Digestive Tract PDF

### **DIGESTIVE TRACT** - Also called the [gastrointestinal (GI) tract or alimentary cana]l - a hollow tube with a lumen of variable diameter and a wall made up of 1. [Mucosa] 2. [Submucosa] 3. [Muscularis,] 4. [Serosa]  - the inner layer of the en...

### **DIGESTIVE TRACT** - Also called the [gastrointestinal (GI) tract or alimentary cana]l - a hollow tube with a lumen of variable diameter and a wall made up of 1. [Mucosa] 2. [Submucosa] 3. [Muscularis,] 4. [Serosa]  - the inner layer of the entire digestive tract forms an important protective barrier between the content of the tract's lumen and the internal milieu of the body's connective tissue and vasculature. - oral cavity - esophagus - stomach - small and large intestines - anus - Associated Glands - salivary glands - liver - pancreas #### **Four main layers** +-----------------------------------+-----------------------------------+ | **[Mucosa]** | - - - and **muscularis | | | mucosae** (separating mucosa | | \- called a mucous membrane. | from submucosa and allowing | | | local movements of the | | | mucosa) | +===================================+===================================+ | **Submucosa** | \- contains denser connective | | | tissue with larger blood and | | | lymph vessels and the | | | **submucosal (Meissner) plexu**s | | | of autonomic nerves. | +-----------------------------------+-----------------------------------+ | **thick muscularis (or** | - - | | | | | **muscularis externa)** | | +-----------------------------------+-----------------------------------+ | **Serosa** | - - - - - - the | | | serosa is replaced by a thick | | | adventitia (a connective | | | tissue layer that merges with | | | the surrounding tissues and | | | lacks mesothelium.) | +-----------------------------------+-----------------------------------+ #### **Small Intestine** - digestion proteins, complex carbohydrates, nucleic acids, and fats are broken down into their small molecule subunits that are easily absorbed. **Large Intestine** - water and electrolytes are absorbed. **STRUCTURES WITHIN THE DIGESTIVE TRACT (IMMSHCAE)** ==================================================== - Ingestion - Mastication - Motility - Secretion - Hormone release - Chemical Digestion - Absorption - Elimination **ORAL CAVITY** =============== - stratified squamous epithelium, which may be keratinized, partially keratinized, or nonkeratinized depending on the location. - Epithelial differentiation and keratinization are **similar to epidermis and dermis**. - Superficial cells undergo continuous **desquamation**. ***[The keratinized cell layers]*** - resist damage from abrasion - best developed in the **masticatory mucosa** on the gingiva (gum) and hard palate #### **[Lamina propria]** - rests directly on the periosteum of underlying bone #### **[Nonkeratinized squamous epithelium]** - predominates in the lining mucosa over the soft palate, cheeks, the floor of the mouth. #### **[Pharynx (or throat)]** - the posterior region of the oral cavity leading to the esophagus. #### **[Lining mucosa]** - overlies a thick submucosa containing many minor salivary glands, - secrete continuously to keep the mucosal surface wet, and diffuse lymphoid tissue ### ### **LIPS OR LABIA** - well-developed core of striated muscle - highly mobile for ingestion, speech, and other forms of communication - #### **Internal mucous surface** - lining mucosa with a thick, nonkeratinized epithelium and many minor labial salivary glands - #### **Red vermilion zone** - covered by very thin keratinized stratified squamous epithelium - transitional between the oral mucosa and skin. - lacks salivary or sweat glands and is kept moist with saliva from the tongue. - underlying connective tissue is very rich in both sensory innervation and capillaries, which impart the pink color to this region - #### **Outer surface** - thin skin - #### keratinized stratified squamous epithelium - consisting of epidermal and dermal layers, sweat glands, and many hair follicles with sebaceous glands. **TONGUE** ========== - mass of striated muscle covered by mucosa - manipulates ingested material during mastication and swallowing - muscle fibers are oriented in all directions, allowing a high level of mobility - penetrated by the lamina propria - **lower surface** of the tongue is smooth, with typical lining mucosa. - **dorsal surface** is irregular, having hundreds of small protruding papillae of various types on its anterior two-thirds and the massed lingual tonsils on the posterior third, or root of the tongue - **Sulcus terminalis** --------------------- - V-shaped groove c - papillary and tonsillar areas of the lingual surface are separated - **Lingual papillae** -------------------- - elevations of the mucous membrane that assume various forms and functions. - Has four types: - Filiform papillae - Fungiform papillae - Foliate papillae - Vallate (or circumvallate) papillae ### **FOUR TYPES** +-----------------------------------+-----------------------------------+ | **Filiform papillae** | - very numerous, | | | | | | - have an elongated conical | | | shape, | | | | | | - heavily keratinized, which | | | gives their surface a gray or | | | whitish appearance | | | | | | - provide a rough surface that | | | facilitates movement of food | | | during chewing | +===================================+===================================+ | **Fungiform papillae** | - much less numerous, | | | | | | - lightly keratinized, | | | | | | - interspersed among the | | | filiform papillae. | | | | | | - mushroom-shaped with well- | | | vascularized and innervated | | | cores of lamina propria | +-----------------------------------+-----------------------------------+ | **Foliate papillae** | - several parallel ridges on | | | each side of the tongue | | | | | | - anterior to the sulcus | | | terminalis, but | | | [a](https://www.studocu.com/p | | | h?utm_campaign=shared-document&ut | | | m_source=studocu-document&utm_med | | | ium=social_sharing&utm_content=hu | | | man-histology-digestive-system-le | | | cture-jungqueras-basic-histology) | | | re | | | rudimentary in humans, | | | | | | - older individuals | +-----------------------------------+-----------------------------------+ | **Vallate (or circumvallate) | - largest papillae with | | papillae** | diameters of 1-3 mm | +-----------------------------------+-----------------------------------+ - #### **Serous salivary (von Ebner) glands** - empty into the deep, moatlike groove surrounding each vallate papilla - provides a continuous flow of fluid over the taste buds that are abundant on the sides of these papillae, washing away food particles so that the taste buds can receive and process new gustatory stimuli. **TASTE BUDS** ============== - are ovoid structures within the stratified epithelium on the tongue's surface, - has 50-100 cells, about half of which are elongated gustatory (taste) cells - sample the general chemical composition of ingested material - **250 taste buds** are present on the lateral surface of each vallate papilla, with many others present on fungiform and foliate (but not the keratinized filiform) papillae - not restricted to papillae - also widely scattered elsewhere on the dorsal and lateral surfaces of the tongue, where they are also continuously flushed by numerous minor salivary glands. **[Detect at least five broad categories of tastes:]** **Salty** **Sodium ions** ------------ ------------------------------------------------- **Sour** **Hydrogen ions from acids** **Sweet** **Sugars and related compounds** **Bitter** **Alkaloids and certain toxins** **Umami** **Amino acids such as glutamate and aspartate** **TEETH** ========= - 32 permanent teeth, arranged in two bilaterally symmetric arches in the maxillary and mandibular bones. - Each quadrant has eight teeth: - two incisors - one canine - two premolars - three permanent molars **Primary teeth (deciduous or milk teeth)** ------------------------------------------- - 20 of the permanent teeth are preceded **Each tooth** -------------- - has a ***crown*** exposed above the [gingiva], a constricted neck at the gum, and one or more roots that fit firmly into bony sockets in the jaws called ***dental alveoli***. - ***Crown*** - covered by enamel, and the roots by cementum - ***Dentin*** - composes the bulk, surrounding the pulp cavity - ***Pulp*** - a vascular, innervated, loose connective tissue. Components: =========== - **Enamel**: Hardest body component, mostly hydroxyapatite. - **Dentin:** 70% hydroxyapatite, secreted by odontoblasts, sensitive to stimuli. - **Cementum:** Covers root dentin, resembles bone, avascular. - **Periodontal Ligament:** Collagen fibers, supports tooth, permits limited movement.) binding the cementum and the alveolar bone - **Pharyngitis** and **Tonsillitis** are caused by *Streptococcus pyogenes* - **White Excrescences** or **Leukoplakia** can be caused by Epstein-Barr virus. - **Oral Thrush** is caused by a yeast (*Candida albicans) infection.* **Periodontium** ================ - Maintains teeth in the bones. - Includes cementum, periodontal ligament, alveolar bone, gingiva. - Gingival sulcus and junctional epithelium around the tooth enamel. - **alveolar bone** lacks the typical lamellar pattern of adult bone but has osteoblasts and osteocytes engaging in continuous remodeling of the bony matrix. It is surrounded by the periodontal ligament, which serves as its periosteum. **THE ESOPHAGUS** ================= - Muscular tube, about 25-cm long in adults, which transports swallowed material from the pharynx to the stomach. - ***The four layers of the GI tract* f**irst become well-established and clearly seen in the esophagus. - ***The esophageal mucosa*** has nonkeratinized stratified squamous epithelium, its muscularis is striated at its superior end with smooth muscle at its inferior end, with mixed fiber types in the middle. - Most of the outer layer of the esophagus is adventitia, merging with other tissues of the mediastinum. - At the esophagogastric junction, stratified squamous