2 Histology of the GI tract: Esophagus and Stomach Flashcards Preview

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Flashcards in 2 Histology of the GI tract: Esophagus and Stomach Deck (23):
1

General design:
The tubular portion of the gastrointestinal tract is organized into four general layers (p.2)

  • the mucosa,
  • the submucosa,
  • the muscularis externa,
  • a serosa

2

Mucosa (p.2)

  • The mucosa/
  • Epithelial lining functions
  • Lamina Propria
  • Muscularis mucosa
  • All three layers of the mucosa/

  • The mucosa lines the inner luminal surface of the gut tube
  • Epithelial lining functions
    • Protection - ex: The stratified squamous epithelia in the esophagus and anus provide protection from friction.
    • Secretion - ex: The mucous surface cells and enzyme producing pits in the stomach, and individual goblet cells in the large and small intestines all release secretions.
    • Absorption - ex: Villi and microvilli augment the surface area of the simple columnar small intestinal epithelium for absorption.
    • Accessory gland ducts - ducts are outgrowths from the epithelial lining to develop and connect to the salivary glands, liver and pancreas.
    • can be folded into the lumen as villi, or into the connective tissue below as crypts and glands
  • Lamina Propria
    • connective tissue layer
    • supports the epithelium both physically and biochemically,
    • attaches it to the underlying muscle layer.
    • projects into the villi
    • contains fenestrated capillaries and lymphatic capillaries to facilitate absorption. 
    • lymphocytes and lymphatic nodules act as a defensive barrier against organisms that may breach the epithelium.
  • Muscularis mucosa
    • smooth muscle layer
    • usually circumferential;
      • the notable exception is the esophagus, where the muscularis mucosa is longitudinally oriented.
    • Individual smooth muscle cells are found in the lamina propria all the way to the tips of the villi.
    • Contraction of the muscularis mucosa results in the independent movement and folding of the mucosa, and therefore, aids in processes of digestion and absorption.
  • All three layers of the mucosa fold to produce rugae (stomach) or plicae circulares (small intestine).
    • These folds have a central core of submucosal tissue.

3

Submucosa (2.3, p.2+4)

  • ?
  • extends into/
  • Two segments of the GI tract which secrete large amounts of mucous
  • Meissner's (submucosal) plexus of nerves and gangila 
    • usually found in/
    • gathers/
    • regulates/
    • consists of/

  • ?
    • loose connective tissue layer with blood vessels and elastin fibers.
  • extends into the core of the plicae circulares and the rugae.
  • Two segments of the GI tract which secrete large amounts of mucous
    • the esophagus and duodenum
    • have mucous glands that penetrate into the submucosa.
  • Meissner's (submucosal) plexus of nerves and gangila
    • usually found in the submucosa, adjacent to the muscularis externa.
    • gathers sensory information from luminal contents
    • regulates the function of the muscularis mucosa and digestive gland secretion.
    • consists of
      • local sensory, motor and interneurons all of the enteric nervous system (ENS),
      • unmyelinated sympathetic postganglionic fibers that originate from the prevertebral group of ganglia,
      • parasympathetic ganglion cell bodies which synapse with preganglionic fibers from the vagus nerve

4

Muscularis externa (propria) (2.4, p.2-3+5)

  • The muscularis externa
  • When examined at the level of the electron microscope/
  • The smooth muscle of these layers is innervated by/
    • what synapses here
  • Autonomic innervation of the myenteric plexus is via /

  • The muscularis externa
    • two layers of perpendicularly oriented smooth muscle, an inner circular and an outer longitudinal.
  • When examined at the level of the electron microscope, smooth muscle cells of the muscularis externa are connected by gap junctions.
  • The smooth muscle of these layers is innervated by the Auerbach's or myenteric ("within muscle") plexus that lies at the junction of the two muscle layers.
    • local sensory, motor and interneurons of the enteric nervous system synapse here.
  • Autonomic innervation of the myenteric plexus is via
    • vagal parasympathetic preganglionic fibers terminating on ganglia cells,
      • increase digestive secretion and motility
    • sympathetic postganglionic fibers from the prevertebral ganglia.
      • cause GI vasoconstriction that indirectly inhibits digestive functions.

