Lesson 20 - Digestive System (II) (notes)_PARTE1.docx

Lesson 20 DIGESTIVE SYSTEM (II) STOMACH The fragmentation of food is completed in the stomach and the digestion begins. The product obtained is called chyme. The stomach is a tubular organ located between the oesophagus and the duodenum that performs mechanical (in the non-glandular portion)...

Lesson 20 DIGESTIVE SYSTEM (II) STOMACH The fragmentation of food is completed in the stomach and the digestion begins. The product obtained is called chyme. The stomach is a tubular organ located between the oesophagus and the duodenum that performs mechanical (in the non-glandular portion) and enzymatic (in the glandular portion) digestion of food. The development of these two portions of the stomach, mechanical and glandular, presents anatomical and morphological variations in the different species of domestic animals related to the type of feeding. Thus, ruminants have a great development of the non-glandular stomach, which is organized into three different compartments, the rumen (“panza” in Spanish), the reticulum (“redecilla” in Spanish) and the omasum (“librillo” in Spanish), while the glandular stomach is the fourth compartment, and it is called abomasum (“cuajar” in Spanish). The horse and pig have a single compartment or diverticulum, but with a non-glandular portion of keratinized flat stratified epithelium, located around the cardia, which is wide in the horse and small in the pig, and another glandular portion of simple columnar epithelium. Carnivores have only one compartment or diverticulum, exclusively glandular. Stomach compartments of ruminants They are also called gastric or stomach diverticula and are the rumen, the reticulum and the omasum, which in this order are available between the oesophagus and the glandular stomach or abomasum. The first three have a typical histological constitution of a tubular organ, with a keratinized stratified squamous epithelium that protects the mucosa from fibrous foods eaten by ruminants and also plays an important role in the absorption of nutrients. Rumen It is the largest of the three compartments. Its mucosa has numerous papillae of two types: 1) long and thin papillae, with the shape of a padle racket, and approximately 1.5 cm length in bovids (although both the length of the folds and the thickness of the epithelium are greater the more fibrous the food is); and 2) short papillae (Figure 1). In the rumen there is no muscularis mucosae, so within the papillae there is only loose connective tissue (lamina propria and submucosa). The muscularis is made up of two layers, an internal circular and an external longitudinal smooth muscle, externally surrounded by the peritoneal serosa. Reticulum It is located between the rumen and the omasum. The mucosa has primary folds that are connected to each other to form a reticulum like a honeycomb. Each "honeycomb" is called a bonnet. There are also numerous secondary folds smaller in size and randomly scattered throughout the mucosa (Figure 1). The epithelium is keratinized stratified squamous, and the lamina propria and submucosa are made up of loose vascularized connective tissue. Only at the apical end of the primary folds is there smooth muscle corresponding to the muscularis mucosae. The long primary folds have conical papillae with keratinized tips. The muscularis and the serosa are similar to those of the rumen. Omasum It is located after the reticulum and presents very long and thin laminar primary folds that are parallel to each other, occupying most of the lumen of the organ (Figure 1). Numerous small papillae emerge from them. The mucosal epithelium is similar to that of the rumen and reticulum. In the primary folds there is a highly developed muscularis mucosae, as well as muscle fibres from the internal muscular tunic, which run the entire length of the folds or long papillae. The submucosa is similar to that of the reticulum, but poorly developed. The muscularis and serosa layers are the same as in the previous organs. A B C Figure 1.- Photomicrographs of the histological constitution of the rumen (A), with long papillae and short papillae; the reticulum (B), with conical papillary ridges on a primary fold; and the omasum (C), with folds with conical papillae. HE, 4x. Function of stomach compartments in ruminants The rumen, the reticulum and the omasum fulfil several functions aimed at digesting food: Mechanical action: This action is carried out through the papillae and folds of the mucosa and through the motility of the strong muscular layer, particularly the rumen. The stimulation caused by a high amount of coarse fibre close to the cardia causes an additional anti-peristaltic movement by means of which the animal regurgitates the ruminal content to the palate and promotes rechewing or rumination (the food returns to the mouth to be suitably ripped and salivated) and eructation or belching (the gas produced during food fermentation is expelled). With this mechanical action it is possible to mix and crumble fibrous foods, and it is especially important in the reticulum and, above all, in the omasum, which with its sheets reduces the intake to porridge so that it can pass to the abomasum. Chemical digestion: The reticulum and especially the rumen act as a fermentation