Download Digestive System: Esophagus and Stomach

Digestive System:
Esophagus and Stomach
General Plan of the Digestive System
The digestive (gastrointestinal) tract is a long hollow tube that extends from the esophagus to the
rectum. It includes the esophagus, stomach, small intestine (duodenum, jejunum, ileum), large
intestine (colon), and rectum. The wall of the digestive tube exhibits four layers that show a basic
histologic organization. The layers are the mucosa, submucosa, muscularis externa, and serosa or
adventitia. Because of the different functions of the digestive organs in the digestive process, the
morphology of these layers exhibits variations.
The mucosa is the innermost layer of the digestive tube. It consists of a covering epithelium
and glands that extend into the underlying layer of loose connective tissue called the lamina pro­
pria. An inner circular and outer longitudinal layer of smooth muscle, called the muscularis
mucosae, forms the outer boundary of the mucosa.
The suhmucosa is located below the mucosa. It consists of dense irregular connective tissue
with numerous blood and lymph vessels and a submucosal (Meissner's) nerve plexus. This nerve
plexus contains postganglionic parasympathetic neurons. The neurons and axons of the submu­
cosal nerve plexus control the motility of the mucosa and secretory activities of associated
mucosal glands. In the initial portion of the small intestine, the duodenum, the submucosa con­
tains numerous branched mucous glands.
The muscularis externa is a thick, smooth muscle layer located inferior to the submucosa.
Except for the large intestine, this layer is composed of an inner layer of circular smooth muscle
and outer layer of longitudinal smooth muscle. Situated between the two smooth muscle layers of
the muscularis externa is connective tissue and another nerve plexus called the myenteric
(Auerbach's) nerve plexus. This plexus also contains some postganglionic parasympathetic neu­
rons and controls the motility of smooth muscles in the muscularis externa.
The serosa is a thin layer of loose connective tissue that surrounds the visceral organs. The
visceral organs may or may not be covered by a thin outer layer of squamous epithelium called
mesothelium. If mesothelium covers the visceral organs, the organs are within the abdominal or
pelvic cavities (intraperitoneal) and the outer layer is called serosa. The serosa covers the outer
surface of the abdominal portion of the esophagus, stomach, and small intestine. It also covers
parts of the colon (ascending and descending colon) only on the anterior and lateral surfaces
because their posterior surfaces are bound to the posterior abdominal body wall and are not cov­
ered by the mesothelium (Overview Figure 12).
When the digestive tube is not covered by mesothelium, it then lies outside of the peritoneal
cavity and is called retroperitoneal. In this case, the outermost layer adheres to the body wall and
consists only of a connective tissue layer called adventitia.
The characteristic features of each layer of the digestive tube and their functions are dis­
cussed in detail with each illustration of the different organs.
Esophagus
The esophagus is a soft tube approximately 10 inches long that extends from the pharynx to the
stomach. It is located posterior to the trachea and in the mediastinum of the thoracic cavity. After
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PART II - ORGANS
descending in the thoracic cavity, the esophagus penetrates the muscular diaphragm. A short sec­
tion ofthe esophagus is present in the abdominal cavity before it terminates at the stomach.
In the thoracic cavity, the esophagus is surrounded only by the connective tissue, which is
called the adventitia. In the abdominal cavity, a simple squamous mesothelium lines the outer­
most wall ofthe short segment ofthe esophagus to form the serosa.
Internally, the esophageal lumen is lined with moist, nonkeratinized stratified squamous
epithelium. When the esophagus is empty, the lumen exhibits numerous but temporary longitudinal
folds ofmucosa. In the lamina propria ofesophagus near stomach are the esophageal cardiac glands.
In the submucosa are small esophageal glands. Both glands secrete mucus to protect the mucosa and
to facilitate the passage offood material through the esophagus. The outer wall ofthe esophagus, the
muscularis externa, contains a mixture of different types of muscle fibers. In the upper third of the
esophagus, the muscularis externa contains striated skeletal muscle fibers. In the middle third ofthe
esophagus, the muscularis externa contains both skeletal and smooth muscle fibers, while the lower
third ofthe esophagus is composed entirely ofsmooth muscle fibers ( see Overview Figure 12).
Stomach
The stomach is an expanded hollow organ situated between the esophagus and small intestine. At
the esophageal-stomach junction, there is an abrupt transition from the stratified squamous
epithelium of the esophagus to the simple columnar epithelium of the stomach. The luminal
surface of the stomach is pitted with numerous tiny openings called gastric pits. These are
formed by the luminal epithelium that invaginates the underlying connective tissue lamina pro­
pria of the mucosa. The tubular gastric glands are located below the luminal epithelium and
open directly into the gastric pits to deliver their secretions into the stomach lumen. The gastric
glands descend through the lamina propria to the muscularis mucosae.
Below the mucosa ofthe stomach is the dense connective tissue submucosa containing large
blood vessels and nerves. The thick muscular wall of the stomach, the muscularis externa,
exhibits three muscle layers instead of the two that are normally seen in the esophagus and small
intestine. The outer layer ofthe stomach is covered by the serosa or visceral peritoneum.
