Lesson 20 - Digestive System (II) (notes)_PARTE2.docx

INTESTINE It is the longest portion of the digestive tract. It transports the food components to the anus, but in this way the chemical and mechanical digestion of food continues and the absorption of the products of this degradation takes place. Small intestine The small intestine has...

INTESTINE It is the longest portion of the digestive tract. It transports the food components to the anus, but in this way the chemical and mechanical digestion of food continues and the absorption of the products of this degradation takes place. Small intestine The small intestine has three parts, the duodenum, jejunum, and ileum. Its functions are digestion and absorption of food. Digestion is carried out by pancreatic juice, bile, and by the small intestine's own secretions, which contain enzymes and mucus to protect epithelial cells from mechanical damage and irritants. Intestinal enzymes come from the mucosal and submucosal glands as well as from the glycocalyx of the microvilli of enterocytes. The absorption is carried out by the enterocytes. To increase the absorption surface, the small intestine has the following specializations (Figure 4): Folds of the mucosa, which are circularly arranged. In ruminants these folds are permanent but in the rest of domestic animals these folds disappear when the small intestine is distended. Villi, which are finger-like projections of the mucosa; they vary in length depending on the region of the small intestine in which we are and the species. Microvilli, which are specializations of the plasma membrane located at the apical border of cells of the simple columnar lining epithelium (enterocytes). The structure of the small intestine is like that of any tubular organ, and consists of a mucosa, a submucosa, a muscularis and a serosa. Mucosa The mucosa of the small intestine is constituted by the lining epithelium, the lamina propria and the muscularis mucosae. Lining epithelium: It is a simple columnar epithelium of absorbent cells (enterocytes) between which goblet cells are interspersed. There are numerous occluding junctions that are located on the lateral surface of the epithelial cells, near the intestinal lumen, preventing the content from diffusing towards the lamina propria. Columnar absorbent cells have an ovoid nucleus in the basal portion and numerous microvilli in the apical border that are called "brush border". Enterocytes have a well-developed smooth endoplasmic reticulum (Figure 5) to synthesize triglycerides from fatty acids that are absorbed, which are then coated by glycoproteins in the Golgi complex to form chylomicrons that pass into the circulation. Amino acids and monosaccharides are absorbed by active transport, and immunoglobulins (colostrum) by endocytosis. Figura 5.- Microfotografías electrónicas de enterocitos. Among the enterocytes of the lining epithelium there are goblet cells, which synthesize mucinogen, a precursor to mucus that accumulates in the apical portion of the cell, distending it, and displacing the nucleus and organoids to the basal position. The mucus stains very little with the haematoxylin-eosin technique but is strongly PAS+. Mucus forms a film on the intestinal surface (glycocalyx) protecting it from enzymes and facilitating the digestion and absorption of substances. M cells are cells located in the parts of the epithelium where the lymphoid follicles of the lamina propria are prominent (Figure 6) and their function is to capture antigens from the intestinal lumen so that they are later processed and presented by other cells to the lymphocytes of the lymphoid follicles. They are wider cells than enterocytes with thicker microvilli. Lamina propria: It is constituted by richly vascularized and innervated loose connective tissue. It contains abundant free cells of defensive function, mainly plasma cells, lymphocytes, mast cells and eosinophils. There is also diffuse lymphoid tissue or lymphoid follicles throughout the small intestine that increase distally (Figure 6). In addition, in the lamina propria of the villi there is a lymphatic capillary called the “central chyliferous vessel”. This vessel has a blind end at the tip of the villi and is the origin of the lymphatic vessels that form the lymphatic plexuses of the lamina propria. The intestinal glands, also called crypts of Lieberkühn or mucosal glands, are found in the lamina propria of the mucosa and are simple unbranched tubular glands that begin in the depressions between the base of the villi (Figure 6). These glands are lined by different types of cells. At the base of the crypts there are cuboidal (regenerative) cells that are undifferentiated and have great mitotic capacity. These cells multiply, differentiate and migrate to the villi, where they give rise to absorbent columnar cells and goblet cells. The cells that make up the villi are pushed towards the apex of the villi by successive waves of new cells that are forming in the crypts. When they reach the apex of the villi, these cells die and shed into the intestinal lumen, a physiological death process (apoptosis) that