chamber in which various microorganisms (protozoa, bacteria and fungi) ferment the food that they can use (the cellulose in the fibre) for energy. The degradation of cellulose produces volatile fatty acids. The proteins of the ruminal microorganisms themselves are digested in the abomasum to amino acids and peptides and then absorbed in the small intestine. Absorption: Volatile fatty acids are absorbed in the rumen. The most abundant are acetic acid, propionic acid and butyric acid, which cover 70% of the energy needs of ruminants. Sodium, potassium, ammonia, urea and water ions are also absorbed, the latter especially in omasum, where the intake has a dry appearance, while in the rumen and reticulum it presents a fluid appearance for fermentation to take place. Oesophageal groove It is a functional fold that communicates the oesophagus with the omasum and the abomasum only during the first weeks of the ruminant's life. In this way, liquids that mainly ingest breast milk can evade the bacterial action of the ruminal compartment and the movements of the reticulum. This groove is formed by a reflex related to the action of suckling and disappears after weaning. The oesophageal groove is made up of two folds that contain a muscularis mucosae and a muscular layer that is a continuation of the muscularis of the oesophagus. When the muscle layers contract, they close the groove forming a tube. When the tube formation mechanism is inhibited, such as when feeding calves with cold milk, the milk falls into the rumen where it ferments, producing ruminal acidosis that endangers the life of the animal. Glandular stomach The glandular stomach is responsible for the enzymatic digestion of food through the enzymes produced by the gastric glands. After this digestion process, the resulting product (chyme) is propelled into the duodenum through the pylorus. This organ presents a typical tubular organ histological architecture. When empty, the mucosa presents longitudinal folds that disappear when the organ is distended, allowing it to increase several times in size, especially in carnivores (Figure 2). The mucosa has a simple mucosecretory columnar lining epithelium, that is, a lining epithelium that also secretes mucus in order to protect itself from self-digestion produced by the hydrolytic enzymes and hydrochloric acid produced by the gastric glands. The epithelium presents a series of invaginations called foveolae or gastric pits that increase the surface of the gastric mucosa and into which 5 to 7 of the glands of the lamina propria flow (Figure 2). The lamina propria of the mucosa is made up of loose connective tissue rich in plasma cells, lymphocytes, mast cells, fibroblasts, and occasional smooth muscle fibres. But its most important characteristic is the presence of numerous branched tubular glands (Figure 3). The gastric mucosa has three areas that differ from each other in the cellular composition of the glands of the lamina propria: Cardial region: It is located around the cardia, usually has little extension and the glands present little development containing few parietal and chief cells. Fundic region and body: It is the most extensive and in which the gastric glands present the greatest development. Pyloric region: It is restricted to areas close to the pylorus and has underdeveloped gastric glands in which mucous cells predominate. The gastric glands of the lamina propria of the mucosa are simple branched tubular glands and have three parts: neck, body and base. The lumen is narrow and the cell types are as follows: Mucous cells of the neck They can constitute the main area of the gland in the cardia and in the pylorus. They have cuboidal morphology, and like the mucous cells of the lining epithelium, they have a very pale cytoplasm due to the large amount of mucopolysaccharides that they synthesize and secrete. For this reason, they are PAS+. With electron microscopy, they present a nucleus in a basal position, around which there is abundant rough endoplasmic reticulum, a supranuclear Golgi complex, and many polyribosomes, while in an apical position, numerous large, electron dense and homogeneous secretion granules are observed, which contain mucopolysaccharides that secrete through exocytosis to the glandular lumen (Figure 3). The function of these cells is, like that of the superficial mucous cells, to produce mucus to protect the mucosa from the action of hydrolytic enzymes and HCl. They also replace superficial mucous cells, which have a short half-life. Parietal or oxyntic cells They are located mainly in the neck and the body of the gastric glands at the level of the fundus and the body of the stomach. They have a polyhedral morphology, a large size, a basophilic central nucleus, and a broad and intensely acidophilic (oxyntic) cytoplasm. Electron microscopy reveals numerous invaginations of the plasma membrane that give rise to intracytoplasmic canaliculi in which there are many microvilli. In the cytoplasm adjacent to these canaliculi there are a large number of mitochondria that give these cells acidophilic staining (Figure 3). The function of parietal cells is to synthesize HCl, a process they carry out using carbonic acid (H2CO3) from which they produce H+ ions that are secreted by active transport to the