epithelium changes abruptly to simple columnar epithelium invaginating into the lamina propria as many branched tubular glands. **ESOPHAGEAL GLANDS** ===================== - Small mucus-secreting glands in the submucosa. - Lubricates and protects the mucosa. **ESOPHAGEAL CARDIAC GLANDS** ============================= - A group of glands that secretes additional mucus. - They secrete neutral mucin that protects the esophagus from acidic gastric juices. **Swallowing** -------------- - begins with voluntary muscle action but finishes with involuntary peristalsis **THE STOMACH** =============== - greatly dilated segment of the digestive tract - main functions: - To continue the digestion of carbohydrates initiated by the amylase of saliva. - To add an acidic fluid to the ingested food and mixing its contents into a viscous mass called chyme by the churning activity of the muscularis. - To begin digestion of triglycerides by a secreted lipase. - To promote the initial digestion of proteins with the enzyme pepsin **Four major regions make up the stomach** +-----------------------------------+-----------------------------------+ | **CARDIA** | - narrow transitional zone, | | | 1.5-3 cm wide, between the | | | esophagus and the stomach | +===================================+===================================+ | **PYLORUS** | - the funnel-shaped region that | | | opens into the small | | | intestine. | +-----------------------------------+-----------------------------------+ | **FUNDUS AND BODY** | - identical in microscopic | | | structure and are the sites | | | of gastric glands releasing | | | acidic gastric juice | | | | | | - ***Rugae** -* The mucosa and | | | submucosa of the empty | | | stomach that have large, | | | longitudinally directed folds | | | which flatten when the | | | stomach fills with food | | | | | | - gastric glands themselves | | | fill most of the mucosa | +-----------------------------------+-----------------------------------+ **MUCOSA (STOMACH)** ==================== - Changing abruptly at the esophagogastric junction - simple columnar epithelium that invaginates deeply into the lamina propria - ***The surface mucous cells*** ------------------------------ - that line the lumen and gastric pits - secrete a thick, adherent, and highly viscous mucous layer that is rich in bicarbonate ions - protects the mucosa from both abrasive effects of intraluminal food and the corrosive effects of stomach acid. - ***Gastric Pits*** ------------------ - lead to long, branched, tubular glands that extend through the full thickness of the lamina propria - ***Stem cells*** ---------------- - the epithelium that lines the glands, pits, and stomach lumen are found in a narrow segment (isthmus) between each gastric pit and the gastric glands. - ***Pluripotent Stem cells*** ---------------------------- - divide asymmetrically, producing progenitor cells for all the other epithelial cells. - Some of these move upward to replace surface mucous cells, which have a turnover time of 4-7 days. - ***Vascularized Lamina Propia*** -------------------------------- - that surrounds and supports the gastric pits and glands contains smooth muscle fibers, lymphoid cells, capillaries, and lymphatics. - Separating the mucosa from the underlying submucosa is a layer of smooth muscle, the muscularis mucosae **Four major types** +-----------------------------------+-----------------------------------+ | **Mucous neck cells** | - present in clusters or as | | | single cells among the other | | | cells in the necks of gastric | | | glands and include many | | | progenitor and immature | | | surface mucous cell. | | | | | | - Less columnar than the | | | surface mucous cells lining | | | the gastric pits, mucous neck | | | cells are often distorted by | | | neighboring cells, but they | | | have round nuclei and apical | | | secretory granules. | | | | | | - less alkaline than that of | | | the surface epithelial mucous | | | cells | +===================================+===================================+ | **Parietal (Oxyntic) cells** | - produce hydrochloric acid | | | (HCl) | | | | | | - present among the mucous neck | | | cells and throughout deeper | | | parts of the gland | | | | | | - large cells, usually | | | appearing rounded or | | | pyramidal, each with one | | | (sometimes two) central round | | | nucleus. | | | | | | - secrete intrinsic factor, a | | | glycoprotein required for | | | uptake of vitamin B12 in the | | | small intestine | | | | | | - cytoplasm is intensely | | | eosinophilic due to the high | | | density of mitochondria | | | | | | - secretory activity is | | | stimulated both by | | | parasympathetic innervation | | | and by paracrine release of | | | histamine and the polypeptide | | | gastrin from enteroendocrine | | | cells | | | | | | - Form intracellular | | | canaliculus with a large | | | surface area produced by | | | | | | - thousands of microvilli | +-----------------------------------+-----------------------------------+ | **Chief (zymogenic) cells** | - predominate in the lower | | | regions of the gastric glands | | | | | | - have all the characteristics | | | of active protein-secreting | | | cells. | | | | | | Ultrastructurally chief cells | | | show abundant RER and numerous | | | apical secretory granules | | | | | | - granules contain inactive | | | enzyme pepsinogens, | | | precursors which are | | | converted in the acid | | | environment of the stomach | | | into active pepsins | | | | | | - Pepsins are endoproteinases | | | with broad specificity and | | | maximal activity at a pH | | | between 1.8 and 3.5 | | | | | | - produce gastric lipase | | | (digests many lipids.) | +-----------------------------------+-----------------------------------+ | **Enteroendocrine cells** | - scattered epithelial cells in | | | the gastric mucosa with | | | endocrine or paracrine | | | functions | | | | | | - release peptide hormones to | | | regulate activities of | | | neighboring tissues during | | | food digestion | +-----------------------------------+-----------------------------------+ **SMALL INTESTINES** ==================== - site where the digestive processes are completed and where the nutrients (products of digestion) are absorbed by cells of the epithelial lining. - covered by a thin serosa with mesothelium continuous with that of mesenteries **1. Duodenum** ----------------- -- **2. Jejunum** **3. Ileum** - - lining of the small intestine shows a series of permanent circular or semilunar folds (plicae circulares) - - covers the entire mucosa. - it is covered by a simple columnar epithelium. - its core has loose connective tissue that extends from the lamina propria. - Between the villi are the openings of short tubular glands called intestinal glands or crypt ***[\*\*\*Celiac disease (celiac sprue)-]*** is a disorder of the small intestine mucosa that causes malabsorption and can lead to damage or destruction of the villi. +-----------------------------------+-----------------------------------+ | ##### **DIFFERENTIATING CELLS AND | | | PLURIPOTENT STEM CELLS** | | +===================================+===================================+ | **Enterocytes** | - The absorptive cells | +-----------------------------------+-----------------------------------+ | **Goblet cells** | - They secrete glycoprotein | | | mucins. | | | | | | - main function is to protect | | | and lubricate the lining of | | | the intestine. | +-----------------------------------+-----------------------------------+ | **Paneth cells** | - granules release lysozyme, | | | phospholipase A2, and | | | hydrophobic peptides called | | | defensins (bind and break | | | down membranes of | | | microorganisms and bacterial | | | cell walls) | | | | | | - located in the basal portion | | | of the intestinal crypts | | | below the stem cells, are | | | exocrine cells with large, | | | eosinophilic secretory | | | granules in their apical | | | cytoplasm. | +-----------------------------------+-----------------------------------+ | **Enteroendocrine cells** | - secrete various peptide | | | hormones. | | | | | | | | | | | | - e "open" type, in which the | | | constricted apical end of the | | | cell contacts the intestinal | | | lumen | +-----------------------------------+-----------------------------------+ | **M (microfold) cells** | - are specialized epithelial | | | cells in the mucosa of the | | | ileum overlying the lymphoid | | | follicles of Peyer patches. | | | | | | - Selectively endocytose | | | antigens and transport them | | | to the underlying lymphocytes | | | and dendritic | +-----------------------------------+-----------------------------------+ - ***Mucosal lamina propria*** ---------------------------- - extensive blood and lymph microvasculature, nerve fibers, smooth muscle cells, and diffuse lymphoid tissue. - penetrates the core of each intestinal villus, bringing with it microvasculature, lymphatics, and nerves - ***Muscularis mucosae*** ------------------------ - produce rhythmic movements of the villi that increase the absorption efficiency. - Its fibers produce local movements of plicae circulares that help propel lymph from the lacteals into submucosal and mesenteric lymphatics. - ***Submucosa*** --------------- - larger blood and lymph vessels and the diffuse, interconnected neurons of the submucosal (Meissner) nerve plexus. - ***Duodenal (or Brunner) glands*** ---------------------------------- - small excretory ducts opening among the intestinal crypts - Mucus from these glands is distinctly alkaline - neutralizes chyme entering the duodenum from the pylorus, protecting the mucous membrane - bringing the intestinal contents to the optimum pH for pancreatic enzyme action - ***Mucosa-Associated Lymphoid Tissue (MALT)*** - In the ileum both the lamina propria and submucosa - consisting of the large lymphoid nodule aggregates known as Peyer patches underlying the epithelial M cells. - ##### ***Neurons of both the submucosal and myenteric plexuses*** - largely autonomic and collectively make up the large