5

Muscularis externa (propria) (2.4, p.2-3+5)

  • Interstitial cells of Cajal (ICC)
  • Peristalsis
  • Segmentation

  • Interstitial cells of Cajal (ICC)
    • a specialized cell that serve as pacemakers for the muscularis externa smooth muscle
    • interact with sensory and motor neurons of the myenteric plexus
    • the origin of electrical slow waves of depolarization in the GI tract (not necessarily action potentials) 
  • Peristalsis
    • a set of sequential contractions of the circular muscle with relaxation at the leading edge
    • A shortening of the longitudinal muscle draws this dilated edge back over the luminal contents, pushing them aborally
  • Segmentation
    • divides the small intestine into small sections by a sequence of circular muscle contractions that close off segments of gut.
    • Content mixes within each segment to allow contact with digestive enzymes and transporters. 

6

Serosa or Adventitia

  • Much of the GI tract is suspended by/
  • Serosa
  • mesothelium
  • some portions of the GI tract are embedded in other tissue, such as/
  • Adventitia

  • Much of the GI tract is suspended by mesentery.
  • Serosa
    • The thin layer of connective tissue that envelopes these organs
    • There is a small amount of loose connective tissue through which blood vessels and nerves travel.
  • mesothelium
    • a simple squamous epithelium on the outer surface
  • some portions of the GI tract are embedded in other tissue, such as the esophagus in the mediastinum, or retroperitoneal portions of the large intestine.
  • Adventitia
    • The outer layer of these tubes
    • blends into surrounding connective tissue.

7

Esophagus:
Mucosa (p.7-11)

  • The esophagus 
  • The mucosa of the esophagus
    • epithelium
    • keratin
    • rete pegs
  • In the upper and lower thirds of the esophagus, mucous secreting glands are found in/
    • These relatively small glands are named/
    • \frequently observed in the esophageal lamina propria
    • This layer of connective tissue borders an epithelium contiguous with/
    • Exposure to outside pathogens/
  • A prominent muscularis mucosa is composed of/
    • \will readily identify a tissue section as esophagus.

  • The esophagus
    • a tube that extends from the epiglottis to the cardiac orifice of the stomach.
    • a conduit for swallowed food,
    • not involved in the storage or significant digestion of food.
  • The mucosa of the esophagus
    • stratified squamous epithelium.
      • protects against the friction of swallowed food and insulates against extremes of hot and cold
      • projects down into the lamina propria as rete pegs
    • keratin
      • In humans this lining is not keratinized,
      • in some species that consume a diet high in roughage, such as bovine, keratinization continues through the oral cavity and the esophagus.
    • rete pegs
      • typically seen in stratified squamous epithelia subjected to friction, such as the esophagus or finger tips.
  • In the upper and lower thirds of the esophagus, mucous secreting glands are found in the lamina propria.
    • These relatively small glands are named cardiac glands due to their similarities to the mucous glands of the cardiac portion of the stomach.
    • Isolated lymphocytes as well as lymphatic nodules are frequently observed in the esophageal lamina propria.
    • This layer of connective tissue borders an epithelium contiguous with the external body surface.
    • Exposure to outside pathogens is potentially greater here; therefore, the lymphocyte defenders.
  • A prominent muscularis mucosa is composed of longitudinally oriented smooth muscle fibers.
    • The combination of a stratified squamous epithelium and longitudinally oriented muscularis mucosa will readily identify a tissue section as esophagus.

8

Esophagus:
Submucosa (p.12-14)

  • The sumucosa/
  • esophageal (submucosal) glands
    • These tubuloacinar glands are present/
    • The esophageal glands also secrete/
  • sumucosal venous plexus
    • If the pressure in the portal system increases/

  • The sumucosa is a loose network of collagen, elastin, mucous glands and blood vessels.
  • The process of swallowing chewed food is facilitated by mucous production of the esophageal (submucosal) glands.
    • These tubuloacinar glands are present throughout the length of the esophagus, although most of the liquid in the esophagus originates as saliva from the oral cavity.
    • The esophageal glands also secrete lysozyme, an enzyme to lyse bacterial cell walls.
  • In the lower esophagus, a sumucosal venous plexus can drain into both the systemic or portal venous systems.
    • If the pressure in the portal system increases, this plexus dilates forming esophageal varices.

