BAU Histology of the Upper GI Tract, 2024 PDF

Histology of the Upper Digestive Tract (Upper Gastrointestinal System) Dr.Emine Özçınar BAU Med. School-2024 Gastrointestinal System The alimentary canal; -Mouth (oral cavity) -Pharynx -Esophagus -Stomach -Small intestine -Large intestine -Anus The accessory digestive organs; -Teeth & tongue -Saliva...

Histology of the Upper Digestive Tract (Upper Gastrointestinal System) Dr.Emine Özçınar BAU Med. School-2024 Gastrointestinal System The alimentary canal; -Mouth (oral cavity) -Pharynx -Esophagus -Stomach -Small intestine -Large intestine -Anus The accessory digestive organs; -Teeth & tongue -Salivary glands -Gallbladder -Liver -Pancreas The Digestive Process Ingestion; taking in food through the mouth Propulsion(movement of food); swallowing, peristalsis Mechanical digestion (breakdown); chewing, churning in stomach and segmentation in small int. Chemical digestion; by secreted enzymes Absorption; transport of digested end products into blood and lymph in wall of canal Defecation; elimination of indigestible substances from body as feces Upper Digestive Tract (Oral Cavity, Pharynx & Esophagus) ❖Oral Cavity (Mouth); lips, teeth, palate, tongue Oral Mucosa; It is composed of a wet stratified squamous epithelium (nonkeratinized, parakeratinized, or orthokeratinized) and an underlying dense irregular collagenous connective tissue. It may be divided into three classifications: ❖Masticatory mucosa; (gingiva, dorsal surface of the tongue, and hard palate) parakeratinized and completely keratinized epith. and dense irregular connective tissue. ❖Specialized mucosa; (dorsal surface of the tongue and patches of the soft palate and pharynx) bear taste buds, specialized to perceive taste. ❖Lining mucosa; (the remainder of oral cavity) nonkeratinized str. squamous epith., overlying a looser type of dense irregular conn. tissue. Oral Mucosa; Ducts of the three pairs of major salivary glands (parotid, submandibular, and sublingual) open into the oral cavity, delivering saliva to moisten the mouth. These glands also manufacture and release the enzyme salivary amylase (to break down carbohydrates), lactoferrin and lysozymes, antibacterial agents, and secretory immunoglobulin (IgA). In addition, minor salivary glands, located in the connective tissue elements of the oral mucosa, add to the flow of saliva into the oral cavity. It is in the oral cavity that food is moistened with saliva, chewed, and isolated by the tongue, ultimately forming spherical masses about 2 cm in diameter. These spherical masses, each known as a bolus, are forced by the tongue into the pharynx to be swallowed. Lips Each lip has three regions: ❖ The skin (external) aspect; covered with thin skin containing sweat glands, hair follicules and sebaceous glands. ❖ The vermilion zone; covered by thin skin, lack of sweat glands and hair follicules. Occasionally nonfunctional sebaceous glands are present. Capillary loops of the dermal papillae are close to the surface of the skin, imparting a pink color to the vermilion zone. ❖ The mucous (internal) aspect; always wet and is lined by stratified squamous nonkeratinized epithelium. The subepithelial connective tissue is of the dense, irregular collagenous type and houses numerous, mostly mucous, minor salivary glands. Teeth Each tooth, whether deciduous or permanent, has a crown, a neck (cervix) , and a root. Humans have two sets of teeth: 20 deciduous (milk) teeth, which are replaced by 32 permanent (adult) teeth composed of 20 succedaneous teeth and 12 molars (accessional teeth). Both the deciduous and permanent dentitions are evenly distributed between the maxillary and mandibular arches. Teeth The various teeth have different morphologic features numbers of roots, and functions, such as seizing prey, cutting smaller pieces from large chunks, and macerating the chunks to form a bolus. Each tooth is suspended in its bony socket, the alveolus, by a dense, irregular collagenous connective tissue, the periodontal ligament. The gingiva also supports the tooth, and its epithelium seals the oral cavity from the subepithelial connective tissue spaces Mineralized Components; Enamel Dentin Cementum Enamel Dentin Cementum Enamel; ❖Overlies dentin of the crown and it is composed of 95-96% calcium hydroxyapatite + 4-5% organic material and water. ❖It is the hardest substance in the body. ❖It is translucent, and its coloration is due to the color of the underlying dentin. ❖The calcified portion of enamel is composed of large crystals coated with a thin layer of organic matrix. ❖The organic constituents of enamel are the keratin-like, high molecular weight glycoproteins, tyrosine-rich enamelins as well as a related