Introduction of GIT

Lecture Objectives
By the end of this lecture, students are expected to:
1. List and describe the layers of the GIT.
2. Outline the histological features of the three layers of submucosa.
3. Compare the local function and histological features of Meissner and Auerbech plexuses.
4. Outline the different types of epithelium within oral cavity and link this difference to functional adaptation.
5. Summarize the functional and histological structure of submucosal gland.
6. Relate the functional to histological feature of the of layer muscularis externa of the esophagus in a process of swallowing.
7. Outline the structural and functional adaptations of the gastroesophageal glands
8. Outline the difference in histological features of outer layer of esophagus along its course.
Digestive Tract: Introduction
The digestive system consists of the digestive tract; oral cavity, esophagus, stomach, small , large intestines, rectum, and anus and its associated glands; salivary glands, liver, gallbladder and pancreas. Its function is to obtain the molecules necessary for the maintenance, growth, and energy needs of the body from ingested food.
Large molecules such as proteins, fats, complex carbohydrates, and nucleic acids are broken down into small molecules that are easily absorbed through the lining of the digestive tract, mostly in the small intestine. Water, vitamins, and minerals are also absorbed from ingested food. In addition, the inner layer of the digestive tract is a protective barrier between the content of the tract s lumen and the internal milieu of the body.
The first step in the complex process known as digestion occurs in the mouth, where food is moistened by saliva and ground by the teeth into smaller pieces; saliva also initiates the digestion of carbohydrates. Digestion continues in the stomach and small intestine, where the food transformed into its basic components (e.g, amino acids, monosaccharaides, free fatty acids, monoglycerides) is absorbed. Water absorption occurs in the large intestine, causing the undigested contents to become semisolid.
Dr.Hameda abdulmahdi College of Medicine /Dep. of anatomy & histology
2nd stag e
2
General Structure of the digestive
Based on its histological organization, the wall of the digestive tract can be divided into four tunics (Fig.1).
1. Mucosa is the innermost layer of the digestive wall. It includes epithelium, lamina propria, and muscularis mucosae.
a. The epithelium consists of simple columnar epithelium lining most of the tract and stratified squamous epithelium lining the two ends, the esophagus and anal canal.
b. The lamina propria is a loose connective tissue that contains abundant ground substance, many fibers, and numerous connective tissue cells such as fibroblasts, macrophages, mast cells, plasma cells, and leukocytes. Various types of glands are found in the lamina propria depending on the region of the digestive tract.
c. The muscularis mucosae is a very thin layer of smooth muscle, which is the boundary between the mucosa and the submucosa. It is usually arranged in an inner circular and outer longitudinal layer. However, the muscularis mucosae varies in different regions, and it is often diffi cult to distinguish between the muscle layers.
2. Submucosa is a thick layer of dense irregular connective tissue ,this layer contains blood vessels, lymphatic vessel and submucosal (Meissner) plexuses, which contain nerve fibers and neurons of the enteric nervous system. In some regions of the digestive tract, this layer is characterized by mucous glands or lymphatic nodules.
3. Muscularis externa is composed of two or three oblique, circular, and longitudinal muscle layers, which vary from region to region. Most of the muscularis externa consists of smooth muscle fibers, but the upper and middle esophagi contain some skeletal muscle. The myenteric (Auerbach) plexuses (nerve fibers and neurons of the enteric nervous system) are located between the muscle layers. They innervate and control contraction of the muscularis externa.
4. Serosa and adventitia are coverings of the outermost wall of the digestive tract. Most parts of the digestive tract are covered by serosa, a thin layer of loose connective tissue lined by mesothelium.