The University of Western Australia

Department of Anatomy and Human Biology

HA235 - Histology - Gastrointestinal Tract

Topics Lab Guides and Images
Stomach Stomach - Cell Types - H&E
Small Intestine Duodenum - H&E
Jejunum and Ileum - H&E, Paneth cells
Large Intestine Colon - H&E
Junctions Ano-rectal Junction - van Gieson

Recommended readings from Histology, A Text and Atlas, M.H. Ross et al, 3rd edition

GASTROINTESTINAL TRACT

The gastro intestinal tract (GIT) comprises the stomach, duodenum, jejunum, ileum, colon, rectum and anal canal. The GIT and oesophagus form the alimentary canal. The basic structure of the walls of the alimentary canal has been described already (see HA235 - Histology - Oral Cavity, Oesophagus).

The Stomach

The stomach functions both as a reservoir and as a digestive organ. It empties its contents in small portions (suitable for continued digestion) into the small intestine.

Anatomically, the stomach is divided into

Histologically, most of the layers of the wall of the stomach appear similar in its different parts. Regional differences are mainly restricted to the appearance of the gastric mucosa.

The Mucosa (epithelium, lamina propria, muscularis mucosae)

The mucosa is thrown into longitudinal folds (gastric folds or rugae), which disappear when the stomach is fully distended. A network of shallow grooves divides the mucosa into gastric areas (1-5 mm). On the mucosal surface we see small, funnel-shaped depressions (gastric pits). Almost the entire mucosa is occupied by simple, tubular gastric glands which open into the bottom of the gastric pits.

The structure and cellular composition of the surface epithelium (simple, tall columnar) does not change throughout the stomach. It contains mucus-producing cells, which form a secretory sheath (glandular epithelium). The mucus is alkaline and adheres to the epithelium. The mucus forms an ~ 1 mm thick layer, which protects the mucosa from the acidic contents of the stomach. The surface epithelium is renewed approximately every third day. The source of the new cells is the isthmus, i.e. the upper part of the neck, of the gastric glands, where cells divide and then migrate towards the surface epithelium and differentiate into mature epithelial cells.

In contrast to the surface epithelium, cellular composition and function of the gastric glands are specialized in the different parts of the stomach.

Cardiac glands

Cardiac glands are heavily branched tubular glands (similar to the cardiac glands of the esophagus), which contain mainly mucus-producing cells. A few of the secretory cells characteristic for the corpus-fundic glands (see below) may be present.

Corpus-fundic (or principal) glands

Each glandular tubule (oriented more or less perpendicular to the surface of the epithelium) consists of three parts: a deep body, an intermediary neck and an upper isthmus. In corpus-fundic glands we find four cell types: chief cells, parietal cells, mucous neck cells and endocrine cells.

Chief cells (or zymogenic cells)

are the most numerous of the four types. They occur primarily in the body of the glands. They produce pepsinogen, which is a precursor of the proteolytic enzyme pepsin.
The pH optimum of of pepsin is about 2. This enzyme is able to break down collagen.

Parietal cells (or oxyntic cells)

occur most frequently in the neck of the glands, where they reach the lumen of the gland. They are situated deeper, between and below chief cells, in lower parts of the gland. Parietal cells secrete the hydrochloric acid of the gastric juice. Aside from activating the pepsinogen the hydrochloric acid also effectively sterilizes the contents of the stomach.

Note that so far only one type of bacteria has found which can live happily in the stomach - Helicobacter pylori. Unfortunately these bacteria are involved in the pathogenesis of gastritis and gastric ulcers.

Parietal cell also secrete intrinsic factor, which is necessary for the resorption of vitamin B12.

Vitamin B12 is a cofactor of enzymes which synthesise tetrahydrofolic acid, which, in turn, is needed for the synthesis of DNA components. An impairment of DNA synthesis will affect rapidly dividing cell populations, among them the haematopoietic cells of the bone marrow, which may result in pernicious anemia. This condition may result from a destruction of the gastric mucosa by e.g. autoimmune gastritis or the resection of large parts of the lower ileum, which is the main site of vitamin B12 absorption, or of the stomach.

Mucous neck cells

are found between the parietal cells in the neck of the gland.
They are difficult to distinguish from chief cells in plain H&E stained section.

Endocrine cells

Endocrine cells are scattered, usually solitary, throughout the epithelium of the gastro-intestinal tract. They are part of the gastro-entero-pancreatic (GEP) endocrine system. The best characterized endocrine cells in the gastric mucosa are gastrin-producing cells (G cells) and somatostatin-producing cells (D cells). G cells are most frequent in the middle third of the glands. They stimulate the secretion of acid and pepsinogen. G cell function is stimulated by nervous input, the distension of the stomach or secretagogues. D cells are found mainly in glands of the pyloric antrum. They inhibit G cells and thereby acid production. D cell function is stimulated by acid in the lumen of the stomach and duodenum.