protein, tuftleins. Enamel; ❖Enamel is produced by cells known as ameloblasts, which elaborate enamel daily in 4to 8-μm segments known as rod segments. ❖Successive rod segments adhere to one another, forming keyhole-shaped enamel rods (prisms), which extend over the complete width of the enamel from the dentinoenamel junction to the enamel surface. ❖The calcium hydroxyapatite crystal orientation within rods varies, permitting a subdivision of the enamel rod into a cylindrical head to which a tail (interrod enamel), in the shape of a rectangular solid, is attached. ❖Enamel is a nonvital substance; because the ameloblasts die before the tooth erupts into the oral cavity, the body cannot repair enamel. ✓ Clinical Correlations: Cavities (caries); fluoride increases the hardness of enamel, especially in young individuals, making the enamel more resistant to caries. Dentin; ❖It forms the bulk of the tooth and it is the second hardest substance in the body. ❖It is yellowish in color, composed of 65% to 70% calcium hydroxyapatite, 20% to 25% organic materials, and about 10% bound water. Most of the organic substance is type I collagen associated with proteoglycans and glycoproteins. ❖The cells that produce dentin are known as odontoblasts. ❖Unlike ameloblasts, they maintain their association with dentin for the life of the tooth. ❖These cells are located at the periphery of the pulp, and their cytoplasmic extensions, odontoblastic processes, occupy tunnel-like spaces within dentin. These extracellular fluid–filled spaces, known as dentinal tubules, extend from the pulp to the dentinoenamel (in the crown) and to the dentinocemental (in the root) junctions. Dentin; ❖During dentinogenesis, odontoblasts manufacture about 4 to 8μm of dentin every day. ❖The quality of dentin, as of enamel, varies with the health of the mother prenatally and of the child postnatally. Thus, along the length of the dentinal tubule, dentin displays alternating regions of normal calcification and hypocalcification. ❖Because odontoblasts remain functional, dentin has the capability of self-repair, and reparative dentin is elaborated on the surface of preexisting dentin within the pulp chamber, thus reducing the size of the pulp chamber with age. ✓ Clinical Correlations: Dentin sensitivity is mediated by sensory nerve fibers that are closely associated with odontoblasts, their processes, and the dentinal tubules. Disturbance of the tissue fluid within dentinal tubules is thought to depolarize the nerve fibers somehow, sending a signal to the brain, where the signal is interpreted as pain. Cementum; ❖Cementum overlies dentin of the roots. It is composed of about 50% calcium hydroxyapatite and 50% organic matrix and bound water; therefore, it is approximately as hard as bone. ❖Most of the organic material is composed of type I collagen with associated proteoglycans and glycoproteins. ❖The apical region of cementum is similar to bone in that it houses cells, cementocytes within lenticular spaces, known as lacunae. ❖Processes of cementocytes extend from lacunae within narrow canaliculi that extend toward the vascular periodontal ligament. Because of the presence of cementocytes, this type of cementum is called cellular cementum. The coronal region of cementum is without cementocytes, and thus this type of cementum is called acellular cementum. Cementum; ❖Both cellular cementum and acellular cementum have cementoblasts. These cells, which are responsible for the formation of cementum, cover cementum at its interface with the periodontal ligament and continue to elaborate cementum for the life of the tooth. ❖Collagen fibers of the periodontal ligament, known as Sharpey’s fibers, are embedded in cementum and in the alveolus, and in this fashion the ligament suspends the tooth in its bony socket. ❖Cementum can be resorbed by osteoclast-like cells known as odontoclasts. During exfoliation, the replacement of deciduous teeth by their succedaneous counterparts, odontoclasts resorb cementum (and dentin) of the root. ✓ Clinical Correlations: Cementum does not resorb as readily as does bone, a property that orthodontists use to their advantage in moving improperly positioned teeth. By placing the correct force on a tooth, the orthodontist reshapes the bony socket and consequently causes the tooth to be moved into its correct position. Pulp ❖It is a richly vascularized and innervated loose connective tissue, surrounded by dentin and communicates with the periodontal ligament via the apical foramen. ❖It is customary to subdivide the pulp into three concentric zones around a central core: ▪ The outermost odontoblastic zone of the pulp is composed of a single layer of odontoblasts, whose processes extend into the