Department of Anatomy & Histology
The esophagus :
The esophagus is the upper part of the digestive tract,
connecting 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
non keratinized stratified squamous epithelium. Mucous glands
called esophageal glands are located in the submucosa of the
esophagus. The muscularis externa consists of two layers of
muscle: inner circular and outer longitudinal layers. Both
skeletal and smooth muscle fibers are found in the muscularis
externa of the esophagus. The proportions of skeletal and
smooth muscle fibers are different in different regions of the
esophagus. The esophagus can be divided into three regions:
1. The upper esophagus connects the oropharynx to the middle
esophagus. This segment contains numerous esophageal glands
in the submucosa. They are small, compound, tubuloalveolar
glands. The upper esophagus contains only skeletal muscle
fibers in the muscularis externa .
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 muscularis
externa contains both skeletal and smooth muscles.
3. The lower esophagus connects the esophagus to the cardia 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 damaged by reflux of
acidic gastric juices from the stomach .The lower esophagus
contains only smooth muscle fibers in the muscularis externa.
In the esophagus , the muscularis mucosa are usually a single
layer of longitudinal smooth muscle fiber , whereas in the
stomach a second layer of smooth muscle is added ,called inner
circular layer .
The outermost layer around the esophagus is the connective
tissue adventitia with adipose tissue, nerve and blood vessels.
Mucosal and submucosal glands of the esophagus secrete
mucus to lubricate and protect the luminal wall and facilitates
smooth passage of bolus through the esophagus to the stomach .
The Stomach :
The stomach is a “J”-shaped sac (hollow) organ or a dilated,
hollow organ of the digestive tube that is situated between the
esophagus and the small intestine. Anatomically, the stomach is
divided into the cardia, fundus, body or corpus, and pylorus.
Because the fundus and body have identical histology, the
stomach has only three histologically distinct regions.
The cardia, the most superior region in the stomach, surrounds
the entrance of the esophagus into the stomach. The fundus and
body form the major portions of the stomach; their mucosa
contains deep gastric glands that produce most of the gastric
secretions or juices for digestion of food. The pylorus, the most
inferior region in the stomach, ends at the border of the
duodenum of the small intestine.
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 stratified squamous epithelium of the
esophagus to the simple columnar epithelium of the stomach.
A thickened smooth muscle ring called the gastroesophageal
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 tubular glands with
coiled secretory portions. The cardiac gland contains mainly
mucus-secreting 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 .
2. The fundic and body regions form the largest portions of the
stomach. Their mucosa has similar histological characteristics,
including short gastric pits (are not deep and extend into the
mucosa about one fourth of its thickness) 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,
mucous neck cells, and enteroendocrine cells.
Parietal cells (stain acidophilic - pink) 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), In humans, the parietal cells produce gastric
intrinsic factor, aglycoprotein that is necessary for vitamin B l2
absorption in the small intestine. Vitamin Bl2 is necessary for
red blood cell production in the red bone marrow; deficiency of
this vitamin leads to anemia.
Chief (zymogenic) 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 into simple more
absorbable compounds . Chief cells also secrete precursors of
lipases, which help in lipid digestion .
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 (extend into the mucosa to
about one half or more of its thickness) and short, coiled
secretory portions. A circualar 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
mucus-secreting cells and the enteroendocrinen cells, These
enteroendocrine cells regulate gastric (HCL) secretion.
Layers of the stomach.
This image shows the wall of the
body of the stomach at low
power which identify the three
major layers seen here - the
mucosa, submucosa and muscularis
externa.
The mucosa is full of gastric glands
and pits, and there is a prominent layer of smooth muscle - the muscularis
mucosa. The contraction of this muscle helps to expel the contents of the
gastric glands.
The mucosa of the empty stomach exhibits temporary folds called rugae .
Rugae are formed during the contractions of the smooth muscle layer,the
muscularis mucosa .As the stomach fills ,the rugae disappear and form
sooth mucosa.
The muscularis externa layer has three layers of muscle. An innner
oblique layer , a middle circular and an external longitudinal layer. The
outer layer of the stomach is covered by the serosa.
Food can stay in the stomach for 2 hours or more. Food is broken down
chemically, by gastric juice, and mechanically, by contraction of the
three layers of smooth muscle in the muscular externa layer. The broken
up food at the end of this process is called chyme.