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1 | Page: Digestive Histology Part 1 Swailes Histology of the Digestive Histology of the Digestive System I Oral cavity, Esophagus and Stomach N. Swailes, Ph.D. Department of Anatomy and Cell Biology Rm: B046A ML Tel: 5-7726 E-mail: [email protected] Required reading Mescher AL, Junqueira’s Basic Histology Text and Atlas, 12th Edition, Chapter 15: pp249-259 Ross MH and Pawlina W, Histology: A text and Atlas, 6th Edition, Chapter 16: pp526-567 Learning objectives 1) Recognize and name the major layers and components of the generalized gut tube. 2) Examine oral mucosa and determine how it is structurally different from both skin and the generalized gut tube. 3) Determine that different regions of the oral cavity have different types of oral mucosa and these are reflective of the function of each region. 4) Identify the histological features of the esophagus and explain the functional role of each in the process of digestion. 5) Identify the histological features of the stomach and explain the functional role of each in the process of digestion. Key terms mucosa esophageal cardiac gland parietal (oxyntic) cell submucosa esophageal gland intrinsic factor Meissner’s plexus stomach chief (zymogenic) cell muscularis externa cardia pepsinogen Auerbach’s (myenteric) fundus pepsin plexus body rennin/chymosin adventitia pylorus somatostatin serosa gastric pit ghrelin oral cavity gastric gland gastrin masticatory mucosa cardiac gland D-cell specialized mucosa principal gland G-cell lining mucosa pyloric gland Ghr-cell lingual papilla surface mucous cell enteroendocrine cell esophagus neck mucous cell cardio-esophageal junction 1 | Page: Digestive Histology Part 1 Swailes Introduction During this lecture you will discover the general structure of the digestive tract and examine the first part of the digestive tract where the primary role is mastication. A1: General structure of the digestive tract A. Mucosa The mucosa is composed of an epithelium and its basement membrane, the underlying lamina propria and a muscularis mucosa. 1. Epithelium - is either stratified squamous (protective) or simple columnar (absorptive). - may contain specialized cells that impart specific functional properties to a region. - often invaginates to form glands for secretion. - often evaginates to form ‘villi’ to maximize surface area for absorption. 2. Lamina propria - composed of loose irregular connective tissue - contains blood vessels, lymphatic vessels and glands - has Mucosa Associated Lymphoid Tissues (MALT) in the form of lymphoid nodules. 3. Muscularis mucosae - is a layer of smooth muscle that moves independently of the muscularis externa. - it is absent from the oral cavity and anal canal. 1 A 2 3 B 4 5 C 6 7 2 | Page: Digestive Histology Part 1 Swailes D 8 B. Submucosa A region of dense irregular connective tissue which contains the blood vessels and lymphatic vessels that supply and drain the lamina propria. In addition to glands and MALT it contains: 4. Meissner’s (submucosal) plexus - contains sympathetic fibers and post-ganglionic parasympathetic neurons (ganglion cells) - innervates muscularis mucosa, secretory and sensory cells of the epithelium. C. Muscularis externa 5. Inner circular - typically a layer of smooth muscle - oral cavity and esophagus are completely or partially composed of skeletal muscle instead. 6. Auerbach’s (myenteric) plexus - located between the two layers of m. externa and innervates them. - it has the same components as Meissner’s plexus. Clinical Correlation: Hirschprung disease Hirschprung disease is a congenital disease (the ganglia are derived from the neural crest) of the bowel where Meissner’s plexus and Auerbach’s plexus are absent from a length of the distal colon As a result there is an imbalance of smooth muscle contractility in the affected region of bowel and it is unable to relax. The result is extreme constipation and dilation of the region proximal to the affected bowel (e.g. megacolon). Treatment involves surgical resection of the affected area. 7. Outer longitudinal - typically a layer of smooth muscle. - oral cavity and esophagus are completely or partially composed of skeletal muscle instead. - forms a thickened region of skeletal muscle in anal canal called the external anal sphincter. Note: The stomach has an additional oblique layer of smooth muscle in its externa. D. Serosa or Adventitia The outer region of the gut tube consists of loose irregular connective tissue. - if the region of the tube is covered in peritoneum, the peritoneal mesothelium can be seen adhered to this connective tissue and it is termed a ‘serosa’. - if the region of the tube is retroperitoneal the mesothelium will be absent and the layer is termed an ‘adventitia’. 