Anatomically, the stomach is divided into the narrow cardia, where the esophagus termi­
nates, an upper dome-shaped fundus, a lower body or corpus, and a funnel-shap ed, terminal
region called the pylorus.
The fundus and the body comprise about two thirds of the stomach and have identical his­
tology. As a result, the stomach has only three distinct histologic regions. The fundus and body
form the major portions ofstomach. Their mucosae consist ofdifferent cell types and deep gastric
glands that produce most ofthe gastric secretions or juices for digestion. Also, all stomach regions
exhibit rugae, the longitudinal folds ofthe mucosa and submucosa. These folds are temporary and
disappear when the stomach is distended with fluid or solid material (Overview Figure 12).
CHAPTER 12 - Digestive System: Esophagus and Stomach
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layer (skeletal)
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12 Artery
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glands proper
Outer longitudinal
muscle layer (skeletal)
FIGURE 12.2 • Upper esophagus (transverse section). Stain: hematoxylin and eosin. Low magnification.
Mucosa:
--- - a. Epithelium---b. Lamina propria -------------........
c. Muscularis mucosae
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esophageal glands proper
5 Esophageal glands proper
8 Serosa (mesothelium)
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Muscularis externa:
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layer (smooth)
b. Outer longitudinal
muscle layer (smooth)
FIGURE 12.3 • Lower esophagus (transverse section). Stain: hematoxylin and eosin. Low magnification.
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PART II - ORGANS
FUNCTIONAL CORRELATIONS: Esophagus
The major function of the esophagus is to convey liquids or a mass of chewed food (bolus)
from the oral cavity to the stomach. For this function, the lumen of the esophagus is lined by
a protective nonkeratinized stratified squamous epithelium. Aiding in this function are
esophageal glands located in the connective tissue of the wall. There are two types of glands in
the wall of the esophagus. The esophageal cardiac glands are present in the lamina propria of
the upper and lower regions of the esophagus. These glands have a similar morphology to
those found in the cardia of the stomach, where the esophagus terminates. Esophageal glands
proper are located in the connective tissue of the submucosa. Both types of glands produce the
secretory product mucus, which is conducted in excretory ducts through the epithelium to
lubricate the esophageal lumen. The swallowed material is moved from one end of the esoph­
agus to the other by strong muscular contractions called peristalsis. At the lower end of the
esophagus, a muscular gastroesophageal sphincter constricts the lumen and prevents regur­
gitation of swallowed material into the esophagus.
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PART II - ORGANS
FUNCTIONAL CORRELATIONS: Gastric Pits and Cells of Gastric Glands
The cardia and pylorus are located at opposite ends of the stomach. The cardia surrounds the
entrance of the esophagus into the stomach. At the esophageal-stomach junction are the car­
diac glands. The pylorus is the most inferior region of the stomach. It terminates at the border
of the initial portion of the small intestine called the duodenum. In the cardia, the gastric pits
are shallow, whereas in the pylorus, the gastric pits are deep. However, gastric glands in these
two regions have similar histology and their cells are predominantly mucus-secreting.
In contrast, the gastric glands in the fundus and body of the stomach contain three major
cell types. Located in the upper region of gastric glands near the gastric pits are mucous neck
cells. The large polygonal cells with a distinctive eosinophilic cytoplasm are the parietal cells.
These cells are primarily located in the upper half of the gastric glands and are squeezed
between other gastric gland cells. Located predominantly in the lower region of the gastric
glands are basophilic staining cuboidal chief (zymogenic) cells.
In addition to cells that are present in gastric glands, the mucosa of the digestive tract also
contains a wide distribution of enteroendocrine or gastrointestinal endocrine cells. These cells
are widely distributed in different digestive organs and are located among and between exist­
ing exocrine cells. Unless sections of digestive organs are prepared with special stains, these
cells are poorly seen in normal histologic sections.
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PART II - ORGANS
FUNCTIONAL CORRELATIONS
Stomach
The stomach has numerous functions. The stomach receives, stores, mixes, and digests
ingested food products and secretes different hormones that regulate digestive functions.
Some functions are mechanically and chemically specifically designed to reduce the mass of
ingested food material, or bolus, to a semiliquid mass called chyme. The mechanical reduction
of the bolus is performed by strong, muscular peristaltic contractions of the stomach wall
when food enters the stomach. With the pylorus closed, the muscular contractions churn and
mix the stomach contents with gastric juices produced by the gastric glands. Neurons and
axons located in the submucosal nerve plexus and myenteric nerve plexus of the stomach
wall regulate the peristaltic activity. The stomach also performs some absorptive functions;
however, these are primarily limited to absorption of water, alcohol, salts, and certain drugs.
Gastric Gland Cells in the Body and Fundus of the Stomach
Chemical reduction or digestion of food in the stomach is the main function of gastric secre­
tions produced by different cells in the gastric glands, especially cells located in the fundus and
body regions of the stomach. The main components of the gastric secretions are pepsin,
hydrochloric acid, mucus, intrinsic factor, water, lysozyme, and different electrolytes.