Other types of endocrine cells encompass VIP-producing cells (or D1 cells; vasoactive intestinal peptide) and serotonin-containing cells (enterochromaffin cells). Endocrine cells in the gastrointestinal tract are alternatively named APUD-cells: amine precursor uptake and decarboxylation cells.

Pyloric glands

Pyloric glands are more coiled than corpus-fundic glands, and they may be branched. Endocrine cells, in particular gastrin-producing cells, are more frequent than in corpus-fundic glands. A few parietal cells may be present but chief cells are usually absent.

The lamina propria is formed by a very cell-rich loose connective tissue (fibroblasts, lymphocytes, plasma cells, macrophages, eosinophilic leucocytes and mast cells). The muscularis mucosae of the stomach contains both circular and longitudinal layers of muscle cells. Its organization is somewhat variable depending on the location in the stomach.

Large blood vessels, lymph vessels and nerves are located in the submucosa which consists of loose connective tissue.

Note that the muscularis externa consists of three layers of muscles: an inner oblique layer, a middle circular layer and an outer longitudinal layer.

Lab: Slides STOMACH CAT H&E. It is helpful to find a spot of the gastric mucosa where the glands have been cut along their long axis.
Draw part of the stomach wall, and identify in your drawing the mucus-producing cells of the secretory sheath, the light, undifferentiated looking mucous neck cells, the slightly pink, fairly large parietal cells and the somewhat darker bluish-violet chief cells.


Small Intestine

The small intestine is divided into duodenum (25-30 cm), jejunum (about first two-fifths of the rest) and ileum. The three segments merge imperceptibly and have the same basic histological organization.

The Mucosa

The mucosa of the small intestine has various structural features which considerably increase the luminal surface area and consequently support the main function of the small intestine - the absorption of the degraded components of the food.

Plicae circulares (of Kerkering) are macroscopically visible, crescent-shaped folds of the mucosa and submucosa. Plicae circulares extend around one-half to two-thirds of the circumference of the lumen of the small intestine.

Plicae circulares

The entire intestinal mucosa forms intestinal villi (about one mm long), which increase the surface area by a factor of ~ ten. The surface of the villi is formed by a simple columnar epithelium. Each absorptive cell, or enterocyte, of the epithelium forms numerous microvilli (1 µm long and about 0.1 µm wide), which increase the surface area by a factor of ~ 20.

Between the intestinal villi we see the openings of simple tubular glands, the crypts of Lieberkühn. They extend through the lamina propria down to the muscularis mucosae. Undifferentiated cells close to the bottom of the crypts regenerate the epithelium (epithelial cell turnover time is less than one week). Other epithelial cells in the crypts correspond largely to those in the epithelium of the intestinal villi. One exception are Paneth cells which are located at the bottom of the crypts. They release a number of anti-bacterial substances, among them lysozyme, and are thought to be involved in the control of infections.

One function of the crypts of Lieberkühn is the secretion of "intestinal juice" (about 2 liter/day), which in its composition closely resembles extracellular fluid and which is rapidly reabsorbed. The only enzymes which can be demonstrated in the intestinal juice are enteropeptidase (or enterokinase), which activates the pancreatic enzyme trypsin, and small amounts of amylase. In addition to enterocytes, the epithelium is composed of mucus-secreting goblet cells and endocrine cells.

The lamina propria is, similar to the lamina propria of the stomach, unusually cell rich. Lymphocytes often invade the epithelium or form solitary lymphoid nodules in the lamina propria. Lymph nodules may form longitudinal aggregations of 30-50 nodules in the lamina propria of the ileum. These large arggregations are called Peyer's patches.

The muscularis mucosae has two layers and extends into the intestinal villi, where the smooth muscle cells form a longitudinal bundle in the centre of the villi.

The Submucosa

The submucosa contains glands only in the duodenum. Submucosal glands of the duodenum are also called Brunner's glands. Their secretion is mucous and slightly alkaline due to bicarbonate ions (pH 7-8). The amount of bicarbonate is however too low to neutralize the acidic contents of the duodenal lumen. Instead, the secretion of Brunner's glands protects the duodenal mucosa - similar to the mucus which protects the gastric mucosa.

In addition to gastrin- and somatostatin-producing cells, we also find endocrine cells secreting cholecystokinin and secretin. Cholecystokinin stimulates the secretion of digestive enzymes in the pancreas and the contraction of the gall bladder. Secretin stimulates the pancreas to release "pancreatic juice", which is rich in bicarbonate ions. Secretin also amplifies the effects of cholecystokinin.

Lab: Slide DUODENUM H&E. Take a close look at the epithelium lining the villi and crypts of the duodenum, and note the absence of plicae circulares. Where in the duodenum are we? You will see this tall columnar epithelium which is composed of enterocytes, goblet cells and endocrine cells throughout the remainder of the GIT. Now identify the lamina propria and muscularis mucosae and the "packages" of glandular tissue (Brunner's glands) in the connective tissue between the muscularis mucosae and muscularis externa, i.e. in the submucosa. Occasionally you will see ducts of Brunner's gland which penetrate the muscularis mucosae and ascend through the lamina propria. Note that goblet cells are absent from these ducts.
You will not be able to identify endocrine cells in the H&E stained sections.
Draw a section of the duodenal wall in which these structures are visible, and identify them in your drawing.


