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Digestive System Part-2

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Digestive System Part-2 Digestive system part-2- General Histological Structure of the Digestive System The long continuous tube that is the digestive tract is about 9 meters in length. It opens to the outside at both ends, through the mouth at one end and through the anus at the other. Although there are variations in each region, the basic structure of the wall is the same throughout the entire length of the tube. The wall of the digestive tract has four layers or tunics: Mucosa Submucosa Muscular layer . Serous layer or serosa The mucosa, or mucous membrane layer, is the innermost tunic of the wall. It lines the lumen of the digestive tract. The mucosa consists of epithelium, an underlying loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosa. In certain regions, the mucosa develops folds that increase the surface area. Certain cells in the mucosa secrete mucus, digestive enzymes, and hormones. Ducts from other glands pass through the mucosa to the lumen. In the mouth and anus, where thickness for protection against abrasion is needed, the epithelium is stratified squamous tissue. The stomach and intestines have a thin simple columnar epithelial layer for secretion and absorption. The submucosa is a thick layer of loose connective tissue that surrounds the mucosa. This layer also contains blood vessels, lymphatic vessels, and nerves. Glands may be embedded in this layer. The smooth muscle responsible for movements of the digestive tract is arranged in two layers, an inner circular layer and an outer longitudinal layer. The myenteric plexus is between the two muscle layers. 1 Above the diaphragm, the outermost layer of the digestive tract is a connective tissue called adventitia. Below the diaphragm, it is called serosa Esophagus Throughout the remainder of the digestive system, the histological composition of the alimentary canal. The lumen is lined by an epithelium, which rests on a vascular connective tissue, the lamina propria. The lamina propria is in turn surrounded by a narrow band of smooth muscle (muscularis mucosae). These three tissues are collectively referred to as the mucosa of the alimentary canal. In the oesophagus the mucosa is formed by a stratified squamous epithelium (non- keratinised) and a well-defined lamina propria and muscularis mucosae. esophageal glands are located in the submucosa. These submucosal glands produce a mucous secretion, which lubricates the epithelium and aids the passage of food. In the part of the esophagus closest to the stomach there may be mucosal mucus- producing glands, which resemble the glands in the adjacent mucosa of the stomach. The muscularis externa is somewhat unusual in that it contains striated muscle in its upper one third, a mixture of striated muscle and smooth muscle in its middle one-third and smooth muscle in its lower one-third. The adventitia consists only of a layer of loose connective tissue. Only the lowest part of the oesophagus (approx. the lowest 2 cm) enters the peritoneal cavity. A serosa forms the outermost part of the adventitia of this short intraperitoneal segment of the oesophagus. 2 The Stomach The stomach functions both as a reservoir and as a digestive organ. It empties its contents in small portions (suitable for continued digestion) into the small intestine. Anatomically, the stomach is divided into a cardiac part, fundus, body or corpus, and a pyloric part (pyloric antrum and pyloric canal) 3 Histologically, most of the layers of the wall of the stomach appear similar in its different parts. Regional differences are mainly restricted to the appearance of the gastric mucosa. The Mucosa (epithelium, lamina propria, muscularis mucosa) The mucosa is thrown into longitudinal folds (gastric folds or rugae), which disappear when the stomach is fully distended. A network of shallow grooves divides the mucosa into gastric areas. On the mucosal surface we see small, funnel- shaped depressions (gastric pits). Almost the entire mucosa is occupied by simple, tubular gastric glands which open into the bottom of the gastric pits. Simple columnar epithelium: secrete bicarbonate-buffered mucus . Gastric pits opening into gastric glands . Mucus neck cells . Parietal cell secrete . HCL & Intrinsic factor (for B12 absorption) . Chief cells secrete Pepsinogen (activated to pepsin with HCL), & Stimulated by gastrin: a stomach hormone The lamina propria is formed by a very cell-rich loose connective tissue (fibroblasts, lymphocytes, plasma cells, macrophages, eosinophilic leukocytes and mast cells). The muscularis mucosae of the stomach contains both circular and longitudinal layers of muscle cells. Its organization is somewhat variable depending on the location in the stomach. Note that the muscularis externa consists of three layers of muscles: an inner oblique layer, a middle circular layer and an outer longitudinal layer. 