Download GASTROINTESTINAL SYSTEM I

GASTROINTESTINAL SYSTEM (GI Tract).
Esophagus and Stomach
Roger Bick, Ph.D., MMEd
Learning Objectives:
 Understand the organization of the four layers of the Gastrointestinal tract.
 Describe the 4 layers of the esophagus, with emphasis on the epithelium and the location of the
gland types in the underlying layers.
 Describe unique features of the tunica muscularis of the esophagus and stomach.
 List histologic characteristics common to glands throughout the stomach.
 Name gland types in the different zones of the stomach.
 List their differences based on pit and gland length. gland shape, cell types, and function.
 List cell types, found in gastric glands, and know the functions of each.
Key Words: Tunica mucosa, lamina propria, submucosa, muscularis, adventitia, serosa, Meissner’s
plexus, Auerbach’s plexus, esophageal and mucosal glands, parietal cells and chief cells.

General organization
The gastrointestinal tract (digestive tract), is a tube and associated glands and organs from the
epithelium of the primitive gut.
(The following notes refer only to the esophagus, stomach, small intestine, large intestine, appendix
and anal canal).
FOUR BASIC LAYERS:
Tunica Mucosa
Tunica Submucosa
Tunica Muscularis
Tunica Adventitia (or Serosa)
It will be easier to study the specific regions if you remember that the basic composition of the each
of the four layers reflects the function of each region.


Tunica Mucosa. Epithelium with basement membrane, lamina propria and muscularis mucosa.
Epithelium and basement membrane. The type of epithelium varies according to local
functions. Esophagus is flexible to accommodate fluids and solid food while protecting the
underlying tissue from temperature and abrasion. The stomach must be protected from its own
acid.
The intestines must have a high absorptive capacity, so in many regions the epithelium gives rise
to ducts and glands that are deeper in the wall of the GI tract.

Lamina propria. Loose connective tissue usually with large numbers of immune cells, a rich
vascular supply and Lymphatic vessels and glands may be present.

Muscularis mucosa. A very thin layer of smooth muscle close to the mucosa, with both an inner
circular and outer longitudinal layer which are often difficult to identify.

Tunica Submucosa. Under the mucosa and is loose connective tissue, and much less cellular
than the mucosa. Contains larger blood vessels and lymphatics, often glands and an important
network of nerves and ganglia belonging to Meissner's plexus. The submucosa act as padding
for the mucosa and together they sometimes throw the inner surface into permanent folds.

Tunica Muscularis/ Muscularis Externa. The main bulk of muscle for the alimentary canal.
With a few exceptions it consists of smooth muscle with inner circular and outer longitudinal
layers. Between these layers is another network of nerves and ganglia known as Auerbach's
plexus. The significance of Meissner's and Auerbach's plexuses will be discussed later. The
muscularis is responsible for peristalsis of the alimentary canal.

Tunica Adventitia or Serosa. Connective tissue always surrounds the muscularis, padding and
protecting it. When facing the peritoneal cavity, i.e. distal esophagus, stomach, jejunum, ileum
and parts of the large intestine, this connective tissue is encased by a thin layer of simple
squamous epithelium known as the mesothelium which, when it forms the outermost layer of the
gut+ connective tissue+ mesothelium is known as the serosa. The moist serosa performs an
important function, allowing adjacent portions of the GI to come into contact and to slide over
each other. In non-peritonealized or retroperitoneal surfaces, such as thoracic esophagus,
duodenum and ascending and descending colon there are some areas that don’t have a serosa,
only connective tissue. Here the outer layer is the adventitia.

The Enteric Nervous System (ENS) consists of nerve cell bodies and their processes in the wall of
the gut. The nerve cell bodies are found singly or in groups in two major plexuses;
 Meissner's or Submucosal Plexus, in the submucosa mainly in the small and large intestines.
 Auerbach's or Myenteric plexus, between the inner circular and outer longitudinal layers of the
muscularis externa. The ENS controls peristaltic movement by contracting and relaxing the
muscularis mucosa and the muscularis externa. It is considered to be a part of the autonomic
nervous system, though it is separate, it is modulated by the sympathetic and parasympathetic
systems. The ENS has as many neuron cell bodies here as in the spinal cord.

Enteroendocrine System: A number of small cells located in various parts of the G1 tract, which
have a several names – APUD cells, chromaffin, argentaffin, and neuroendocrine cells. Dispersed
among epithelial cells from the stomach distally. These are endocrine cells that secrete products
across the basement membrane into the underlying capillaries. Their placement lets them respond to
stimuli in luminal solutions, with production of amines and.

Digestive tract enteroendocrine system in general: Serotonin and substance P – increase gut
motility; Vasoactive Intestinal polypeptide. – ion and water secretion, increase gut motility.