Lab: Slides JEJUNUM H&E and ILEUM H&E. Have a "no-scope" look at the slides. Why is this a puzzling set of slides? Try to explain to yourself why the slides may look the way they look. Now have a proper look at both slides and identify surface epithelium, villi, crypts and the layers of the walls. The muscularis externa may not be present around all of the outer surfaces - artefact! Identify lymphoid tissue in both the jejunum and ileum. Note that the accumulations of lymphocytes (dense accumulations of small violet dots in the mucosa/submucosa) are much more extensive (Peyer's patches) in the ileum than in the jejunum, which would allow you to distinguish these two parts of the small intestine independent of other features that you may observe.
Draw part of the wall of the ileum in which a patch of lymphocytes is visible. A schematic presentation of other features should be sufficient.


Large Intestine

The large intestine constitutes the terminal part of the digestive system. It is divided into three main sections: cecum including the appendix, colon, and rectum with the anal canal. The primary function of the large intestine is the reabsorption of water and inorganic salts. The only secretion of any importance is mucus, which acts as a lubricant during the transport of the intestinal contents.

The surface of the mucosa is relatively smooth as there are no plicae circulares or intestinal villi. Crypts of Lieberkühn are present and usually longer and straighter than those of the small intestine. Goblet cells account for more of the epithelial cells than in the small intestine.

There is only little lamina propria squeezed between the glands. The muscularis mucosae again forms two layers.

Considerable amounts of fat may be found in the submucosa.

The appearance of the muscularis externa is different from that of the small intestine. The inner circular layer of muscle forms the usual sheath around the large intestine, but the outer longitudinal muscle layer forms three flattened strands, the taenia coli. Only a thin layer of longitudinal muscle surrounds the inner circular muscle layer between the taenia coli .

The adventitia forms small pouches (appendices epiploicae) filled with fatty tissue along the large intestine.

Specialized Sections of the Large Intestine

The vermiform appendix

is a small blind-ending diverticulum from the cecum. The most important of its features is the thickening of its walls primarily due to large accumulations of lymphoid nodules in the lamina propria. Intestinal villi are usually absent, and crypts are comparatively rare. There is often fatty tissue in the submucosa. The muscularis externa is thinner than in the remainder of the large intestine.

The anal canal

is the terminal part of the digestive tract (2.5-4 cm long). The mucosa has a characteristic surface relief of 5-10 longitudinal folds, the anal columns, each of which contains a terminal branch of the superior rectal artery and vein.
Small folds between the anal columns (anal valves) form the pectinate line. This line defines sections of the anal canal with different arterial and nerve supplies, different venous and lymphatic drainages and different embryological origins. Crypts disappear below the pectinate line and the epithelium changes from the tall, columnar type seen in other parts of the large intestine to a stratified squamous epithelium. The muscularis externa gradually becomes thicker and forms the involuntary internal anal sphincter.

Lab: Slide COLON H&E. Again look at the slide without the aid of the microscope. Note that the bundles of longitudinal muscle are clearly visible on the outside of the colon and that plicae circulares are absent from the luminal side of the colon.
Draw a small schematic figure of these features.
Now have a closer look at the components of the wall of the colon. Villi are absent and the crypts appear deeper than the ones you observed in the small intestine. Goblet cells are numerous. The lamina propria and muscularis mucosae may be difficult to distinguish. Note also that a thin layer of longitudinal muscle is found between the taenia coli on the outside of the inner circular muscle layer.
Draw a small segment of the wall of the colon and label specialized features of the colon in your drawing.


Junctions

Junctions between the major parts of the alimentary canal share a rapid transition from tissues characteristic of one part to those characteristic for the next part, e.g. the transition from the stratified squamous epithelium of the oesophagus to the glandular epithelium of the stomach. Many junctions are in additon accompanied by morphological specializations, e.g. the pylorus at the gastro-duodenal junctions or the ileo-caecal valve at the ileo-caecal junction. You will have a chance to examine these specializations in the DR.

Compared with these rapid changes, "junctions" between parts of the small intestine are very gradual morphological transitions.

Lab: Slides GASTRO-OESOPHAGEAL JUNCTION BABOON, GASTRO-DUODENAL JUNCTION RAT, ILEO-CAECAL JUNCTION RAT and  ANO-RECTAL JUNCTION HUMAN. Identify in each of these sections histological features characteristic for the adjacent parts of the alimentary canal. Spend a couple of extra moments on the gastro-oesophageal junction and have a look at the cardiac glands of the stomach. Note that the tubules of the glands branch and that they are lined by an almost homogenous population of mucus-producing cells. As you move away from the opening of the oesophagus both chief cells and parietal cells gradually appear in the tubules.

Page content and construction: Lutz Slomianka
Last updated: 28/02/00