3 | Page: Digestive Histology Part 1 Swailes A2: Oral cavity The oral cavity is the site of mastication, mixing of food with saliva and commencement of the breakdown of carbohydrate. It is composed of the following areas: 1. Hard palate 2. Soft palate 3. Tongue 4. Teeth 1 5. Gingiva (gums) 6. Lip 2 A. Oral mucosa There are three regions of oral mucosa (stratified squamous epithelium + lamina propria) found in the oral cavity: 3 1. Masticatory - keratinized or parakeratinized - withstand the forces of chewing 4 - hard palate and gingiva 2. Specialized 5 - keratinized with papillae - dorsal surface of the tongue 3. Lining 6 - non-keratinized - robust and flexible - cheeks, inner lip, soft palate etc. ** Note that oral mucosa DOES NOT have a Clinical Correlation: Aphthous stomatitis muscularis mucosa! Aphthous stomatitis (also known as canker sores) occur only in lining mucosa (non-keratinized). The cause of these ulcers is unknown but citrus fruits, physical trauma and stress have been implicated in Clinical Correlation: Oral squamous cell carcinoma their development. Oral squamous cell carcinoma affects around 30,000 Americans each year. It can occur anywhere in the oral cavity, but it is most common in certain areas of Clinical Correlation: Cold sores the lining mucosa: floor of the mouth, ventral and Cold sores are a result of herpes simplex virus-1 lateral surfaces of the tongue and the soft palate. The infection which only occurs in keratinized chief risk factors for this cancer are smoking and epithelium. It presents as visible sores or ulceration. alcohol use. Surgery and radiation therapy are the A cure for the condition has not been developed. treatments of choice. Once infected the virus remains in the body for life and recurrent outbreaks may occur. 4 | Page: Digestive Histology Part 1 Swailes B. Tongue The tongue is composed mostly of intrinsic skeletal muscle fibers arranged in longitudinal, transverse and vertical bundles. Its dorsum is covered in 1 specialized mucosa and the remainder is covered in lining mucosa. Lingual papillae 1. Filiform - most numerous 2 - no taste buds - very thick keratin layer 2. Fungiform - less numerous and irregularly arranged - contain taste buds - lightly keratinized 3. Circumvallate - 7-12 papillae in front of terminal sulcus 3 - contain taste buds - serous glands of von Ebner cleanse taste buds 4. Foliate - poorly developed in humans - contain taste buds - located in postero-lateral surface of tongue Taste buds Specialized regions of the oral mucosa that contain: 4 5. Taste pore - molecules dissolved in saliva enter the taste pore 6. Gustatory microvilli - bind to receptors on the gustatory microvilli 7. Gustatory cell 7 - depolarizes the gustatory cell 6 9 8. CN VII (chordae tympani) or CNIX - transmit signals to the brain 5 9. Basal cell - stem cell population that replaces old/damaged gustatory cells 8 10. Serous glands of von Ebner (not shown) - secretions from these submucosal glands wash away food particles around the taste buds so that they can process new uncontaminated tastes 5 | Page: Digestive Histology Part 1 Swailes A3: Esophagus The esophagus is a muscular tube through which food is carried from the pharynx to the stomach. i. Mucosa 1. Non-keratinized stratified squamous epithelium 1 - resists abrasive forces of bolus during swallowing. 2. Lamina propria 2 Loose irregular connective tissue containing: Cardiac esophageal glands (not shown) - mucous secreting glands located in distal region of the esophagus near stomach. Lymphoid aggregates (not shown) - MALT (Mucosa Associated Lymphoid Tissue 3. Muscularis mucosae - typical layer of circumscribing smooth muscle. 3 ii. Submucosa Dense irregular connective tissue containing: 4. Esophageal glands - small mucous-secreting glands - lubricates epithelium and assists in the 4 passage of the bolus. iii. Muscularis externa 5. Inner circular muscle 6. Outer longitudinal muscle 5 Note the type of muscle in the esophagus varies: rd - upper 1/3 is skeletal only. 6 rd - middle 1/3 is mixed smooth and skeletal. rd - lower 1/3 is smooth only. This organization reflects the fact that swallowing begins with a controlled motion (skeletal) and progresses to involuntary peristalsis (smooth). iv. Adventitia/Serosa 7. Mesothelium (or not) At the esophageal hiatus (T10) the esophagus becomes covered in peritoneum (mesothelium) this is a serosa. Above this, there is no mesothelium just 7 connective tissue, this is an adventitia. 6 | Page: Digestive Histology Part 1 Swailes B A4: The Stomach The stomach is responsible for both mechanical and chemical digestion. It continues the digestion of carbohydrates, initiates the digestion of proteins and A produces the viscous chyme. D There are four distinct regions of the stomach: A. Cardia B. Fundus C. Body C D. Pylorus Organization of the stomach wall i. Mucosa 1. Simple columnar epithelium - produce
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