The surface or luminal cells that line the stomach secrete thick layers of mucus, whose
main function is to cover, lubricate, and protect the stomach surface from the corrosive actions
of acidic gastric juices secreted by different cells in the gastric glands.
The major component of gastric juice is the hydrochloric acid, produced by parietal cells
that are located in the upper regions of the gastric glands. In humans, parietal cells also pro­
duce gastric intrinsic factor, a glycoprotein that is necessary for absorption of vitamin B12
from the small intestine. Vitamin B 12 is necessary for erythrocyte (red blood cell) production
(erythropoiesis) in the red bone marrow. Deficiency of this vitamin leads to the development
of pernicious anemia, a disorder of erythrocyte formation.
Chief or zymogenic cells are filled with secretory granules that contain the proenzyme
pepsinogen, an inactive precursor of pepsin. Release of pepsinogen during gastric secretion
into the acidic environment of the stomach converts the inactive pepsinogen into a highly
active, proteolytic enzm
y e pepsin. This enzyme digests large protein molecules into smaller
peptides, converting almost all of the proteins into smaller molecules. Pepsin is primarily
responsible for converting the solid food material into fluid chyme. The secretory activities of
the chief and parietal cells are controlled by the autonomic nervous system and the hormone
gastrin, secreted by the enteroendocrine cells of the pyloric region of the stomach.
CHAPTER 12
Summary
Digestive System: Esophagus and Stomach
General Plan of the Digestive System
• Hollow tube extending from oral cavity to rectum
• Wall exhibits basic organization of the entire tube
Mucosa: Composition
• Covering epithelium
• Loose connective tissue called lamina propria
• Smooth muscle layer muscularis mucosae, with inner circu­
lar and outer longitudinal layers
Submucosa
• Dense irregular connective tissue layer with blood vessels,
nerves, and lymphatic vessels
• Contains submucosal nerve plexus that controls muscularis
mucosae
Muscularis Extema
• Thick, smooth muscle layer inferior to submucosa
• Normally contains an inner circular and an outer longitudi­
nal smooth muscle l ayers
• Myenteric nerve plexus located between inner and outer
muscle layers
• Myenteric nerve plexus controls motility of smooth muscles
in muscularis externa
Serosa
• Thin layer of tissue, mesothelium, that covers the visceral
organs
• Covers abdominal esophagus, stomach, small intestine, and
anterior wall of colon
Adventitia
• Covers thoracic part of esophagus and posterior wall of
ascending and descending colon
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Esophagus
• Soft tube that extends from pharynx to stomach, posterior
to the trachea
• Penetrates diaphragm and enters stomach
• Lumen lined by nonkeratinized stratified squamous epithe­
lium
• In the upper third, muscularis externa contains skeletal
muscle
• In the middle, both smooth and skeletal muscle found in
muscularis externa
• In lower third, muscularis externa contains smooth muscle
• Mucous esophageal glands are present in both the lamina
propria and submucosa for lubrication
• Adventitia surrounds the esophagus in the thoracic cavity
• Muscularis mucosae and submucosa from esophagus con­
tinue with those of stomach layers
Stomach
• Transition from esophagus to stomach is abrupt and from
stratified squamous to simple columnar
• Receives, stores, mixes, and digests ingested food products to
form liquid chyme
• Converts bolus of ingested food into semiliquid mass chyme
• Consists of cardia, fundus, body, and pyloric regions
• Surface pitted by gastric pits, which are connected to gastric
glands in the lamina propria
• Surface is lined by mucus-secreting simple columnar epithe­
lium for protection
• Gastric glands produce gastric juices rich in hydrochloric
acid and protein-digesting enzymes
• Muscularis externa shows internal oblique, middle circular,
and outer longitudinal muscle layers
• Fundus and body form the major regions, and are histolog­
ically identical
• Submucosal and myenteric nerve plexuses regulate peri­
staltic activity
CHAPTER 12 - Digestive System: Esophagus and Stomach
Gastric Glands and Cells
• In fundus and body produce the chemicals for digestion of
stomach contents
• In body and fundus, parietal cells are large, acidophilic, and
are in the upper gland region
• Deeper region of the glands contains chief or zymogen cells
• When contracted or empty, temporary rugae seen in the wall
• In cardia and pylorus, surface epithelium and simple tubu­
lar gastric glands produce mucus
Glands in the pylorus produce mucus and bacteria-destroy­
ing enzyme lysozyme
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• Parietal cells in fundus and body produce hydrochloric acid
and gastric intrinsic factor
• Gastric intrinsic factor essential for absorption of vitamin
B 12 and erythropoiesis
• Chief or zymogen cells produce pepsinogen that is con­
verted to pepsin in acid environment
• Enteroendocrine cells secrete variety of polyp eptides and
proteins for digestive functions
In pylorus, gastric pits are deeper than in fundus or body
• Mucus-secreting stomach cells change to intestinal epithe­
lium in the duodenum