4 Smal l Intes 5 tine The small intestine is divided into duodenum (25-30 cm), jejunum (about first two- fifths of the rest) and ileum. The three segments have the same basic histological organization. The Mucosa The mucosa of the small intestine has various structural features which considerably increase the luminal surface area and consequently support the main function of the small intestine - the absorption of the degraded components of the food. Plicae circulares (of Kerkering) are macroscopically visible, crescent-shaped folds of the mucosa and submucosa. Plicae circulares extend around one-half to two- thirds of the circumference of the lumen of the small intestine. The entire intestinal mucosa forms intestinal villi (about one mm long), which increase the surface area by a factor of ~ ten. The surface of the villi is formed by a simple columnar epithelium. Each absorptive cell or enterocyte of the epithelium forms numerous microvilli (1 µm long and about 0.1 µm wide). Microvilli increase the surface area by a factor of ~ 20. 6 Between the intestinal villi we see the openings of simple tubular glands, the crypts of Lieberkühn. They extend through the lamina propria down to the muscularis mucosae. Paneth cells which are located at the bottom of the crypts. They release a number of antibacterial substances, among them lysozyme, and are thought to be involved in the control of infections. One function of the crypts of Lieberkühn is the secretion of "intestinal juice" (about 2 liter/day), which in its composition closely resembles extracellular fluid and which is rapidly reabsorbed. The lamina propria is, similar to the lamina propria of the stomach, unusually cell rich. Lymphocytes often invade the epithelium or form solitary lymphoid nodules in the lamina propria. Lymph nodules may form longitudinal aggregations of 30-50 nodules in the lamina propria of the ileum. These large aggregations are called Peyer's patches. The muscularis mucosae has two layers and extends into the intestinal villi, where the smooth muscle cells form a longitudinal bundle in the centre of the villi. The submucosa contains glands only in the duodenum. Submucosal glands of the duodenum are also called Brunner's glands. Their secretion is mucous and slightly alkaline due to bicarbonate ions (pH 7-8). The amount of bicarbonate is however too low to neutralize the acidic contents of the duodenal lumen. Instead, the secretion of Brunner's glands protects the duodenal mucosa - similar to the mucus which protects the gastric mucosa. 7 Large Intestine The large intestine constitutes the terminal part of the digestive system. It is divided into three main sections: cecum including the appendix, colon, and rectum with the anal canal. The primary function of the large intestine is the reabsorption of water and inorganic salts. The only secretion of any importance is mucus, which acts as a lubricant during the transport of the intestinal contents. The surface of the mucosa is relatively smooth as there are no plicae circulares or intestinal villi. Crypts of Lieberkühn are present and usually longer and straighter than those of the small intestine. Goblet cells account for more of the epithelial cells than in the small intestine. There is only little lamina propria squeezed between the glands. The muscularis mucosae again forms two layers. Considerable amounts of fat may be found in the submucosa. The appearance of the muscularis externa is different from that of the small intestine. The inner circular layer of muscle forms the usual sheath around the large intestine, but the outer longitudinal muscle layer forms three flattened strands, the taenia coli. Only a thin layer of longitudinal muscle surrounds the inner circular muscle layer between the taenia coli . The adventitia forms small pouches (appendices epiploicae) filled with fatty tissue along the large intestine. 8 Specialized Sections of the Large Intestine The vermiform appendix is a small blind-ending diverticulum from the cecum. The most important features of the appendix is the thickening of its wall, which is mainly due to large accumulations of lymphoid tissue in the lamina propria and submucosa. Intestinal villi are usually absent. An extreme proliferation of lymphocytes (lymphoid hyperplasia) as a consequence of bacterial or viral stimulation may lead to the obstruction of the lumen of the appendix and thereby cause appendicitis, but this is only one of many possible causes. The anal canal is the 2.5-4 cm long terminal part of the digestive tract. The mucosa has a characteristic surface relief of 5-10 longitudinal folds, the anal columns. Each column contains a terminal branch of the superior rectal artery and vein. Small mucosal folds between the anal columns (anal valves) form the pectinate line. This line defines sections of the anal canal with different arterial and nerve supplies, different venous and lymphatic drainages and different embryological origins. Crypts disappear below the pectinate line and the epithelium changes from the tall, columnar type seen in other parts of the large intestine to a stratified squamous epithelium.
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