lumen of the canaliculi, and HCO3-. Chief or peptic or zymogenic cells They are located mainly in the body and base of the gastric glands at the level of the fundus and the body of the stomach. They are smaller than the parietal cells and have strongly basophilic cytoplasm, a reflection of the large number of ribosomes they contain. They are recognized by their condensed nuclei from baseline. With electron microscopy they show typical morphology of exocrine secreting cells, nucleus in basal position around which they have abundant rough endoplasmic reticulum and supranuclear Golgi complex (Figure 3). At the apical pole of the cytoplasm, they have numerous electron dense secretion granules of different size (zymogen granules) that contain the precursors of gastric enzymes, especially pepsinogen, which are secreted by exocytosis to the lumen, where in the presence of acidic pH it is transformed into pepsin, which is the active form of the enzyme. Other enzymes produced by major cells are gastric lipase and renin. Endocrine cells These cells are also called argentaffin cells because they stain with silver salts, or cells of the APUD system because they capture and transform amines. They are located at the base of the gastric glands in contact with the basement membrane. With the haematoxylin and eosin technique they do not differentiate from the chief cells. With electron microscopy they show typical morphology of endocrine secreting cells, presenting a pyramidal morphology. At the basal border they contain secretory granules of variable size and electron density, through which they secrete different hormones, such as serotonin, which cross the basement membrane and pass into the lumen of the capillaries present in the lamina propria. Numerous types of enteroendocrine cells have been described that produce different hormones and active substances, among the most important are G cells, which produce gastrin, a hormone that activates the synthesis and secretion of gastric juice and gastric motility. Other substances produced by the enteroendocrine cells of the stomach are cholecystokinin, which stimulates the secretion of pancreatic juice, intestinal vasoactive peptide, somatostatin, motilin, neurotensin, enteroglucagon, etc. All these substances, together with those produced by the enteroendocrine cells of the intestine and pancreas, regulate the secretion and motility of the entire gastrointestinal tract. Beneath the gastric glands of the lamina propria of the gastric mucosa there is a well-developed muscularis mucosae. In carnivores, there is also a thick layer of collagen fibres called the compact stratum, whose function is to prevent the stomach from being punctured by sharp objects that they may ingest, such as bones. The submucosa is made up of loose vascularized connective tissue in which there are usually numerous leukocytes, sometimes forming isolated lymphoid follicles. The muscularis is made up of three layers of smooth muscle: the internal, oblique, the middle, circular, and the external, longitudinal, between which the myenteric plexuses (groups of neurons of the autonomic nervous system that innervate the stomach) are arranged. Externally, the muscular tunic is enveloped by the peritoneal serosa. Regulation of gastric motility and secretion Gastric motility and secretion are regulated by both the autonomic nervous system (the sympathetic system slows them down and the parasympathetic stimulates them) and hormones. Nerve regulation is called the cephalic phase, and it is induced by visual or olfactory stimuli conveyed by the vagus nerve. There is also hormonal regulation, through hormones produced by the endocrine cells of the stomach, intestine and pancreas. Thus, gastrin stimulates gastric secretion, while when the chyme reaches the duodenum, other hormones are produced such as gastric inhibitory peptide and intestinal vasoactive peptide that inhibit gastric motility, or cholecystokinin, which inhibits gastric secretion. Stomach of birds The stomach of birds is made up of two diverticula, the proventriculus or glandular stomach, and the ventriculus or gizzard, which is the mechanical stomach. Unlike other species, in birds the food first passes through the glandular stomach. Proventriculus This compartment presents a mucosa lined by a simple cuboidal epithelium with numerous folds or papillae at the base of which the mucosal glands (simple branching glands with cubic epithelium and producing mucus) flow. There are also highly developed submucosal glands that secrete gastric juice to a central duct of columnar or pseudostratified epithelium. In these glands there is only one cell type that produces both acidic products and hydrolytic enzymes. The muscularis and serosa layers are similar to those of the stomach of mammals. Ventriculus (gizzard) (“molleja” in Spanish) The mucosa is lined by a thick horny layer under which there is a simple columnar epithelium, and under this are the glands that synthesize and secrete a special keratin that forms the corneal substance. The lamina propria and the submucosa are made up of loose connective tissue. The muscularis is highly developed and is made up of thick bands of dense connective tissue interconnected by highly developed layers of smooth muscle. The serosa is typical.