DIGESTIVE SYSTEM: STRUCTURE An organ system consists of several interacting organs which effect a common function. Each organ contributes a part to the overall function of the system to which it belongs and is specialized for this role in the unique arrangement of tissues which form its structure. We will examine how tissues specify organ function and how organs interact to effect a common system function by studying the digestive system which is composed of the mouth, pharynx, esophagus stomach small intestine, large intestine (colon), rectum, anus and associated glands (liver, pancreas and salivary glands). To illustrate the principle of regional differentiation of organs within an organ system we will concentrate on the digestive organs which comprise the alimentary tube. These organs (esophagus, stomach and both intestines) for the most part are suspended in the abdominal cavity supported by mesentaries. Each organ has a unique functional contribution to the process of digestion and is structurally specialized to achieve this function. Furthermore, most of these organs have parts which are also specialized and these subdivisions within organs can be recognized by their particular modifications of the general structure of the organ as a whole. You will be responsible for correlating structure with function not only between the different organs of the alimentary tube, but also between the different parts of the organs themselves. The alimentary tube has a general ground plan which consists of four major regions: mucosa, submucosa, muscularis externa and serosa (or adventitia). Mucosa The mucosa is a mucous membrane which surrounds the lumen of the alimentary tract. The lumen is the central cavity of the tube which contains the undigested food. The part of the mucosa which comes in contact with the food is the epithelial lining. Underlying the epithelium is a bed of connective tissue called the lamina propria. This layer of connective tissue is bounded on the side opposite the epithelium by a thin layer of muscle called the muscularis mucosae. Thus, the mucosa consists of the epithelium, lamina propria and muscularis mucosae. Submucosa The submucosa is a more extensive layer of connective tissue than the lamina propria of the mucosa and it is bounded on the mucosa side by the muscularis mucosae and on the other side by the inner layer of the muscularis externa. Glands may be found in the submucosa and their ducts empty into the lumen. Muscularis externa This tunic consists of two different bands of muscle separated by a thin layer of connective tissue. The inner layer closest to the lumen is circularly arranged about the tube, while the outer layer is lonitudinally arranged, i.e., the fibers run up and down the length of the alimentary tube. In between these two muscle layers can be found nervous tissue among the connective tissue fibers. This nervous tissue, called a plexus, consists of parasympathetic ganglia and both sympathetic and parasympathetic fibers. A second plexus is found near the boundary between the inner circular layer of muscle and the submucosa. Serosa and Adventitia The outside of the tube is surrounded by a reflection of the mesentery which holds the tube in place in the abdominal cavity. This thin layer of simple squamous epithelium and connective tissue is called the serosa. At the beginning and end of the tube, i.e., at the esophagus and rectum, the serosa is replaced by a thick band of connective tissue called the adventitia. The adventitia blends in with the connective tissue around other organs, e.g., the trachea, and so is not a distinct layer. The following specializations exist in the different tunics of the alimentary tube which serve as diagnostic characteristics for recognizing the different organs of the tube. Esophagus The single diagnostic feature which distinguishes the esophagus from all other parts of the alimentary tube (at least those studied here) is the presence of stratified squamous epithelium in the mucosa. All of the other three organs have simple columnar epithelium. In addition the presence of glands in the submucosa also serves as a diagnostic feature since only one other organ (the duodenum) has submucosal glands. The esophagus can be regionally separated into an upper, middle and lower region based on the type of muscel in the muscularis externa and muscularis mucosae. The upper part of the esophagus has striated muscle (structurally the same as skeletal muscle), the middle region a mixture of smooth and striated muscle fibers and the lower part all smooth muscle. Since the esophagus alone has striated muscle (all the other organs have smooth muscle) the presence of striated muscle fibers is diagnostic, but only for the upper and middle parts of the esophagus. Stomach The stomach can be identified by the presence of unspecialized simple columnar epithelium (the intestines have goblet cells in their epithelium), the absence of villi and the presence of a third muscle layer (an innermost oblique layer) in the muscularis externa. The stomach has three regions of specialization: the cardiac stomach which receives food from the esophagus, the fundic stomach (and corpus) which is the largest region, and the pyloric stomach which empties into the duodenum through the pyloric sphincter. The mucosa can be through into folds, called gastric rugae, when the stomach is empty. The rugae allow for distensibility and disappear when the stomach is full. The three different parts of the stomach can be identified structurally by the shape of their glands (or gastric pits) which extend from the epithelium to the lamina propria. The cardiac and pyloric stomachs both have coiled glands which can be recognized in sections of the stomach by their appearance as pieces since they don't all fall in the same plane of sectioning. The corpus of the stomach and the fundic stomach have straight glands which contain two types of cells: large light-staining parietal cells which secrete HCl and smaller basophilic chief cells which secrete pepsinogen. To distinguish between the cardiac and pyloric regions of the stomach, both of which possess coiled glands, note the characteristic epithelium at the junction of the esophagus and cardiac stomach and at the junction of the pyloric stomach and the duodenum. Small Intestine Both the small and large intestines have goblet cells in their epithelium. These cells are large and light due to the fact that the cytoplasm is loaded with mucous. The simple columnar epithelial lining extends into the lamina propria as glands called the crypts of Lieberkuhn. The small intestine, however, has evaginations of the mucous membrane into the lumen. These evaginations are called villi and are diagnostic of the small intestine. The small intestine also has folds in its mucosa called plicae circulares or valves of Kerkring. Although these are similar to gastric rugae, the valves of Kerkring are permanent features which do not flatten out when the intestine is full. The small intestine is subdivided into three regions: the duodenum, jejunum and ileum. Although the shape of the villi may be used to differentiate these parts we will focus on other structural differences. The duodenum borders the stomach and so at the junction of the two there will be an abrupt change in the mucosal lining. The presence of goblet cells and villi will identify the duodenum. In addition, this short section of the small intestine (about 12 inches) has glands in the submucosa which along with villi and goblet cells are distinctive of the duodenum. These glands are called Brunner's glands and are the only submucosal glands found in either intestine. The jejunum, which comprises the next 2/5ths of the small intestine, is unspecialized and so must be identified through a process of elimination. The ileum is the largest part of the small intestine and is characterized by the presence Peyer's patches or aggregated lymphatic nodules in the lamina propria and bordering submucosa. Large Intestine The colon (large intestine) can be identified by the presence of goblet cells and the absence of villi. Summary of the characteristics diagnostic for each digestive organ and part. Esophagus - stratified squamous epithelium, submucosal glands. Upper esophagus: striated muscle in muscularis mucosae and muscularis externa. Middle esophagus: mixture of striated and smooth muscle. Lower esophagus: only smooth muscle in muscle layers. Stomach - simple columnar epithelium without goblet cells. No villi. Third muscle layer in muscularis externa. Cardiac stomach: coiled glands in the lamina propria. Fundic stomach: straight glands in the lamina propria with two cell types (parietal and chief cells). Pyloric stomach: coiled glands in the lamina propria. Small intestine - simple columnar epithelium with goblet cells and mucosa thrown into folds which extend into the lumen (villi). Duodenum: Brunner's glands in the submucosa. Jejunum: neither Brunner's glands nor Peyer's patches. Ileum: Peyer's patches in mucosa and part of submucosa. Large intestine - goblet cells but no villi.