9

Esophagus:
Muscularis externa (propria) (p.12+15-17)

  • Along the entire length of the esophagus, the muscularis externa is formed by two distinct layers
  • The mechanisms of swallowing and propulsion of food down the length of the esophagus is coordinated by/
    • The muscularis externa in the top 1/3 of the esophagus is composed of/
    • The middle third displays/
    • the final third of the esophagus, the muscularis externa is composed/
  • Swallowing
    • involuntary/
    • controlled by/
  • The esophagus is restricted by upper and lower sphincters. 
    • The upper esophageal sphincter/
    • The lower esophageal sphincter (LES)
      • has
        • an internal component/
        • an external component/
      • tonically contracted to/
      • relaxes to/
    • A third supporting structure/

  • Along the entire length of the esophagus, the muscularis externa is formed by two distinct layers
    • an inner circular
    • an outer longitudinal
  • The mechanisms of swallowing and propulsion of food down the length of the esophagus is coordinated by both skeletal muscle and smooth muscle.
    • The muscularis externa in the top 1/3 of the esophagus is composed of skeletal muscle and is under voluntary control.
    • The middle third displays a mixture of skeletal and smooth muscle intermingled,
    • the final third of the esophagus, the muscularis externa is composed entirely of smooth muscle.
  • Swallowing
    • involuntary past the upper third of the esophagus
    • controlled by the deglutination (swallowing) reflex centers in the medulla and lower pons.
  • The esophagus is restricted by upper and lower sphincters.
    • The upper esophageal sphincter is the cricopharyngeal muscle and participates in the initiation of swallowing.
    • The lower esophageal sphincter (LES)
      • has
        • an internal component, a slight thickening of the circular muscularis externa,
        • an external component, the crural portion of the diaphragm.
      • tonically contracted to prevent reflux of stomach contents
      • relaxes to allow a swallowed bolus of food to pass.
    • A third supporting structure is the sling of oblique fibers of muscularis externa of the upper stomach. 

10

Esophagus:
Adventitia

  • the esophagus descends/
  • adventitia
  • A ligament-like thickening/

  • the esophagus descends through the thorax in the central mediastinum.
  • Its outermost layer is therefore an adventitia.
  • A ligament-like thickening attaches the diaphragm to the outer esophageal muscle

11

Stomach:
Functions (p.18-19)

  • storage for ingested food;
    • the organ is a large expansion along the GI tube and is restricted by a tight pyloric (exiting) sphincter.
  • proteolytic digestion.
    • Proteolytic enzymes such as pepsin (proteins), rennin (milk), lipase (fats) and a highly acidic environment facilitate digestion.
  • mixes its contents using thick smooth muscle layers.
  • absorption of water, salts, alcohol, and some drugs takes place across the stomach wall.
  • secretion of Vitamin B12 intrinsic factor,
    • binds and preserves dietary Vitamin B12 for later absorption in the ileum. 

12

Stomach:
General histology (p.20-22)

  • The stomach is composed of/
  • The unique stomach mucosa is characterized by/
  • The surface is lined by/
    • The apical ends of the cells are filled with/
    • Surface mucous is secreted in response to/
  • Agents which increase the rate of HCO3- secretion into the surface mucous
  • Agents which decrease HCO3- secretion

  • The stomach is composed of the same four general layers introduced previously.
  • The unique stomach mucosa is characterized by pits and glands.
  • The surface is lined by a simple columnar epithelium of surface mucous cells.
    • The apical ends of the cells are filled with secretory vesicles of mucous, which is an alkaline carbohydrate-rich glycoprotein.
    • Surface mucous is secreted in response to friction due to roughage in the diet or to chemical stimuli such as ethanol.
  • Agents which increase the rate of HCO3- secretion into the surface mucous
    • Ca+2
    • prostaglandins E and F
    • cholinergic agents
    • dibutyryl cGMP
  • Agents which decrease HCO3- secretion 
    • NSAIDs
    • Aspirin

13

Stomach:
General histology (p.20-22)