occurs every 2 or 3 days. Thus, from the crypts of Lieberkühn, the intestinal epithelium is renewed. Eosinophilic granular cells or Paneth cells are located at the base of the crypts and are present in horses, ruminants and pigs. They are pyramidal in shape, with spherical and acidophilic granules located between the nucleus and the cell apex. They produce peptidase, lysozyme, and other antimicrobial components. The endocrine cells of the APUD system or enteroendocrine cells secrete hormones such as secretin, serotonin, gastrin, glucagon, somatostatin, motilin, etc., and their function is, together with the APUD cells of the stomach and pancreas, to regulate the peristaltic movements of the gastrointestinal tract and its secretions. Muscularis mucosae: It is made up of two layers of smooth muscle, an internal one, which is circularly arranged, and the other external, which is longitudinally arranged. Submucosa It is made up of loose connective tissue that can have special structures: Glands of the submucosa or Brünner's glands (Figure 6), which are compound tubuloalveolar glands that open at the base of the crypts of Lieberkünh, and produce a mucous secretion in dogs and ruminants, serous in the pig and horse and seromucous in cats. In most species they are only in the duodenum but in the horse they reach more distal portions. Lymphoid follicles (Figure 6), which are located in the three segments of the small intestine, although they are considered more characteristic of the ileum, where they form Peyer's patches. The largest lymphoid follicles are those of cattle, although the most numerous are those of horses. Muscularis It consists of an internal circular layer and an external longitudinal layer of smooth muscle (Figure 6). In the connective tissue that exists between the two layers of muscle fibres, a nervous plexus called Auerbach's myenteric plexus is observed, sympathetic in nervous nature and innervating the outer longitudinal layer of the muscularis, which when contracted is responsible for peristaltic movements of the intestine. Serosa It is made up of loose connective tissue lined by a mesothelium, which is a simple squamous epithelium. II. 2. Large intestine It is made up of the caecum, colon and rectum. Along the large intestine the (i) microbial action on the intestinal content, (ii) the absorption of water, electrolytes and vitamins as well as (iii) the secretion of mucus take place. In horses and pigs, fatty acids produced by the microbial digestion of cellulose are also absorbed. The three sections of the large intestine (caecum, colon and rectum) present a similar histological architecture: mucosa, submucosa, muscularis and serosa (Figure 7). The most significant differences with the small intestine are in the mucosa and in the submucosa: In the mucosa of the large intestine there are no villi, although the lining epithelium of the small and large intestine is of the same type, simple columnar, with absorbent, goblet and endocrine cells. The crypts of Lieberkühn in the mucosa of the large intestine are longer, straighter and with more goblet cells than in the small intestine but lack Paneth cells. There are no circular folds of the mucosa in the large intestine, although there are longitudinal folds. In the submucosa there are a higher number of lymphoid follicles than in the small intestine and, on the contrary, there are no glands. In pigs and horses, the outer longitudinal layer of the muscularis of caecum and colon form large flat bands, macroscopically visible, and made up of smooth muscle fibres and a large number of elastic fibres. These bands are called taenia. Regulation of intestinal activity Both motility and intestinal secretion are regulated by the autonomic nervous system (the parasympathetic stimulates it and the sympathetic decreases it), and by hormones produced by the endocrine cells of the APUD system. II. 3. Anum It is the terminal segment of the digestive tract. Simple columnar epithelium abruptly changes to non-keratinized stratified squamous epithelium. At the recto-anal junction there is a smooth muscle sphincter (internal anal sphincter), while the external anal sphincter is made up of skeletal muscles. In some species there are associated glands: Anal glands: They are modified tubuloalveolar sweat glands, present in carnivores and pigs. Circumanal glands: Present in dogs and occasionally in cats. Anal sac: They are bilateral sacs present in carnivores. II. 4. Intestine of birds The small intestine of birds is similar to that of mammals, but the villi are very long and often folded. The large intestine is also similar. It also has a greater number of lymphoid follicles in the submucosa and lamina propria. The cloaca is the terminal compartment of the digestive (coprodeum), urinary and reproductive (urodeum) tracts and a dorsal compartment for the bursa of Fabricius (proctodeum), all of them separated by folds of the mucosa, of simple columnar epithelium. PERITONEUM It is a highly vascularised loose connective tissue lined by a mesothelium that lines all surfaces of the abdominal cavity (parietal and visceral).