Digestive System Introduction and Histology PDF

Digestive system Introduction and histology Digestive system - Introduction Includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body Food is broken down, bit by bit, until the molecules are small enough to be...

Digestive system Introduction and histology Digestive system - Introduction Includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated Digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus Includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Salivary glands, liver, gallbladder and pancreas are major accessory organs that have a role in digestion. Tongue and teeth are accessory structures located in the mouth. Salivary glands Salivary glands produce saliva and they pour their secretions into the oral cavity through the ducts Covered by capsule (connective tissue) Divided into lobules Lobules are separated from one another by the connective tissue septa Salivary glands Lobules Capsule Lobules: Contain secreting units called acini, ducts which drain the acini Acinus is an oval or spherical structure which has a lumen in the center It is enclosed by a basement membrane Inside the basement membrane there is a layer of epithelial cells (acinar cells) which are usually columnar or pyramidal cells Myoepithelial cells lie between the epithelial cells and the basement membrane These cells squeeze out secretion Acini are drained by the intercalated ducts Depending upon the type of cells lining the acini, the acini are classified into 3 types Mucous acinus, serous acinus, and mixed acinus Mucous acinus: Oval shaped structure, has a large lumen Lined by columnar cells (acinar cells) Cells have flat nucleus situated on the basement membrane In the fresh condition they contain large secretory granules of mucinogen near the apex Secretory product has either been dissolved during the staining process or remains unstained Due to this, the cells are clear and transparent Small amounts of cytoplasm which remain between the granules gives the apical part of the cell a distinct "spongy" appearance. Serous acinus: Spherical structure and smaller than mucous acinus Has a small lumen, lined by columnar or pyramidal cells Cells contain a round nucleus which is placed near basal part In an H&E stain, the cytoplasm at the apical part of the cell appear pinkish The apical part contains secretory granules The digestive enzyme αamylase is secreted by the acinar cells Mixed acinus: These are the mucous acini They have a half moon (crescent) shaped cap of serous cells on them These caps of serous cells attach to mucus producing acini and empty their secretory product into interstices between the mucus-producing cell These caps of serous cells are called serous demilunes or crescents of Gianuzzi Ducts of salivary glands: Lobules contain intralobular ducts which are located in between the secretory acini Intralobular ducts are of 2 types – intercalated ducts and striated ducts Intercalated ducts drain the acini and lined by simple cuboidal epithelium Striated ducts formed by the intercalated ducts and lined by simple columnar epithelium Interlobular ducts - embedded in the connective tissue surrounding the lobules of the glands, lined by stratified cuboidal or columnar epithelium Serous salivary gland (parotid salivary gland -slide 54): The lobules of serous salivary glands contain serous acini Serous salivary gland (parotid gland) – Slide 54 Intralobular duct Blood vessel Serous acini Interlobular duct Interlobular septum Mucous (sublingual) salivary gland – slide 56 The lobules of mucous salivary glands contain mucous acini Blood vessel Mucous acinus Interlobular septum Intralobular duct Mucous salivary gland- magnified Mucous acinus Connective tissue Flattened nucleus Mixed salivary (submandibular) gland – slide 55 Interlobular Group of mucous septum acini Intralobular duct Lobules contain mainly serous acini Lobules also contain groups of mixed acini Mixed acini are mucous acini with caps of serous cells (serous Serous demilunes) acinus Mixed salivary (submandibular) gland- magnified Serous demilunes Group of serous acini Group of mucoucs acini Mixed salivary gland (submandibular) – slide 55 Histology of tongue Tongue is a muscular organ It has 2 surfaces – dorsal surface (dorsum) and ventral surface Tongue is covered by the mucous membrane (mucosa) Mucosa is made up of 2 layers Outer epithelium and inner lamina propria (corium) Epithelium - mostly of stratified squamous non keratinized type and it is keratinized over the papillae Lamina propria - connective tissue, blood vessels and serous and mucous glands Deeper part of the tongue contains bundles of skeletal muscle fibers that are oriented transversely, longitudinally and vertically Mucosa covering the dorsal surface of anterior part of the tongue is rough and it is thrown into numerous projections called papillae Papillae