and lymphatic nodules are present, often in proximity to ducts of the esophageal mucous glands (described below). The deep layer of the mucosa, the muscularis mucosae, is composed of longitudinally organized smooth muscle that begins near the level of the cricoid cartilage. It is unusually thick in the proximal portion of the esophagus and presumably functions as an aid in swallowing.
The submucosa consists of dense irregular connective tissue that contains the larger blood and lymphatic vessels, nerve fibers, and ganglion cells. The nerve fibers and ganglion cells make up the submucosal plexus (Meissner's plexus). Glands are also present (described below). In addition, diffuse lymphatic tissue and lymphatic nodules are present mostly in the upper and lower parts of the esophagus where submucosal glands are more prevalent.
The muscularis externa consists of two muscle layers, an inner circular layer and an outer longitudinal layer. It differs from the muscularis externa found in the rest of the digestive tract in that the upper one third is striated muscle, a continuation of the muscle of the pharynx. Striated muscle and smooth muscle bundles are mixed and interwoven in the muscularis externa of the middle third of the esophagus; the muscularis externa of the distal third consists only of smooth muscle, as in the rest of the digestive tract. A nerve plexus, the myenteric plexus (Auerbach's plexus), is present between the outer and inner muscle layers. As in the submucosal plexus (Meissner's plexus), nerves and ganglion cells are present here. This plexus innervates the muscularis externa and produces peristaltic activity.
mucosa submucosa lymphatic nodule mucous gland -
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