Title MOD10A-1-Histology of the GI Tract.pdf ✅

ANAT ● ANATOMY Histology of the GI Tract TRANS 1 Nestor S. Calimbahin, MD, FPSA MODULE 10A January 28, 2023 LECTURE OUTLINE I The Histologic Layers of the Digestive Tract A. Mucosa 1. Epithelium 2. Lamina Propria 3. Muscularis Mucosa B. Submucosa C. Muscularis Externa 1. Auerbach’s Plexus D. Adventitia or Serosa II. Oral Cavity III. Tongue A. Types of Lingual Papillae IV. Taste Buds A. Types of Cells in a Taste Bud B. Types of Cells under Electronmicroscope V. Esophagus VI. Stomach A. Surface Mucous Cells B. Glands of the Stomach C. Fundic Glands / Principal Gatsric Glands 1. Parietal / Oxyntic Cell 2. Zymogenic / Chief Cell 3. Mucous Neck Cells 4. Stem Cells 5. Enterendocrine Cells D. Gastric Mucosal Barrier E. Muscularis Externa of the Stomach 1. Auerbach’s Plexus 2. Serosa LECTURE OBJECTIVES 1. Describe the general histologic organization of the digestive tract. 2. Point out the distinctive features in the histologic layers of the various segments of the digestive tract. 3. Discuss the taste buds. 4. Distinguish the digestive glands that are embedded in the wall of the digestive tract as to their location and cellular composition. 5. Relate the variations in the histologic features of the segments of the digestive tract to their function. I. THE HISTOLOGIC LAYERS OF THE DIGESTIVE TRACT 4 Histologic Layers of the Digestive Tract a Mucosa b Submucosa c Muscularis Externa d Adventitia or Serosa Figure 1. 4 layers of the Digestive Tract Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. The illustration shows the 4 general layers of the digestive tract which includes the mucosa, submucosa, the muscularis externa, and the serosa or adventitia A. MUCOSA 1. Epithelium ● Lines the luminal surface of the digestive tract 2. Lamina Propria ● Loose connective tissue layer that underlies the epithelium ● Contains: ○ Blood ○ Lymphatic vessels ○ MALT/GALT ○ Digestive glands 3. Muscularis Mucosa ● A smooth muscle fiber forming the outermost layer of the mucosa which is composed of 2 layers: ○ INNER CIRCULAR smooth muscle layer ○ OUTER LONGITUDINALLY arranged smooth muscle layer B. SUBMUCOSA ● Lies external to the muscularis mucosa (SM = Submucosa, Meissner’s) ● Made up of loose connective tissue ● Supplied with GALT ● Submucous Plexus of Meissner (autonomic neurons) ○ The submucosa of the ff organs: → Esophagus → Stomach → Intestines Group 6A & 7A | Histology of the GI Tract 1
Figure 2. Photomicrograph showing the Enteric Nervous System, the submucosal plexus, and myenteric plexus Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. C. MUSCULARIS EXTERNA ● Responsible for mechanical digestion of food ● Contains the Interstitial cells of Cajal (ICC) which resemble smooth muscle cells that act as pacemakers for smooth muscle contraction, their intermediate filament is Vimentin, smooth ER is flattened with smaller mitochondria. ● 2 Layers: ○ Inner Circular Smooth muscle layer ○ Outer Longitudinally arranged smooth muscle layer 1. Auerbach’s Plexus ● Located between the Inner Circular and Outer Longitudinal smooth muscle layer of the Esophagus, stomach and intestines. ● Composed of Ganglionated autonomic neurons similar to Meissner’s plexus of the submucosa Figure 3. Inner circular and Outer longitudinal smooth muscle Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. Slide showing the inner circular and outer longitudinal smooth muscle, in between is Auerbach’s plexus.(Alternative Source: https://histology.siu.edu/erg/GI015b.htm) Figure 4. Auerbach’s Plexus Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. Diagram showing the Auerbach’s plexus between the Inner circular and Outer longitudinally arranged smooth muscle in the Muscularis Externa Figure 5. Electron micrograph of the myenteric (Auerbach’s plexus) Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. The plexus is located between the two smooth muscle (SM) layers of the muscularis externa. It consists of nerve cell bodies (CB) and an extensive network nerve fibers (N). A satellite cell (SC), also referred to as an enteric glial cell, is seen in proximity to the neuron cell bodies. These cells have structural and chemical features in common with glial cells of the CNS. BV, blood vessel, x3800. D. ADVENTITIA/SEROSA ● Outermost histologic layer of the digestive tract made up of loose connective tissue ● In areas of digestive tract covered by Peritoneum, the connective tissue is lined by Mesothelium or referred to as SEROSA ○ If tract has no peritoneal cover = cover layer is ADVENTITIA II. ORAL CAVITY ● 2 Regions ○ Vestibule - region of the mouth anterior to the teeth and gums ○ Oral cavity proper - rest of the mouth III. TONGUE ● A Skeletal Muscular organ for mastication, deglutition, speech and taste ● Dorsum is rough because of the presence of lingual papillae ○ Mucosa is atypical because it is made up of dense connective tissue instead of loose Group 6A & 7A | Histology of the GI Tract 2