adjacent dentinal tubules of dentin. ▪ The cell-free zone forms the layer deep to the odontoblastic zone, and as its name implies, it is devoid of cells. ▪ The cell-rich zone, consisting of fibroblasts and mesenchymal cells, is the deepest zone of the pulp, immediately surrounding the pulp core. Pulp ❖The core of the pulp resembles most other loose connective tissues but lacks adipose cells. Another notable difference is that the pulp core is highly vascularized and occasionally houses calcified elements called pulp stones (denticles). ❖The nerve fibers of the pulp are of two types: (1) sympathetic (vasomotor) fibers control the luminal diameters of blood vessels, and (2) sensory fibers are responsible for the transmission of pain sensation. ✓Clinical Correlations: Hemorrhage of the pulp is evident clinically as dark discoloration of the tooth. Odontogenesis (tooth development); Odontogenesis begins with the appearance of the dental lamina (horseshoe-shaped band of epithelial cells) betwen the 6th and 7th weeks of gestation, when the ectodermally derived oral epithelium proliferates. Dental lamina, surrounded by neural crest–derived ectomesenchyme of the mandibular and maxillary arches. The dental lamina is separated from the ectomesenchyme by a well-defined basal lamina. -Bud Stage -Cap Stage -Bell Stage -Appositional Stage -Root formation -Eruption Bud Stage; dental lamina increases on the inferior aspect of epithelial band of each arch. 10 discrete epithelial structures (buds) presage the 10 deciduous teeth of both the maxillary and the mandibular arches. At the inferior tip of each bud, ectomesenchymal cells congregate to form the presumptive dental papilla. Further development, although similar for each bud, is asynchronous, corresponding to the order of emergence of the various teeth of the child. Cap Stage; it is recognized by the three-layered enamel organ. The three layers are: outer enamel epithelium, stellate reticulum, and inner enamel epithelium. ❖ The dental papilla and the enamel organ are collectively called the tooth germ. ❖ Ectomesenchymal cells surrounding the tooth germ form a vascularized membranous capsule, the dental sac, which gives rise to the cementum, periodontal ligament, connective tissue of the gingiva, and alveolus. Cells of the inner enamel epithelium differentiate into preameloblasts, which mature into ameloblasts to form enamel. Except for enamel, the tooth and its associated structures are derived from cells of neural crest origin. Succedaneous lamina proliferate to form a bud, the precursor of the succedaneous tooth that eventually replaces the deciduous tooth being developed. The remaining 12 permanent teeth, known as accessional teeth because they do not replace existing deciduous dentition, arise from the posterior extensions of the maxillary and mandibular dental laminae. Bell Stage and Appositional Stage; the bell stage is recognized by the four-layered enamel organ. The four layers are the outer enamel epithelium, stellate reticulum, stratum intermedium, and inner enamel epithelium. Inner enamel epithelium differentiates into preameloblasts that will mature into enamelproducing columnar cells, known as ameloblasts. In response to the histodifferentiation of the inner enamel epithelial cells, the most peripheral cells of the dental papilla, those in contact with the basal lamina, also differentiate to become preodontoblasts that will mature into dentin-producing columnar cells, known as odontoblasts. Shortly after the odontoblasts begin to elaborate the matrix of dentin into the basal lamina, the ameloblasts also begin to manufacture the matrix of enamel. The dentin and enamel adjoin each other, and the junction between them is called the dentinoenamel junction (DEJ). The tooth germ is now said to be in the appositional stage of odontogenesis. Developing Tooth Ameloblasts Dentin Odontoblasts Ameloblasts Enamel Dentin Odontoblasts Root Formation; root formation begins after the completion of the crown and is organized by the Hertwig epithelial root sheath. Ectomesenchymal cells from the dental sac migrate and differentiate into cementoblasts. These newly differentiated cells manufacture cementum matrix, which subsequently calcifies and is referred to as cementum. As the root becomes longer, the crown approaches and eventually erupts into the oral cavity. Structures Associated with Teeth The structures associated with teeth are the periodontal ligament, alveolus, and gingiva. Periodontal Ligament (PDL); a dense, irregular collagenous connective tissue whose principal fiber groups, composed of type I collagen, suspend the tooth in its alveolus. It is located in the PDL space, defined as the region between the cementum