are the projections of mucous membrane, covered by the epithelium Central core of the papilla is made up of lamina propria or corium Types of papilla are – filiform papillae, fungiform papillae, circum-vallate papillae Filiform papilla: Small conical projections covered by horny keratinized cap All over the dorsum of anterior 2/3 of tongue No taste buds Tongue with filiform papillae Stratified squamous keratinized epithelium Blood vessel Lamina propria Cut sections of skeletal muscle Fungiform papilla: Have rounded top and narrow base Contain taste buds Present at the apex and along the lateral margins of anterior 2/3 of dorsum of tongue Tongue with fungiform papillae Stratified squamous epithelium Lamina propria Blood vessel Cut sections of skeletal muscle fibers Circum-vallate papillae: 8 to 12 in number Largest papilla, situated in a depression Surrounded by the circular (vallate) sulcus Have rounded top and narrow base Contain taste buds on the their lateral wall The suclus receives the opening ducts of serous glands (Von Ebner’s gland) Serous secretion of these glands distribute the substance to be tasted & wash out the debris in the sulcus Tongue with circumvallate papilla Stratified squamous nonkeratinized epithelium Circular sulcus Lamina propria Muscle fibers Blood vessel Taste buds Serous acini Circumvallate papilla showing taste buds Stratified squamous non-keratinized epithelium Taste buds Circular sulcus Lamina propria General plan of basic structure of alimentary canal The histology of alimentary canal consists of four layers -mucosa, sub mucosa, muscularis externa and serosa. These layers are similar throughout the length of digestive tract but there is regional modifications and specializations Layers of GIT (Eg.- esophagus) Lumen Epithelium Lamina propria Muscularis mucosa Submucosa Muscularis externa with Mucous acini inner circular layer & outer longitudinal layers of smooth muscle fibres Mucosa (mucous membrane): Has 3 parts Consists of epithelium, lamina propria and muscularis mucosa. Epithelium in the major part of GIT is of simple columnar type Lamina propria is made up of loose areolar connective tissue, has glands , lymph vessels and occasional lymph nodules Muscularis mucosa, composed of inner circular layer and outer longitudinal layer of smooth muscle Submucosa: Composed of irregular fibroelastic connective tissue with elastic and collagen fibres Has no glands except in the esophagus and the duodenum Also contains blood and lymph vessels and Meissner`s or submucosal plexus of nerves Muscularis externa Contains inner circular and outer longitudinal layers of smooth muscle fibres Auerbach`s or myenteric plexus of nerves are situated between the two layers of smooth muscle layers and regulates the activity of the muscularis externa. Serosa: Is the outermost layer formed by visceral peritoneum If the organ is retroperitoneal , serosa is replaced by connective coat called adventitia. Histolgy of esophagus: Wall of esophagus presents 4 layers From inside to outside they are Mucosa (mucous membrane ) Submucosa Muscularis externa Adventitia Mucous membrane (mucosa): Is the innermost layer Contains 3 parts Epithelium, lamina propria and muscularis mucosa Epithelium: Epithelium at most of the esophagus is of stratified squamous non keratinized type Lowest part of esophagus is lined by simple columnar type epithelium Esophagus –slide 57 Lumen Stratified squamous epithelium Lamina propria Muscularis mucosa Submucosa Muscularis externa Mucous acini Esophagus –slide 57 Lumen Stratified squamous epithelium Lamina propria Mucous acini Muscularis externa Adventitia Submucosa Muscularis mucosa Lamina propria: Made up of loose irregular connective tissue with blood vessels Muscularis mucosa: Thin layer made up of muscle fibres The muscle fibres are arranged in 2 layers Inner layer of circularly arranged muscle fibres and outer layer of longitudinally arranged muscle fibres Submucosa: Contains dense irregular connective tissue Also contains blood vessels and mucous glands (esophageal glands) Larger ducts of the glands extend to the lumen and lined by stratified columnar epithelium Glands secrete mucus, which aids in the passage of food Muscularis externa: Is made up thick layer of muscle fibres Muscle fibres are arranged to form 2 layers Inner layer of circularly arranged and outer layer of longitudinally arranged muscle fibers Muscularis externa in the upper part of esophagus is made of skeletal muscle fibres In the lower part it is made of smooth muscle fibres In the middle part it is made of both skeletal and smooth muscle fibres Adventitia: Is the outermost layer Is made up of loosely arranged connective tissue with blood vessels Adventitia is replaced by serosa in the abdominal part of esophagus Serosa is visceral layer of peritoneum, it is made up of simple squamous epithelium Stomach Stomach is the most dilated