  • Surface mucous cells act as/
  • Gastric pits/
    • These pits are also lined by/
    • Into the base of each pit drain/
  • the lamina propria is filled with/
  • Directly under the base of the gastric glands is a/
  • The stomach wall has/
  • Together, the pits, glands, and rugae serve to/
  • The muscularis externa expands to include three layers of smooth muscle
  • The serosa/

  • Surface mucous cells act as a buffer zone to protect the mucosa from the caustic environment in the stomach lumen.
  • Gastric pits descend from the surface into the lamina propria.
    • These pits are also lined by surface mucous cells.
    • Into the base of each pit drain two or three gastric glands.
      • The gastric glands are different in each anatomical region of the stomach (cardiac, body, pyloric).
  • the lamina propria is filled with gastric pits and glands, and is only evident as small amounts of intervening connective tissue.
  • Directly under the base of the gastric glands is a thin muscularis mucosa composed of circularly oriented smooth muscle.
  • The stomach wall has large, visible folds with a central core of submucosal tissue called rugae.
  • Together, the pits, glands, and rugae serve to increase the surface area of the mucosal layer available for the secretion of digestive enzymes and acid.
  • The muscularis externa expands to include three layers of smooth muscle: inner oblique, middle circular and outer longitudinal.
    • increased thickness of the overall muscularis externa.
  • The serosa is not distinguished in any specialized way

14

Stomach regions (p.23)

  • consists of/
  • Histologically, 
    • the fundus and body/
    • The cardia and the antrum/pylorus/

  • consists of the cardia, fundus, body, antrum and pylorus
  • Histologically,
    • the fundus and body are very similar
    • The cardia and the antrum/pylorus will be considered separately

15

Stomach regions:
Cardiac region (p.24-25)

  • glands
  • The cardia of the stomach/
  • The transition from the esophagus to the cardiac stomach/
    • This is indicative of /
  • The cardiac pits/
  • the cardiac glands/
  • The ratio of the length of the pits to glands/
  • The cardiac glands are composed primarily of/
    • However, just a centimeter into the cardia/

  • Mucous and Mucoparietal Glands
  • The cardia of the stomach is a narrow 2-3 cm zone surrounding the esophageal entrance.
  • The transition from the esophagus to the cardiac stomach is a sharp and dramatic one (stratified squamous mucosa to folded simple columnar).
    • This is indicative of the immediate change in function from conduit to digestion.
  • The cardiac pits are very shallow,
  • the cardiac glands are short and highly coiled.
  • The ratio of the length of the pits to glands is about 1:1.
  • The cardiac glands are composed primarily of small, pale staining mucus producing cells.
    • However, just a centimeter into the cardia, bright pink staining, acid producing parietal cells appear in the glands

16

Stomach regions:
Fundus and body regions (2.7, p.26-27)

  • glands
  • The bulging fundus/
  • The glands of these regions are referred to by a variety of names
    • fundic glands
    • gastric glands
    • oxyntic glands
    • zymogen glands
  • These glands secrete/
  • The mucosa of the fundus and body displays/
    • The ratio of the length of pits to glands/
    • There are from/
  • The gastric glands have an isthmus, neck, and base
    • The isthmus/
      • Mucous neck cells/
      • Precursor cells/
    • The neck (middle) of the gastric gland contains/
    • The base of the gastric gland is made up of/

  • Gastric Glands
  • The bulging fundus has a high compliance so that food can accumulate without increasing stomach pressure.
  • The glands of these regions are referred to by a variety of names:
    • fundic glands (location),
    • gastric glands (major secretors of stomach),
    • oxyntic glands (contain acid secreting cells)
    • zymogen glands (contain stored secretory or “zymogen” granules).
  • These glands secrete the digestive enzymes and acid into the stomach lumen
  • The mucosa of the fundus and body displays deep, straight pits and long glands which are only slightly coiled at their base.
    • The ratio of the length of pits to glands is approximately 1:4.
    • There are from 2-7 glands emptying into the base of each pit.
  • The gastric glands have an isthmus, neck, and base.
    • The isthmus drains into the base of the gastric pit.
      • Mucous neck cells found here secrete an acidic glycoprotein, display cytoplasmic basophilia due to increased amounts of RER, and contain larger secretion granules than surface mucous cells.
      • Precursor cells for both the surface and base of the stomach epithelium arise from the isthmus.
    • The neck (middle) of the gastric gland contains mucous neck cells and acid producing parietal cells.
    • The base of the gastric gland is made up of chief cells, which secrete digestive enzymes, a variety of enteroendocrine cells and a few parietal cells