of the root and the bony alveolus. The ends of the principal fiber groups are embedded in the alveolus and cementum as Sharpey’s fibers, which permit the periodontal ligament to suspend the tooth in its socket. Fibroblasts are the most populous cells of the periodontal ligament. These cells not only manufacture the collagen and amorphous intercellular components of the PDL but also help to resorb collagen fibers, thus being responsible for the high turnover of collagen in the PDL. In addition, mast cells, macrophages, plasma cells, and leukocytes are also present in the PDL. Nerves of the PDL include: (1) autonomic fibers, which regulate the luminal diameter of the arterioles; (2) pain fibers, which mediate pain sensation; and (3) proprioceptive fibers, which are responsible for the perception of spatial orientation. ✓ Clinical Correlations: Proprioceptive fibers in the periodontal ligament are responsible for the jaw-jerk reflex, an involuntary opening of the jaw when one unexpectedly bites down on something hard. This reflex causes relaxation of the muscles of mastication and contraction of muscles responsible for opening the jaw, thus protecting the teeth from fracture. Alveolus; a bony continuation of the mandible and maxilla, is divided into compartments, each known as an alveolus, that house the root or, in the case of multirooted teeth, roots of a tooth. Adjacent alveoli are separated from each other by a bony interalveolar septum. Gingiva (Gums); Since the gingiva is exposed to strenuous frictional forces, its stratified squamous epithelium is either fully keratinized (orthokeratinized) or partially keratinized (parakeratinized). Deep to the epithelium is a dense, irregular collagenous connective tissue whose type I collagen fibers form principal fiber groups that resemble those of the periodontal ligament. As the epithelium of the gingiva approaches the tooth, it forms a hairpin turn, proceeds apically (toward the root tip) for 1 to 2 mm, and then attaches to the enamel surface by the formation of hemidesmosomes. The 1- to 2-mm-deep space between the gingiva and the tooth is the gingival sulcus. The region of the gingival epithelium that attaches to the enamel surface is known as the junctional epithelium, which forms a collar around the neck of the tooth. The junctional epithelium forms a robust barrier between the bacteria-laden oral cavity and the sterile environment of the gingival connective tissue. Palate; comprising the hard palate, the soft palate, and the uvula, separates the oral cavity from the nasal cavity. The masticatory mucosa on the oral aspect of the hard palate is composed of a wet stratified squamous keratinized (or parakeratinized) epithelium underlain by dense, irregular collagenous connective tissue. The connective tissue of the anterior lateral region of the hard palate displays clusters of adipose tissue, whereas its posterior lateral aspect exhibits acini of mucous minor salivary glands. The nasal aspect of the hard palate is covered by respiratory epithelium with occasional patches of stratified squamous nonkeratinized epithelium. The most posterior extension of the soft palate is the uvula, its epithelium is composed of stratified squamous nonkeratinized epithelium. The connective tissue of the uvula is also a dense irregular collagenous type and possesses mucous minor salivary glands and its core is composed of skeletal muscle that is responsible for its movement. Tongue; the tongue is the largest structure in the oral cavity. Its extreme mobility is due to the large intertwined mass of skeletal muscle fibers that compose its bulk. Muscles originate outside the tongue, the extrinsic muscles, and those that originate within and insert into the tongue, the intrinsic muscles. The tongue has a dorsal surface, a ventral surface, and two lateral surfaces. The dorsal surface is observed to have two unequal regions, the larger anterior two-thirds and the smaller posterior onethird separated from one another by a shallow, Vshaped groove, the sulcus terminalis, whose apex points posteriorly and contains a deep concavity, the foramen cecum. The dorsal surface of the posterior one third of the tongue is uneven because of the presence of the lingual tonsil. The most posterior portion of the tongue is known as the root of the tongue. Lingual papillae, most of which project above the surface, cover the anterior two thirds of the tongue’s dorsal surface. ❖Lingual Papillae; filiform, fungiform, foliate, and circumvallate They are all located anterior to the sulcus terminalis on the dorsal or lateral aspect of the tongue. The numerous filiform papillae are slender structures that impart a velvety appearance to the dorsal surface. These