part of GI Tract Stomach has 4 parts – cardia, fundus, body, pyloric part Interior of empty stomach presents longitudinal folds of mucosa – gastric rugae HISTOLOGY OF CARDIA OF STOMACH Has 4 main layers: the inner mucosa, submucosa, muscularis externa, and outer serosa Mucosa made up of 3 layers (epithelium, lamina propria and muscularis mucosa) of the cardia has a simple columnar epithelium with many invaginations that form millions of gastric pits. The gastric pits will comprise about a quarter of the mucosa’s thickness. At the base of these pits, they join with multiple tubular cardiac glands that secrete mucus that protects the esophagus from gastric reflux and is also a part of the stomach’s gastric juice. The cardiac glands extend all the way to its underlying layer called the muscularis mucosa, which is a thin layer of smooth muscle that is still a part of the mucosal layer (inner circular and outer longtitudinal layers) The next main layer of the cardia is the submucosa, which consists mostly of dense irregular connective tissue, but also contains larger blood vessels. The entire stomach’s muscularis externa is unique when compared to the rest of the GI tract because it has 3 layers of smooth muscle instead of only 2 layers. Inner oblique layer on the right side of the muscularis propria, closest to the submucosa. The middle layer is the circular layer, and the outer layer is a longitudinal layer of smooth muscle. The outermost layer of the cardiac stomach is the serosa, which consists of loose connective tissue with large blood vessels, adipose tissue, lymphatic vessels, and nerves. Histology of Fundus of stomach 4 layers - mucosa, submucosa, muscularis externa, serosa Mucosa (mucous membrane): Mucosa form longitudinal folds – rugae (disappear when the stomach is distended) Mucosa is made up of 3 layers Epithelium, lamina propria and muscularis mucosa Epithelium: Is made up of simple columnar type Epithelium Lamina propria Muscularis mcuosa Submucosa Muscularis externa – circular layer of smooth muscle Muscularis externa Serosa – longitudinal layer of smooth muscle Surface epithelium Gastric pit Mucous neck cell Gastric gland Parietal cell Peptic cell Muscularis mucosa Submucosa Fundus of stomach – slide 59 Lumen Epithelium Muscularis mucosa Gastric pit (in lamina propria) Submucosa Gastric (fundic) glands (in lamina propria) Oblique layer Muscularis externa Circular layer Gastric (fundic) glands- magnified (59) Lumen Epithelium Gastric pit Mucous neck cells Parietal cells Chief cells Lamina propria: Loose irregular connective tissue with small blood vessels Contains gastric pits and gastric glands (fundic glands) Gastric pits: Occupy the inner one-third of lamina propria Receive the openings of 2 to 3 gastric glands Lined the by simple columnar epithelium Gastric glands: Elongated tubular glands, located in the deeper 2/3rd of lamina propria Open into the gastric pits Lined mainly by 3 types of cells Mucous neck cells: at the neck of the glands Parietal cells (oxyntic cells) : rounded cells, take acidic (eosin) stain, located mainly at the side walls of the gland, produce HCL Peptic cells (chief or zymogenic cells): take basic stain, located at the basal part of gland, produce gastric enzymes (pepsin) Neuroendocrine cells (secrete serotonin, gastrin) Stem cells – divide to replace all other types of cells Muscularis mucosa: Thin layer of smooth muscle fibers - arranged in 2 layers Inner layer of circularly arranged muscle fibers and outer layer of longitudinally arranged muscle fibers Submucosa: Contains dense irregular connective tissue, blood vessels Muscularis externa: Thick layer of smooth muscle fibers - arranged in 3 layers – Inner oblique layer, middle circular layer and outer longitudinal layer Serosa: Simple squamous epithelium (visceral layer of peritoneum) Pyloric part of stomach Wall is made up of 4 layers Mucosa, submucosa, muscularis externa, serosa Mucosa (mucous membrane): Mucosa contains 3 layers – epithelium, lamina propria and muscularis mucosa Epithelium: Is made up of simple columnar type Lamina propria: Loose irregular connective tissue with small blood vessels Gastric pits and gastric glands (pyloric glands) Gastric pits: Very deep, occupy the inner two-third of lamina propria Receive openings of pyloric glands Lined the by simple columnar epithelial cells Pyloric part of stomach (contd) Pyloric glands: Coiled tubular glands - deeper 1/3rd of lamina propria Open into the gastric pits Lined mainly by mucous secreting cells and few parietal cells, stem cells, neuroendocrine cells (G cells which secrete hormone gastrin) Muscularis mucosa: Thin layer of smooth muscle fibers Arranged in 2 layers – inner circular layer and outer longitudinal layer Submucosa: Dense irregular connective tissue, blood vessels Muscularis externa: Thick layer of smooth muscle fibers - inner layer of circularly arranged