Figure 6. Human Tongue Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. Circumvallate papillae are positioned in a V configuration, separating the anterior two thirds of the tongue from the posterior third. Fungiform and filiform papillae are on the anterior portion of the dorsal tongue surface. The uneven contour of the posterior tongue surface is attributable to the lingual tonsils. The palatine tonsil is at the junction between the oral cavity and the pharynx. A. 4 TYPES OF LINGUAL PAPILLAE ● Projections of the lingual mucosa confined to the dorsum of the anterior tongue Table 1. 4 Types of Lingual Papillae FILIFORM ● Most numerous, slender and tapering. ● Covered by KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM. ● Do not have taste buds on the surface FUNGIFORM ● Flattened surfaces, broader than filiform. ● Connective tissue core penetrates deeply into the very thin keratinized epithelial cover plus it contains capillaries that appear as pinhead sized red dots on the dorsum of the tongue. ● Taste buds - Occasionally present on the surface. ● Covered by KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM CIRCUMVALLATE ● Largest (1-3 mm) but fewest ● Cylindrical, its base is surrounded by canal or moat ● Epithelium on the lateral surface contains 200 TO 300 TASTE BUDS. ● Covered by: ○ slightly KERATINIZED STRATIFIED SQUAMOUS EPITHELIUM FOLIATE ● Covered by NON KERATINIZEDD STRATIFIED SQUAMOUS EPITHELIUM. ● Consists of parallel low ridges separated by DEEP MUCOSAL CLEFTS on the posterior lateral surface of the tongue. ● Contains many taste buds in the epithelium of the facing walls of neighboring papillae. ● Small serous glands empty into the clefts (Serous glands of Von Ebner similar to circumvallate papillae) ● Can be distinguished from fungiform papillae because they appear in rows separated by deep clefts. ● They are covered by STRATIFIED SQUAMOUS NON KERATINIZED EPITHELIUM containing numerous taste buds on their lateral surfaces. Figure 7. 4 Types of Lingual papillae Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1.(Alternative Source: Histology by MIchael H. Ross) IV. TASTE BUDS ● Ovoid structures embedded in the epithelium of the circumvallate, foliate and occasionally in the FUNGIFORM papillae, epiglottis, soft palate, palatoglossal arch and posterior pharyngeal wall. ● Organs responsible for sense of taste ● 50-80 um TALL, 30-50 um WIDE ● Consists of 50-150 fusiform cells whose apices converge on a small opening on the epithelial surface = taste pore Figure 8. Illustration of a taste bud Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1.(Alternative Source: Histology by Michael Ross) Group 6A & 7A | Histology of the GI Tract 3
A. 3 TYPES OF CELLS IN A TASTE BUD ● Sustentacular/supporting cells ○ Slender, spindle shape with microvilli ○ Darker staining ● Neuroepithelial/gustatory cells ○ Slender, spindle shape with microvilli ○ Lighter staining ● Basal cells ○ Round cells at the base of the taste buds. B. 5 TYPES OF CELLS UNDER ELECTRON MICROSCOPE Type I sustentacular cells ● dark staining compared to neuroepithelial cells Type II and III subsets of neuroepithelial cells Type IV basal cells - serve as stem cells Type V cells form the outer boundary of taste buds V. ESOPHAGUS ● 20-25 cm long muscular tube that serves as passageway for food from the pharynx to the stomach. Epithelium non keratinized stratified squamous At the gastroesophageal junction, the epithelium becomes simple columnar epithelium. ● Z line - serrated border formed at the transitional area ● most common site of esophageal cancer Langerhans Cells – antigen presenting cells (APCs) present in the basal area of the esophageal epithelium. ● Esophageal Cardiac Glands ○ mucous secreting simple tubular glands in the lamina propria of the esophagus. (ecg lp - esophageal cardiac glands, lamina propria). ● Esophageal Glands Proper ○ mucus secreting compound tubuloalveolar glands situated in the submucosa of the esophagus (EGPS = Esophageal Glands Proper, Submucosa) Muscularis Externa – ATYPICAL Upper Third composed of skeletal muscle Middle Third mixture of skeletal and smooth muscle fibers Lower Third consist exclusively of smooth muscle fibers NOTE: Skeletal muscle fibers in the esophagus are INVOLUNTARY Adventitia - outermost histologic layer of the esophagus except in the segment inside the abdomen where the layer is serosa Figure 9. Photomicrograph of the esophagus. Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. This low-magnification photomicrograph shows an H&E-stained section of the esophagus with its characteristically folded wall, giving the lumen an irregular appearance. The mucosa consists of a relatively thick stratified squamous epithelium, a thin layer of lamina propria containing occasional lymphatic nodules, and muscularis mucosa. Mucous glands are present in the submucosa; their ducts, which empty into the lumen of the esophagus, are not evident in this section. External to the submucosa in this part of the esophagus is a thick muscularis externa made up of an inner layer of circularly arranged smooth muscle and an outer layer of longitudinally arranged smooth muscle. The adventitia is seen just external to the muscularis externa. X8. (Alternative Source: Histology by Michael Ross) Figure 10. Photomicrograph of an esophageal submucosal gland. Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. This photomicrograph shows a mucicarmine-stained section of the esophagus. An esophageal gland, deeply stained red by the carmine, and an adjacent excretory duct are seen in the submucosa. These small, compound, tubuloalveolar glands produce mucus that lubricates the epithelial surface of the esophagus. Note the stained mucus within the excretory duct. The remaining submucosa consists of irregular dense connective tissue. The inner layer of the muscularis externa (bottom) is composed of circularly arranged smooth muscle. X110 (Alternative Source: Histology by Michael Ross) Group 6A & 7A | Histology of the GI Tract 4
VI. STOMACH ● Mucosa ○ Surface epithelium: simple tall columnar epithelium ● Gastric Foveolae or Pits ○ Grooves formed by epithelial invagination into the lamina propria. At the bottom of these pits is where glands of the stomach open into Surface Epithelium of Stomach ● Composed of two types of cells ○ Surface mucous cells ○ Enteroendocrine cells A. SURFACE MUCOUS CELLS ● Majority of cells in the surface epithelium ● Produce mucus and bicarbonate ions ● Lamina Propria - where millions of gastric glands that open into the gastric pits are embedded ● Muscularis mucosa - inner circular and outer longitudinal smooth muscle fibers - contraction of which facilitate emptying of the glands Figure 11. Photomicrograph of esophagogastric junction Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. Photomicrograph of esophagogastric junction showing non-keratinized squamous epithelium lining - esophagus ; simple columnar epithelium lining - stomach ; tall columnar epithelium - gastroesophageal junction Figure 12. Mucosal surface of the stomach in electron micrograph Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. Figure shows the mucosal surface, gastric pits, and presence of mucus, right side has higher magnification showing the apical surface mucus cells that lines the stomach, and the gastric pits B. GLANDS OF THE STOMACH ● Confined to lamina propria 3 Types of Glands of the Stomach ● Cardiac ● Fundic (Principal Gastric) ● Pyloric NOTE: All are SIMPLE TUBULAR or SIMPLE BRANCHED TUBULAR GLANDS whose ducts are open at the bottom of the gastric pits. Parts of the Gland Isthmus Segment of the gland that opens into the mucosal surface Neck Basal to isthmus Base Rest of the gland Parts of the Stomach Cardia Gastric pits take up the same amount of space in the mucosa as the glands Fundus and Body Shallow gastric pits, Mucosa is mostly occupied by glands Pylorus Deep gastric pits, Narrow area of mucosa is occupied by glands C. FUNDIC GLANDS/PRINCIPAL GASTRIC GLANDS ● Most numerous and longest of the glands of the stomach. ● Populated by 5 Types of Cells ○ Parietal/Oxyntic ○ Zymogenic/Chief ○ Mucous Neck Cell ○ Stem Cell ○ Enteroendocrine Figure 13. Gastric Gland Source: Dr. Calimbahin’s PPT; Digestive Tract Lecture Video Part 1. Figure shows stained specimen on the left; photo on the right side is a drawing showing the different parts of the gastric gland. Group 6A & 7A | Histology of the GI Tract 5