papillae are covered by stratified squamous keratinized epithelium and help to scrape food off a surface. The high degree of keratinization is especially apparent in the sandpaper-like quality of the cat tongue. Filiform papillae do not have taste buds. Each fungiform papilla resembles a mushroom whose slender stalk connects a broad cap to the tongue surface. The epithelial covering of these papillae is stratified squamous nonkeratinized; thus, the blood coursing through the subepithelial capillary loops is evident as red dots distributed randomly among the filiform papillae on the dorsum of the tongue. Fungiform papillae have taste buds on the dorsal aspect of their cap. ❖Lingual Papillae; filiform, fungiform, foliate, and circumvallate Foliate papillae are located along the posterolateral aspect of the tongue. They appear as vertical furrows, reminiscent of pages of a book. These papillae have functional taste buds in the neonate, but these taste buds degenerate by the second or third year of life. Slender ducts of serous minor salivary glands of von Ebner, located in the core of the tongue, empty into the base of the furrows. There are 8 to 12 large circumvallate papillae in a Vshaped arrangement just anterior to the sulcus terminalis. These papillae are submerged into the surface of the tongue so that they are surrounded by an epithelially lined groove, whose base is pierced by slender ducts of glands of von Ebner. The epithelial lining of the groove and the side (but not the dorsum) of these papillae have taste buds. Filiform Papillae No taste buds Fungiform Papillae Foliate Papillae Circumvallate (Vallate) Papillae Taste Buds; taste buds are intraepithelial sensory organs that function in the perception of taste. The surface of the tongue and the posterior aspect of the oral cavity have approximately 3000 taste buds. Each taste bud, composed of 60 to 80 spindle-shaped cells, is an oval structure, 70 to 80 μm long and 30 to 40 μm wide, and is distinctly paler than the epithelium surrounding it. The narrow end of the taste bud, located at the free surface of the epithelium, projects into an opening, the taste pore, formed by the squamous epithelial cells that overlie the taste bud. Four types of cells constitute the taste bud: -Basal cells (type IV cells) -Dark cells (type I cells) -Light cells (type II cells) -Intermediate cells (type III cells) The relationship among the various cell types is not clear, although researchers agree that basal cells function as reserve cells and regenerate the cells of the taste buds, which have an average life span of 10 days. Most investigators believe in the following progression: Basal cells give rise to dark cells, which mature into light cells, which become intermediate cells and die. Nerve fibers enter the taste bud and form synaptic junctions with type I, type II, and type III cells, indicating that all three cell types probably function in the discernment of taste. Each of these cell types has long, slender microvilli that protrude from the taste pore. In the past, these microvilli were noted with the light microscope and were called taste hairs. Tastants, chemicals from food dissolved in saliva, interact either with ion channels or with receptors located on the microvilli of the taste cells, effecting electrical alterations in the resting potentials of these cells resulting in depolarization of the cell and initiating an action potential that is transmitted to the brain where the signals are interpreted as specific taste sensations. There are five primary taste sensations: salty, sweet, sour, bitter, and umami (a savory taste sensed via glutamate receptors). It is believed that although every taste bud can discern each of the five sensations, each taste bud specializes in two of the five tastes. The reaction to these taste modalities is due to the presence of specific ion channels (salty and sour) and G proteincoupled membrane receptors (bitter, sweet, and umami) in the plasmalemma of the cells of the taste bud. Recently, another receptor was localized on taste buds, CD36, a fatty acid transporter, that has the ability to detect fat and some individuals prefer foods that are fatty. The process of complex taste perception is due more to the olfactory apparatus than to the taste buds, as evidenced by the decreased taste ability of people with nasal congestion from colds Esophagus Esophagus It is a muscular tube, approximately 25 cm in length, that conveys the bolus (masticated food) from the oral pharynx to the stomach. Along its entire length, its mucosa presents numerous longitudinal folds with intervening grooves that cause the lumen to appear to be obstructed; however, when the esophagus is distended