Histology of Esophagus
The
esophagus, like other parts of the gastrointestinal tract, is made up of a mucosa,
a submucosa, a muscularis externa, and an adventitia. The
mucosa is subdivided into epithelium, lamina propria, and muscularis mucosae.
The epithelium that lines the lumen of the esophagus is stratified squamous
epithelium, which is continuous with the epithelial lining of the pharynx.
The surface cells of this epithelium are flattened and contain a few
keratohyaline granules, but do not form a keratinized layer. An abrupt
transition takes place between the stratified squamous epithelium of the
esophagus and the simple columnar epithelium of the stomach along an irregular
zigzag line, known as the Z line, situated slightly superior to the
cardiac region of the stomach. The lamina propria is a layer of loose
connective tissue deep to the epithelium. The muscularis mucosae is a
thin layer of smooth muscle deep to the lamina propria and is continuous with
the pharyngeal aponeurosis. A transition from connective tissue to muscular
tissue takes place in this aponeurosis at about the level of the cricoid
cartilage. It contains both longitudinal smooth muscle fibers and some
elastic tissue and is thicker at the lower end of the esophagus.
The submucosa is formed by
dense irregular connective tissue and contains both elastic and type I collagen
fibers, as well as blood vessels and nerves supplying the mucosa. Lymphocytes
are scattered in moderate numbers through both the lamina propria and the submucosa,
and occasionally these may be found in isolated concentric groups. In its
contracted state, the esophageal mucosa is thrown into irregular longitudinal
folds. The submucosa extends into these folds, but the more superficial
muscular layers do not.
The muscularis externa consists
of an inner circular layer and an outer longitudinal layer. A
thin layer of connective tissue exists between the two layers, in which is
embedded the myenteric ganglia and plexus (of Auerbach). Between the
muscularis externa and the submucosa are the submucosal ganglia and plexus (of
Meissner) and several blood vessels. The musculature of the upper one fourth of
the esophagus is generally striated in character, the second fourth contains
both striated and smooth muscle, and the lower half is composed entirely of
smooth muscle. The adventitia is a layer of loose connective
tissue on the surface of the esophagus that anchors the organ to the
surrounding structure.
Two types of glands can be
recognized in the esophagus. The esophageal glands proper (of Brunner)
are irregularly distributed throughout the entire length of the tube. They are
small, compound mucous glands. Their ducts penetrate the muscularis mucosae and
their branched tubules lie in the submucosa. They are some what more prominent
in the superior part of the esophagus. The other type of glands is known as the
esophageal cardiac glands because they closely resemble or are identical
with the cardiac glands of the stomach. They are found just above the cardiac
region of the stomach, in the distal esophagus. They are also occasionally
found proximally, a few centimeters below the level of the cricopharyngeus
muscle. They differ from the esophageal glands proper in that their ducts do
not penetrate the muscularis mucosae and their branched and coiled tubules are
located in the lamina propria, not in the submucosa.