enteric nervous system **LARGE INTESTINE** =================== - bowel - 6-7 cm - absorbs water and electrolytes and forms indigestible material into feces - portion of the digestive tract extending from the ileocecal junction to the anus **FUNCTIONS:** ============== - Feces formation - Mass movements - Defecation reflex +-----------------------------------+-----------------------------------+ | **CECUM** | | +===================================+===================================+ | **COLON** | - Its wall is puckered into | | | haustra (series of large | | | sacs) | | | | | | - contains numerous straight, | | | tubular glands called crypts | | | | | | - longitudinal smooth muscle | | | layer of the colon [does | | | not] completely | | | envelop the intestinal wall | | | but forms three bands called | | | teniae coli (produce the | | | haustra in the colon wall) | | | | | | - display omental appendages | | | (series of suspended masses | | | of adipose tissue) | | | | | | FOUR PARTS: | | | | | | - ***Ascending colon*** | | | | | | - ***Transverse colon*** | | | | | | - ***Descending colon*** | | | | | | - ***Sigmoid colon*** | +-----------------------------------+-----------------------------------+ **Regions:** +-----------------------------------+-----------------------------------+ | **RECTUM** | - - - - | +===================================+===================================+ | **ANAL CANAL** | - - - - | +-----------------------------------+-----------------------------------+ - has only shallow plicae and no villi. - occupied mostly by tubular intestinal glands extending as deep as the muscularis mucosae and by lamina propria rich in MALT. **ORGANS ASSOCIATED WITH THE DIGESTIVE TRACT** - **Major salivary glands** - **Pancreas** - **Liver** - **Gallbladder** - **facilitate transport and digestion of food within the gastrointestinal tract.** +-----------------------------------+-----------------------------------+ | **Pancreas** | - - | +===================================+===================================+ | **Liver** | - - - | +-----------------------------------+-----------------------------------+ **SALIVARY GLANDS** =================== - **moisten and lubricate ingested food and the oral mucosa** - **initiate the digestion of carbohydrates and lipids with amylase and lipase** - **secrete innate immune components such as lysozyme and lactoferrin.** - have secretory units of either protein-secreting - - normal pH of 6.5-6.9 - secrete about 10% of the total saliva volume. - surrounded by connective tissue capsule - secretion of each gland is either serous, seromucous, or mucous, depending on its content of the glycoprotein mucin. - minor glands is mostly mucous. - ***Saliva*** ------------ - modified by the cells of the duct system draining the secretory units, with much Na+ and Cl− reabsorbed - from the parotids is serous and watery. - #### **The submandibular and sublingual glands** - produce e a seromucous secretion - ***Exocrine glands*** --------------------- - in the mouth produce saliva, which has digestive, lubricating, and protective functions. - ##### Three Bilateral Pairs 1. Parotid Gland 2. Submandibular Gland 3. Sublingual Gland - produce about 90% of saliva - produce 0.75-1.50 L of saliva daily +-----------------------------------+-----------------------------------+ | **SEROUS CELLS** | - - | +===================================+===================================+ | **MUCOUS CELLS** | - - | +-----------------------------------+-----------------------------------+ | **MYOEPITHELIAL CELLS** | - - | +-----------------------------------+-----------------------------------+ **PANCREAS** ============ - mixed exocrine-endocrine gland that produces both digestive enzymes and hormones - elongated retroperitoneal organ, with a large head near the duodenum and more narrow body and tail regions that extend to the left - has a thin capsule of connective tissue, from which septa extend to cover the larger vessels and ducts and to separate the parenchyma into lobules - lacks striated ducts and the parotid glands lack islets of endocrine tissue pancreatic islets (islets of Langerhans) **LIVER** ========= - 75% of the blood entering the liver is nutrient rich (but O2 poor) blood - 25% comes from the hepatic artery and supplies the organ's O2. ##### **HEPATOCYTES** - the key cells; most functionally diverse cells of the body - Functions: - tough, durable form of supporting connective tissue, - lack nerves - characterized by an extracellular matrix (ECM) with high concentrations of GAGs and proteoglycans, interacting with collagen and elastic fibers. - forms the framework supporting softer tissues. (In the respiratory tract, ears, and nose) - provides cushioning and sliding regions within skeletal joints (Because of its resiliency and smooth, lubricated surface) - facilitates bone movements. - All types of cartilage lack vascular supplies and chondrocytes receive nutrients by diffusion from capillaries in surrounding connective tissue (the perichondrium)