17

Stomach regions:
Fundus and body regions:
Parietal or oxyntic cells (p.28-30)

  • Parietal or oxyntic cells secrete/
  • In H&E stained sections parietal cells are/
  • Ultrastructure visualized with the electron microscope reveals/
  • intracellular canaliculi
  • tubulovesicular system
  • Together, the intracellular canaliculi and the tubulovesicular system/
  • In a stimulated parietal cell, 80% of the proteins of the apical plasma membrane are /
  • The cytoplasm is also densely packed with/
  • The basolateral plasma membrane of the parietal cell contains/
  • Acid secretion is decreased by/

  • Parietal or oxyntic cells secrete 0.1N HCl and intrinsic factor for binding vitamin B12.
  • In H&E stained sections parietal cells are plump round cells with a central nucleus and a homogeneous, eosinophilic cytoplasm.
  • Ultrastructure visualized with the electron microscope reveals specializations that permit secretion of the strong solution of HCl.
  • The apical plasma membrane folds into a deep trenchlike invagination with numerous microvilli called intracellular canaliculi.
  • The surrounding cytoplasm is filled with closed smooth surfaced vesicles, the tubulovesicular system;
    • these vesicles fuse with the canaliculi when acid secretion is stimulated.
  • Together, the intracellular canaliculi and the tubulovesicular system augment the surface area for H+K+ATPase pumps to release H+ ions into the stomach lumen.
  • In a stimulated parietal cell, 80% of the proteins of the apical plasma membrane are H+K+ATPase pumps.
  • The cytoplasm is also densely packed with mitochondria, which accounts for the cytoplasmic eosinophilia.
    • Mitochondria provide ATP for the active transport occurring at the canalicular surface.
  • The basolateral plasma membrane of the parietal cell contains receptors for histamine, gastrin and acetylcholine, the three factors that stimulate acid secretion.
  • Acid secretion is decreased by somatostatin and by prostaglandins.

18

Stomach regions:
Fundus and body regions:
Chief cells (p.31-32)

  • Chief cells secrete/
  • Chief cells store/
  • The basal cytoplasm is packed with/
  • Chief cells are regulated secretors, and therefore/
  • Chief cell secretion
    • stimulated by/
    • inhibited by/
  • Chief cells are absent from/

  • Chief cells secrete pepsinogen, rennin, and lipase.
  • Chief cells store abundant basophilic secretory granules in the apical end of the cell.
  • The basal cytoplasm is packed with basophilic RER, understandable for a cell which is a very active synthesizer of secretory proteins.
  • Chief cells are regulated secretors, and therefore store their secretory products in condensed zymogen granules.
  • Chief cell secretion
    • stimulated by acetylcholine
    • inhibited by somatostatin.
  • Chief cells are absent from the cardiac glands and seldom found in the pyloric glands.

19

Stomach regions:
Fundus and body regions:
Enteroendocrine (APUD) cells (p.33-34)

  • acronym
  • found in/
  • secrete/
  • found/
  • identified in/
  • secretions 
    • released at /
    • move by/
    •  

  • amine precursor uptake and decarboxylation
  • the third type of cells found in base of the gastric glands.
  • single cells within the epithelium that secrete substances that act as hormones or paracrine agents.
  • found throughout the stomach, small intestine and a few in the colon.
  • identified in electron micrographs by basal secretory granules.
  • secretions
    • released at the basal end of the cell
    • move by diffusion through the interstitial fluid or via the blood stream. 

