286
UNIT 3
Organ Systems
Esophagus
Submucosa
Muscularis
externa
Mucosa
Mucosa
Mucosa
Lumen
Lumen
Lumen
Esophageal
glands
Muscularis
mucosae
A
Figure 15-4A.
Overview of the esophagus.
H&E,
3
11
The
esophagus
can be divided into three regions: the
upper
,
middle
, and
lower esophagus
. The esophagus is a long tube
that connects the oropharynx to the stomach. Like other parts
of the digestive tract, the esophagus has
mucosa
,
submucosa
,
muscularis externa
, and
adventitia/serosa
. (1) The
mucosa
is
composed of
epithelium
,
lamina propria
, and
muscularis muco-
sae
. The mucosa of the esophagus has folds extending into the
lumen. StratiF ed squamous epithelium covers the inner surface of
the esophagus. The
muscularis mucosae
is composed of a single
layer of longitudinal smooth muscle. (2) The
submucosa
contains
mucous glands called
esophageal glands
, which secrete mucus and
provide lubrication that aids in swallowing. (3) The
muscularis
externa
is composed of two layers of muscle organized into
inner
circular
and
outer longitudinal
layers. (4) The outermost wall of
the esophagus is commonly covered by
adventitia
(±ig. 15-4B),
but the lower esophagus is covered by
serosa
.
Inner circular
Inner circular
(skeletal) muscle
(skeletal) muscle
Inner circular
(skeletal) muscle
Outer longitudinal
Outer longitudinal
(skeletal) muscle
(skeletal) muscle
Outer longitudinal
(skeletal) muscle
Esophageal
Esophageal
glands
glands
Esophageal
glands
Esophageal
Esophageal
glands
glands
Inner circular muscle
Inner circular muscle
Outer longitudinal
muscle
Outer longitudinal muscle
Adventitia
Adventitia
Mucosa
Mucosa
Submucosa
Submucosa
Duct of
Duct of
glands
glands
Duct of
glands
B
Esophageal
glands
Inner circular muscle
Outer longitudinal
muscle
Adventitia
Mucosa
Submucosa
Figure 15-4B.
Upper esophagus.
H&E,
3
17; (
upper
inset)
3
40; (
lower
insets)
3
216
The
upper esophagus
, connecting to the oropharynx, is the F
rst
segment of the esophagus. The muscularis externa of the upper
esophagus contains two layers of
skeletal muscle
. The skeletal
muscle is a voluntary muscle that helps to initiate swallowing.
This muscle is innervated by the glossopharyngeal nerve. The
muscularis externa
of the esophagus has distinguishable muscle
components, which vary in different regions. The upper esopha-
gus has skeletal muscle, the middle esophagus has mixed skeletal
and smooth muscles, and the lower esophagus has only smooth
muscle in the muscularis externa. The muscularis externa plays
an important role in producing contractions of the esophagus
that transport food from the oral cavity to the stomach. The
major movement of the esophagus is
peristalsis
(waves of invol-
untary contraction), as in other parts of the digestive tract. The
initiation of swallowing is voluntary because the upper esophagus
contains only skeletal muscle F bers in the muscularis externa.
Epithelium
Epithelium
Epithelium
Muscularis
Muscularis
mucosae
mucosae
Muscularis
mucosae
Epithelium
Epithelium
Epithelium
Lamina
Lamina
propria
propria
Lamina
propria
Mucosa
M
u
c
o
s
a
Mucosa
Inner circular muscle
Inner circular muscle
Inner circular muscle
Outer longitudinal
muscle
Outer longitudinal muscle
Outer longitudinal
muscle
Submucosa
Submucosa
Submucosa
Smooth muscle
Smooth muscle
Smooth muscle
Skeletal
Skeletal
muscle
muscle
Skeletal
muscle
C
Figure 15-4C.
Middle esophagus.
H&E,
3
34; insets (
left
)
3
208, (
right
)
3
116
The
mucosa
of the esophagus is covered by a thick layer of
nonkeratinized strati±
ed squamous epithelium
, which refl ects
its function in resisting abrasion and friction. The epithe-
lium of the esophagus is continuously renewed by basal cells
that migrate and differentiate from the basal layer. Most of
the digestive tract is covered by
simple columnar epithelium
;
only the two ends (esophagus and anal canal) are covered by
stratiF
ed squamous epithelium. The
muscularis externa
of the
middle esophagus
contains mixed
skeletal
and
smooth muscle
± bers
, which are organized in inner circular and outer longitu-
dinal muscle bundles. Each skeletal muscle F
ber is larger, has
multiple nuclei
positioned peripherally, and stains darker than
the smooth muscle F
ber. Each smooth muscle F
ber has a
single
nucleus
located in the center of the muscle F
ber.
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