Stomach: Rennin – aspartic proteinase (curdles milk); Gastrin - stimulates production of
hydrochloric acid; Histamine – stimulates HCl secretion.

Gut: Somatostatin – inhibition of endocrine and exocrine secretions.

Small Intestine - Pyloric segment:
Gastrin – stimulates production of hydrochloric acid
Somatostatin – inhibits release of gastrin when duodenal contents become acidified.
Gastric Inhibitory polypeptide (Urogastrone from K cells) - Released in response to glucose, amino
acids and fatty acids in the gut lumen. Stimulates insulin secretion from pancreatic islets.
Lysozyme - bactericide
Tumor necrosis factor – α (TNF-α). (Pro)inflammatory factor.
Defensins -Increase permeability in bacteria and parasites by forming ion channels.

Small intestine the rest:
Gastrin - stimulates gastric acid secretion, gastric mucosal growth
Secretin – alkaline pancreatic and biliary water secretion.
Cholecystokinin – duodenum and jejunum – triggered by small peptides, amino acids and fat in the
small intestine – stimulates gall bladder contraction and secretion from pancreatic acinar cells.
Motilin – stimulates gastrointestinal motility.
Glucagon like peptide (L cells). Released in response to glucose, amino acids and fatty acids in the
gut lumen. Stimulates insulin secretion and inhibits glucagon secretion.
ESOPHAGUS.
GENERAL STRUCTURE. Thick muscular tube with numerous folds that allows distension for passage
of fluid and solids to the stomach. Mucous glands and muscular coats facilitate passage.
Cross section. Muscular, between pharynx and stomach, mostly intrathoracic. Empty esophagus is
collapsed and thrown into longitudinal folds, but has the same basic GI structure with important
differences.

Mucosa:
 Epithelium - Thick non-keratinizing stratified squamous epithelium able to withstand abrasion
but also flexible to accommodate food boli.
 Lamina propria - Very thin layer of loose connective tissue with lymphocytes clusters. At the
distal junction with the stomach, mucus-secreting glands, cardiac glands, are found.
 Muscularis mucosa - Mostly longitudinally-oriented smooth muscle bundles.

Submucosa - Mucus secreting glands found in the submucosa, submucosal glands or esophageal
glands proper. Can be distributed through all levels of the esophagus. In the distal esophagus veins
anastomose with the portal vein.

Muscularis - The proximal one third to one quarter is skeletal muscle that gradually changes to
smooth muscle. The distal one third of the esophagus is smooth muscle. The inner circular muscle
layer of the distal esophagus is tonically contracted, preventing reflux of gastric contents.

Adventitia/serosa –is the outermost layer of the thoracic esophagus; loose connective tissue that
blends into surrounding tissues and is therefore an adventitia. After passing through the
diaphragm, mesothelium covers the connective tissue, therefore a serosa.

At the junction with the stomach is the lower esophageal sphincter controlling passage of material
from esophagus into the stomach.
THE STOMACH.
GENERAL FEATURES - major function is food storage, mixing, and acidic breakdown for production
of a semisolid paste known as chyme. Anatomically subdivided into four zones – cardia (C), fundus (F),
body (B), and pylorus (P).

Mucosa comprised of:
 Epithelium
 Gastric glands
 Lamina propria
 Muscularis mucosae

Submucosa comprised of:
 Loose connective tissue.
 Rich vascular supply.

Tunica Mucosa.
Stomach mucosa is very complex with differences seen in different zones. The underlying submucosa
throws stomach into folds (rugae). Surface is covered with a thick protective layer of mucus. Gastric
pits and long gastric glands packed together making this tunic very thick and complex to study.
 Epithelium
The epithelium lining the surface pits and plus cells in the glands are renewed by undifferentiated
stem cells in the neck of each gland. Neck cells secrete alkaline mucus for protecting the mucosa
from the high acidity of the stomach contents. The gastric pits branch into 5-8 tubular glands.
 Lamina propria is difficult to delineate as branching of the glands, crowding of cells and
complexity of the glands tends to obscure the lamina propria.
 Muscularis mucosa. Found at the base of the glands.

Tunica Submucosa. Characteristic loose connective tissue with coarse collagen bundles.
Increased thickening of this layer forms the rugae of the mucosa. Nerve supply includes
Meissner’s plexus

Muscularis externa: Three layers of smooth muscle.
1. Inner oblique.
2. Middle circular.
3. Outer longitudinal
4. Auerbach’s plexus is between the two layers.

Serosa.

GASTRIC MUCOSA.
 Very complex with differences seen in various gastric zones.
 Glands invaginate from surface deep into the lamina propria.
 Surface mucous cells produce the protective layer of mucous, as well as bicarbonate ions, to
protect stomach surface.