the folds disappear and the lumen becomes patent. Layers; ❖ Mucosa:The esophageal mucosa is composed of a stratified squamous epithelium, fibroelastic lamina propria, and a smooth muscle layer that is the longitudinally disposed muscularis mucosae. -The epithelium is regenerated at a much slower rate than the remainder of the gastrointestinal tract; the newly formed cell in the basal layer of the epithelium reaches the free surface in about 3 weeks after formation. Interspersed within the keratinocytes of the epithelium are antigen-presenting cells, known as Langerhans cells, which phagocytose and degrade antigens into small polypeptides known as epitopes. These cells also synthesize major histocompatibility complex (MHC) II molecules, attach the epitopes to these molecules, and place the MHC II–epitope complex on the external aspect of their plasmalemmae. Langerhans cells then migrate to lymph nodes, where they present the MHC II–epitope complex to lymphocytes. -The lamina propria is unremarkable. It houses esophageal cardiac glands, which are located in two regions of the esophagus, one cluster near the pharynx and the other near its juncture with the stomach. It also houses occasional lymphoid nodules, members of the MALT system. -The muscularis mucosae is unusual in that it consists only of a single layer of longitudinally oriented smooth muscle fibers that become thicker in the vicinity of the stomach. ✓ The esophageal cardiac glands produce mucus that coats the lining of the esophagus, lubricating it to protect the epithelium as the bolus is passed into the stomach. Because these glands resemble glands from the cardiac region of the stomach, some investigators suggest that they are ectopic patches of gastric tissue. Esophagus ❖Submucosa: The submucosa of the esophagus is composed of a dense, fibroelastic connective tissue, which houses the esophageal glands proper. The esophagus and the duodenum are the only two regions of the alimentary canal with glands in the submucosa. Electron micrographs of these tubuloacinar glands indicate that their secretory units are composed of two types of cells, mucous cells and serous cells. Mucous cells have basally located, flattened nuclei and apical accumulations of mucus-filled secretory granules. The second cell type is serous cells, with round, centrally placed nuclei. The secretory granules of these cells contain the proenzyme pepsinogen and the antibacterial agent lysozyme. The ducts of these glands deliver their secretions into the lumen of the esophagus. The submucosal plexus is in its customary location within the submucosa, in the vicinity of the inner circular layer of the muscularis externa. Esophagus ❖Muscularis Externa and Adventitia: The muscularis externa of the esophagus is arranged in two layers, inner circular and outer longitudinal. However, these muscle layers are unusual in that they are composed of both skeletal and smooth muscle fibers. -The muscularis externa of the upper third of the esophagus has mostly skeletal muscle; the middle third has both skeletal and smooth muscle; and the lowest third has only smooth muscle fibers. -Auerbach’s plexus occupies its usual position between the inner circular and outer longitudinal smooth muscle layers of the muscularis externa. The esophagus is covered by an adventitia until it pierces the diaphragm, after which it is covered by a serosa. Esophagus ✓ Clinical Correlations: As the esophagus passes through the diaphragm, it is reinforced by fibers of that muscular structure. In some people, development is abnormal, causing a gap in the diaphragm around the wall of the esophagus that permits herniation of the stomach into the thoracic cage. This condition, known as hiatal hernia, weakens the gastroesophageal sphincter, allowing reflux of the stomach contents into the esophagus. Barrett’s syndrome is probably a premalignant condition due, initially, to gastroesophageal reflux. Part of the stratified squamous nonkeratinized epithelium of the esophagus, usually in the lowest region, is replaced by a simple columnar epithelium that resembles the lining of the stomach. Endoscopically, this metaplastic area is reddish in color, and at least 3 cm of the esophagus must be involved to be considered as Barrett’s syndrome. If there are numerous red patches in the lower esophagus, esophageal resection may be necessary. Study Questions: 1) Ameloblasts are originated from ……………. 2) Odontoblasts are originated from ……………. 3) Taste buds are found on which type of tongue papillae? 4) Esophageal mucosal glands are ……… and submucosal glands are ……. 5) Submucosal glands are found only in ……. and …………….. along alimentary canal. 6) Esophageal wall layers: ………………………..