muscle fibers and outer layer of longitudinally arranged muscle fibers Serosa: Simple squamous epithelium (visceral layer of peritoneum) Gastric pit Pyloric glands Muscularis mucosa Pylorus of stomach –slide 60 Lumen Mucosal folds Pyloric glands Muscularis mucosa Submucosa Thick circular layer of muscularis externa Pylorus of stomach- magnified – slide 60 Lumen Mucosal fold Gastric pits (in lamina propria) Gastrtic (pyloric) glands (in lamina propria) Muscularis mucosa Submucosa Circular layer of muscularis externa Pyloric glands- magnified Gastric pit Simple columnar epithelium Pyloric glands ~2/3 of lamina propria Small intestine Has 3 parts – duodenum, jejunum and ileum Wall has 4 layers – mucosa, submucosa, muscularis externa and serosa Mucosa: forms tall projections called villi. Core of the villus is formed by connective tissue, blood vessels and lymph vessels of the lamina propria. Epithelium: Villi are covered by simple columnar epithelium (enterocytes) with some goblet cells in between. Small intestine Lamina propria: Contains connective tissue, blood vessels and lymph vessels and intestinal glands (crypts of Lieberkuhn) which open into the lumen between the villi. Crypts are mainly lined by simple columnar cells with microvilli (enterocytes) which are absorptive in function. Other cells lining the crypts include – goblet cells (produce mucin), paneth cells (defensive in function), neuroendocrine cells (produce locally acting hormones that regulate motility and secretion of GIT) and stem cells (replace other cells) Small Intestine Lamina propria of ileum contains solitary lymphoid follicles called Peyer’s patches. At some regions they break the muscularis mucosa and extend into the submucosa. Muscularis mucosa: It is made up of inner circular and outer longitudinal layer of smooth muscle fibers. Some of these fibers extend into the core of the villus. Submucosa: Contains connective tissue, blood vessels, lymph vessels and nerve fibers. It is characterized by presence of mucous glands (Brunner’s glands) in the duodenum Contains Peyer’s patches (aggregated lymphoid follicles) in the ileum Muscularis externa: Has outer longitudinal and inner circular layer of smooth muscle fibers. Serosa Single layer of squamous cells (visceral peritoneum) Duodenum – slide 61 Lumen Villus Intestinal glands Muscularis mucosa Brunner’s glands Muscularis externa Duodenum – slide 61 Villus Intestinal glands (crypts of Lieberkuhn) In lamina propria Brunner’s glands in submucosa Muscularis externa Muscularis mucosa Serosa Jejunum – slide 62 Lumen Villus Intestinal glands Muscularis mucosa Submucosa Ileum – slide 63 Lumen Villus Intestinal glands Muscularis mucosa Muscularis externa Peyer's patches Ileum – slide 63 Histology of Colon Mucosa: Epithelium: plenty of goblet cells and few simple columnar cells line the lumen and crypts of Liberkuhn. Lamina propria: Contains crypts of Lieberkuhn, connective tissue, blood vessels Muscularis mucosa: inner circular and outer longitudinal layer of smooth muscle fibers. Submucosa: Thin with connective tissue, blood vessels, lymph vessels and nerve fibers. Muscularis externa: Outer longitudinal and inner circular layer of smooth muscles. Serosa: Visceral peritoneum (single layer of squamous cells) Colon Epithelium (simple columnar with goblet cells) Intestinal glands in lamina propria Muscularis mucosa Submucosa Muscularis externa Serosa Colon Muscularis mucosa Colon Intestinal glands in lamina propria Muscularis mucosa Submucosa Colon- magnified Lumen Intestinal glands with goblet cells (in the lamina propria) Muscularis mucosa Submucosa Histology of Appendix Mucosa: Epithelium: Simple columnar epithelium with lots of goblet cells. Lamina propria: Contains aggregation of lymphocytes, which extend into the submucosa breaking muscularis mucosa. Also contains few intestinal glands Muscularis mucosa: Inner circular and outer longitudinal layer of smooth muscle fibers. Submucosa: It is thin with connective tissue, blood vessels, lymph vessels and nerve fibers. Muscularis externa: Has outer longitudinal and inner circular layer of smooth muscles. Serosa: It is made up of single layer of squamous cells (visceral peritoneum) Appendix Epithelium (simple columnar with goblet cells) Intestinal glands in lamina propria Lymphoid follicles in Lamina propria and submucosa Submucosa Muscularis externa Serosa Appendix – slide 64 Longitudinal layer Circular layer Lumen Muscularis externa Lymphoid aggregation Appendix Epithelium (simple columnar with goblet cells) Intestinal glands in lamina propria Muscularis mucosa Lymphoid follicles in Lamina propria and submucosa Submucosa Muscularis externa Serosa Appendix- magnified –slide 64 Lumen Intestinal glands Muscularis externa Serosa Muscularis mucosa Lymphoid aggregation Appendix- magnified Intestinal glands Muscularis mucosa Lymphoid aggregaton Muscularis externa Serosa