20

Stomach regions:
Fundus and body regions:
Enteroendocrine (APUD) cells:
Major types, secretions, and actions of enteroendocrine cells in the stomach

  • Gastrin, (G cell)
  • Histamine, (ECL cell)
  • Somatostatin, (D cell)
  • Vasoactive intestinal peptide (VIP), (D1 cells)
  • Ghrelin, Fundic cells

  • Gastrin, (G cell)
    • Stimulates secretion of HCl by parietal cells
    • Increases histamine secretion by ECL cells
    • Proliferation of gastric and intestinal mucosa
    • Promotes peristalis of the gastro-colic reflex
    • Acts via G-proteins (Gq), IP3, DAG & Ca+2 second messengers.
  • Histamine, (ECL cell)
    • Binds histamine H2-receptors on parietal cells to stimulate and potentiate secretion of HCl.
    • Acts via a cAMP second messenger.
  • Somatostatin, (D cell)
    • Inhibits release of gastrin (from G-cells) and acid (from parietal cells) after stomach luminal pH has dropped below 3.0
    • Inhibits chief cell secretion
  • Vasoactive intestinal peptide (VIP), (D1 cells)
    • Increase pancreatic bicarbonate and water secretion
    • Relax lower esophageal sphincter, fundic stomach & gall bladder
    • Decrease gastric acid secretion
    • Increase intestinal secretion of water & electrolytes
  • Ghrelin, Fundic cells
    • Secreted during periods of fasting
    • Binds to a hypothalamic receptor to cause secretion of growth hormone
    • In the brain, ghrelin also stimulates the sensation of hunger
    • In adipose tissue, ghrelin inhibits mobilization of fats

21

Stomach regions:
Fundus and body regions:
Gastric gland renewal (p.35)

  • \the renewal rate of the gastric surface mucous cells/
    • Surface cells turnover/
  • the cells of the gastric gland turnover/
  • The isthmus of the fundic gland serves as a source of/
  • This renewal process can be damaged by/
    • However, when the epithelium is damaged and cells are lost, /
    • Adjacent cells/

  • Because of the extreme conditions within the lumen of the stomach, the renewal rate of the gastric surface mucous cells is rapid.
    • Surface cells turnover every 2-6 days.
  • the cells of the gastric gland turnover at a much slower rate of approximately twice a year (190 days).
  • The isthmus of the fundic gland serves as a source of stem cells for both the surface mucous cells as well as the cell types deep in the base of the gland.
  • This renewal process can be damaged by aspirin, bile salts, and alcohol.
    • However, when the epithelium is damaged and cells are lost, reepithelialization will begin within minutes as long as the basement membrane remains intact.
    • Adjacent cells thin and extend lamellipodia via microfilament action to cover the exposed areas

22

Stomach regions:
Antral region (p.36-37)

  • glands
  • This region is a transition between/
  • Physiologically, the antrum/
  • The antrum 
    • devoid of /
    • enriched in/
  • The major stimulus for gastrin secretion is/
    • These G cells are also stimulated by/
    • The G cells appear like /
  • The antral epithelium also contains/

  • Modified Gastric Glands
  • This region is a transition between the body and the pylorus.
  • Physiologically, the antrum contracts with a powerful churning that triturates the stomach contents into a pureed “chyme.”
  • The antrum
    • devoid of parietal cells,
    • enriched in gastrin secreting G cells.
  • The major stimulus for gastrin secretion is peptides in the stomach lumen.
    • These G cells are also stimulated by an intermediate neuron via the neurotransmitter bombesin, or gastrin-releasing peptide.
    • The G cells appear like pale staining “fried eggs” in histological images.
  • The antral epithelium also contains D cells that secrete somatostatin to inhibit parietal and chief cell secretion.

23

Stomach regions:
Pyloric region (p.38-39)

  • glands
  • characterized by/
  • The pit to gland ratio
  • The glands 
  • The G cells/
  • The rate of renewal of surface mucous cells/
  • frequent site of/
  • A pyloric sphincter is formed by/

  • Mucous glands
  • characterized by deep pits (equal to those in the body) but short glands.
  • The pit to gland ratio is close to 1:1.
  • The glands
    • coiled, unlike the straight glands of the body
    • contain mucous-secreting cells and enteroendocrine cells, but rarely chief or parietal cells.
  • The G cells that secrete gastrin are enriched in the pyloric glands.
  • The rate of renewal of surface mucous cells is slower in the pylorus, with life spans up to two months.
  • frequent site of peptic ulcers.
  • A pyloric sphincter is formed by a thickening of the middle circular layer of the muscularis externa.
    • This substantial tonic constriction prevents leakage of the stored stomach chyme.