CHAPTER 15
Digestive Tract
281
and
submucosal
(
Meissner
)
plexuses
, which contain nerve
f bers and neurons oF the enteric nervous system. In some
regions oF the digestive tract, this layer is characterized by
mucous glands or lymphatic nodules.
3.
Muscularis externa
is composed oF two or three oblique,
circular, and longitudinal muscle layers, which vary From
region to region. Most oF the muscularis externa consists
oF smooth muscle f bers, but the upper and middle esophagi
contain some skeletal muscle. The
myenteric
(
Auerbach
)
plexuses
(nerve f bers and neurons oF the enteric nervous sys-
tem) are located between the muscle layers. They innervate
and control contraction oF the muscularis externa.
4.
Serosa and adventitia
are coverings oF the outermost wall
oF the digestive tract. Most parts oF the digestive tract are
covered by
serosa
, a thin layer oF loose connective tissue
lined by
mesothelium
. The mesothelium produces a lubri-
cating fl
uid that reduces Friction during movement oF the
organs against each other (see Chapter 3, “Epithelium and
Glands,” ±ig. 3-2A,B). The serosa is the visceral layer oF the
peritoneum and covers the wall oF the digestive tract where
it connects to the mesentery in the peritoneal cavity (intrap-
eritoneal organs). The
adventitia
is a layer oF loose connec-
tive tissue without mesothelium that covers the upper region
oF the esophagus, part oF the duodenum, and the lower part
oF the digestive tract, such as the rectum and anal canal.
Adventitia covers regions oF the digestive tract where it is
connected to other organs or to the body wall (e.g., retro-
peritoneal organs).
Esophagus
The
esophagus
is the upper part oF the digestive tract, con-
necting the oral cavity to the stomach. The major Function oF
the
esophagus is to provide passage For Food From the mouth
to the stomach. The luminal surFace oF the esophagus is lined
by nonkeratinized stratif ed squamous epithelium. Mucous
glands called
esophageal glands
are located in the submucosa
oF the esophagus. The muscularis externa consists oF two lay-
ers oF muscle: inner circular and outer longitudinal layers. Both
skeletal and smooth muscle f bers are Found in the muscularis
externa oF the esophagus. The proportions oF skeletal and
smooth muscle f bers are diFFerent in diFFerent regions oF the
esophagus. The esophagus can be divided into three regions:
the
upper esophagus
,
middle esophagus
, and
lower esophagus
(±igs. 15-4A to 15-5A).
1.
The
upper esophagus
connects the oropharynx to the mid-
dle esophagus. This segment contains numerous esophageal
glands in the submucosa. These glands secrete mucus to
lubricate the esophageal wall so that Food will pass through
easily. The upper esophagus contains only skeletal muscle
f bers in the
muscularis externa. These are voluntary muscle
f bers and are innervated by the
glossopharyngeal nerve
(
cra-
nial nerve
[
CN
]
IX
) (see ±ig. 15-4B).
2.
The
middle esophagus
has mucosa similar to that oF the
upper esophagus. The esophageal glands in the submucosa
are less numerous than in the upper esophagus. The mus-
cularis externa contains both skeletal and smooth muscles
(±ig. 15-4C).
3.
The
lower esophagus
connects the esophagus to the car-
dia oF the stomach. This region contains large numbers oF
mucous glands in the lamina propria and submucosa. These
are called
esophageal cardiac glands
and produce mucous
secretions to protect the lower esophagus From being dam-
aged by refl
ux oF acidic gastric juices From the stomach.
The lower esophagus contains only smooth muscle f
bers in
the muscularis externa. These are controlled by the enteric
branches oF the
vagus nerve
(
CN X
) (see ±ig. 15-5A).
Stomach
The
stomach
is a “J”-shaped sac (hollow) organ. It temporarily
stores Food, mixes Food with gastric juice, and initiates the pro-
cessing oF Food by breaking it down into simpler substances that
are easier to digest. The stomach can be divided into the
cardia
,
fundus
,
body
, and
pylorus
. The inner surFace oF the stomach
is lined by simple columnar epithelium composed mainly oF
surFace mucous cells. The surFace epithelium oF the stomach is
invaginated into the lamina propria to Form
gastric pits
. These
pits serve as ducts For the glands in the lamina propria, which
vary From region to region in the stomach.
1.
The cardiac region
connects to the lower esophagus at the
esophagogastric junction, which is characterized by a change
From the nonkeratinized stratif
ed squamous epithelium oF
the esophagus to the simple columnar epithelium oF the
stomach. A thickened smooth muscle ring called the
gastroe-
sophageal sphincter
(
lower esophageal sphincter
) or
cardiac
sphincter
surrounds the opening at the junction oF the lower
esophagus and cardiac region oF the stomach. This smooth
muscle contracts to prevent the acidic stomach contents From
entering the esophagus. The glands in the lamina propria oF
the cardia are called
cardiac glands
and are branched tubu-
lar glands with coiled secretory portions. The cardiac gland
contains mainly
mucus-secreting cells
and some
stem cells
,
enteroendocrine cells
, and, occasionally,
parietal cells
. The
mucus-secreting cells mainly produce mucus and lysozymes.
The mucus protects the stomach wall From acidic gastric
juices; lysozymes destroy bacterial membranes, preventing
bacterial inFections (±ig. 15-7A).
2.
The
fundic and body regions
Form the largest portions oF the
stomach. Their mucosa has similar histological characteris-
tics, including short gastric pits and long branched tubular
glands in the lamina propria. The glands are called
fundic
or
gastric glands
in both the Fundus and the body regions. The
gastric glands contain mainly
parietal cells
and
chief cells
,
along with some
stem cells
,
mucous neck cells
, and
enteroen-
docrine cells
.
Parietal cells
are more numerous in the superior
regions oF the glands; these cells produce large quantities oF
hydrochloric acid
(
HCl
), creating an acidic environment to
help digestion.
Parietal cells also secrete
intrinsic factor
(
IF
),
which is required For the absorption oF
vitamin B12
.
Chief
cells
are located in the more inFerior regions oF the glands;
they secrete precursor enzymes such as
pepsinogen
, which is
activated by HCl and becomes pepsin. Pepsin helps to break
down proteins (particularly protein collagen) into simpler,
more absorbable compounds. ChieF cells also secrete precur-
sors oF
lipases
, which help in lipid digestion (±ig. 15-7B,C).
3.
The
pyloric region
is the lower end oF the stomach, which
connects with the duodenum. Its mucosa is similar to that oF
the cardia, with long gastric pits and short, coiled secretory
portions. A
circular smooth muscle ring called the
pylorus
sphincter
(
pyloric valve
) surrounds the end oF the pylorus
region. This valve controls the entry oF stomach contents
into the duodenum. The glands in the lamina propria oF
the pylorus are called
pyloric glands
and contain primarily
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