GASTRIC GLANDS.
 Neck Region - The top (luminal) portion of every gland opens into a gastric pit; this junction is
known as the neck of the gland. This is the beginning of the gastric gland and is characterized by
having a mixed population of columnar mucous neck cells and stem cells.

Cell Types.



Stem Cells: Undifferentiated cells that will give rise to the other cell types in the glands and
the gastric pits every 5-7 days.
Mucous neck cells. Secrete mucous.
Parietal cells (Oxyntic cells). Secrete the following:
 Hydrochloric acid into gland lumen. Action of carbonic anhydrase and Na+ K+ ATPase
for H+ and Cl- secreted at the same time.
 HCO3- into blood stream. Causing a rise in pH.
 Gastric intrinsic factor. Essential for absorption of vitamin B12.
 Light pink cells showb below

Chief Cells. Secrete inactive pepsinogen that becomes pepsin when in the acidic environment of
the stomach lumen. Darker cells in this image
Parietal and Chief cells

Enteroendocrine cells.
 G cells. In response to meals secrete gastrin. Gastrin stimulates parietal cells to secrete
hydrochloric acid.
 EC cells. Secrete serotonin and histamine. Serotonin increases gut motility. Histamine
stimulates HCl secretion.
 D cells. Secrete somatostatin that inhibits secretion of enteroendocrine cells in the region.
 A cells. Secrete glucagon that stimulates glycogen degradation in the liver and increases
blood glucose levels.
_____________________________________________________________________________________
DIGESTIVE SYSTEM I LABORATORY.
Begin by examining your slides with the naked eye, holding them against a white background to see gross
differences. Then use low power to make sure you have a complete cross section or just a piece of the
wall. The innermost mucosal layer will often appear thicker and more basophilic than the outermost layer,
and will frequently be thrown into folds. Be able to identify every structure that is in bold font.
THE ESOPHAGUS.
Proximal Esophagus Slide 8. Cross section- Identify the four layers.
Tunica Mucosa - stratified squamous non-keratinized epithelium, lamina propria, and muscularis
mucosa. The function of the epithelium in protection against mechanical damage dictates that it is thick,
and therefore unsuitable for allowing migration of lymphocytes. Some lymphocytes and other immune
cells might be found in the epithelium but many less than in other parts of the GI tract. The majority of
cells in the lamina propria are fibroblasts. The muscularis mucosa usually appears to be a discontinuous
layer, but more like discrete bundles of longitudinal smooth muscle.
The submucosa is loose connective tissue with numerous medium-sized blood vessels and collagen
bundles. Mucus secreting esophageal glands are found here; you should be able to distinguish glands
from ducts (lined with simple or stratified cuboidal epithelium). The ducts can sometimes be seen
merging with surface epithelium for emptying mucous onto the surface. Meissner's plexus can
occasionally be seen here. Occasional lymphatic nodules may also be found.
Muscularis externa is completely striated muscle - rapid passage of the food through this portion.
Striated in proximal one third of the esophagus. Find Auerbach's plexus.
The outermost layer an adventitia.
Distal Esophagus - Esophageal-Cardiac Junction Slide 68
Same basic feature as previous slide, but important differences.
 Look for thick stratified squamous non-keratinized epithelium of the esophagus stopping abruptly
at the junction with the stomach.
 Look for prominent veins to the submucosa and lamina propria.
 Find cardiac glands in the lamina propria.
 Note thick muscularis mucosa.
 Check the muscle composition of the inner circular and outer longitudinal layers of the
muscularis externa. What is it?
 There should be a serosa covering this portion but in many slides it has been lost during
preparation.
THE STOMACH.
Cardiac Stomach Slide 68.
Find the following:
 Surface epithelium made of surface mucous cells (NOT goblet cells).
 Long pits, going at least halfway down the mucosa and frequently branching.
 Coiled, mucous-secreting pyloric glands; find the neck region and parietal and chief cells.
 Lamina propria containing numerous plasma cells and maybe eosinophils.
 Well defined muscularis mucosa.
 Thick acellular submucosa of loose connective tissue.
 Inner oblique, middle circular, outer longitudinal layers of the muscularis externa.
 Auerbach's plexus.
 Serosa with intact mesothelial lining.
Fundus/ Body of Stomach, Slide 69.Just have a quick look, but notice difference in gland length to
#68
 Surface epithelium of surface mucous cells.
 Long pits, going at least halfway down the mucosa and frequently seen branching.
 Lamina propria containing numerous plasma cells.
 Well defined muscularis mucosa.
 Thick open submucosa of loose connective tissue.