General Histology of the Alimentary Canal
The wall of the alimentary canal has the same basic structure throughout its length. It consists of four basic layers. Listed from the lining epithelium to the covering tissue, they are 1) the mucosa, 2) submucosa, 3) muscularis externa, and 4) the serosa or adventita.
The mucosa contains three sublayers:
- the lining epithelium,
- mucosa glands in most parts of the alimentary canal
- the lamina propria, which consists of a loose connective tissue supporting the epithelium, and a
- muscularis mucosa, which is a double-layer of smooth muscle surrounding the lamina propria. The inner layer is a layer of circularly-arranged smooth muscle cells, and the outer layer is a layer of longitudinally-arranged smooth muscle cells.
The submucosa is a relatively thick layer of dense irregular connective tissue. It not only binds and supports the other layers of the canal wall, but also supports vascular, lymphatic, and nerve supplies of the wall.
The muscularis externa consists of multiple layers of muscle. Contractions of the muscularis externae provide the movements that mix the luminal contents of the canal and move the contents along its length. Peristalsis, the common waves of contraction that move lumenal contents, come from the contractions of the muscularis externa.
The serosa is the outermost tissue layer of those portions of the alimentary canal that are suspended in the peritoneal cavity by mesentery. It is the visceral peritoneum of gross anatomy. It is continuous with mesentery and consists of a thin layer of loose connective tissue covered by mesothelium. The loose connective tissue layer supports large blood vessels, lymphatic vessels, and contains variable numbers of adipocytes.
Regions of the alimentary canal that are not suspended by mesentery, but rather are attached directly to the body wall, are surrounded by an adventitia rather than a serosa. These regions include the esophagus, which is actually physically attached to the aorta, the duodenum, the ascending colon, and the descending colon. The adventitia is a thicker layer of loose connective tissue that is continuous with the loose connective tissue of other structures.
The wall of the alimentary inside the muscularis externa is generally folded. Folds of the mucosa, the so-called mucosal folds, can be identified by their core of lamina propria. Submucosal folds can be identified by their core of submucosa.
The wall of the alimentary canal also contains a number of different exocrine glands that deliver substances to the lumen or the lining epithelium of the alimentary canal. These glands are classified according to their position in the wall. Mucosal glands are those that extend into and are supported by the lamina propria. Submucosal glands extend into and are supported by the submucosa.
Stomach
Structure of the stomach
Food starts to be digested and absorbed in the stomach, although absorption is mostly limited to water, alcohol and some drugs. The stomach is an expandable, muscular bag, and it keeps swallowed food inside it by contracting the muscular pyloric sphincter. 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.
Gastric juice is secreted by gastric mucosal glands, and contains hydrochloric acid, mucus, and proteolytic enzymes pepsin (which breaks down proteins), and lipase(which breaks down fats).
When the stomach is empty, and not distended, the lining is thrown up into folds called rugae. After eating, these folds flatten, and the stomach is able to distend greatly.
The stomach has three anatomical regions:
- cardiac, which contains mucous secreting glands (called cardiac glands) and is closest to the oesophagus
- fundus, the body or largest part of the stomach which contain the gastric (fundic) glands
- pyloric, which secretes two types of mucus, and the hormone gastrin.
The pyloric region ends at the pyloric sphincter. This sphincter relaxes when the formation of chyme is completed, and the chyme is squirted into the duodenum.
Layers of the stomach.
This shows an image through the wall of the body of the stomach at low power. You should be able to 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 muscularis externa layer has three layers of muscle. An innner oblique layer , a middle circular and an external longitudinal layer. The contraction of these muscle layers help to break up the food mechanically.
Oesophageo-gastric junction.
The structure of the oesophagus was covered in the topic ‘oral’. The oesophagus forms a junction with the stomach, which is called the oesophageo-gastric junction.
The photograph on the right hand side shows this junction.
Can you identify the oesophageo-gastric junction, where the oesophagus ends, and the stomach begins? This region of the stomach is called the ‘cardiac’ region. How does the mucosa in this region change?
Gastric Glands in the fundus (body) of the stomach
The epithelium of the mucosa of the fundus and body of the stomach forms invaginations called gastric pits. The lamina propria contains gastric glands, which open into the bases of the gastric pits. These glands are responsible for the synthesis and secretion of the gastric juice.
The lining epithelium of the stomach, and gastric pits is entirely made up of mucous columnar cells. These cells produce a thick coating of mucus, that protects the gastric mucosa from acid and enzymes in the lumen. Even so, these cells have to be replaced after 4-6 days.
This diagram shows the structure of a gastric gland, a simple tubular gland.
The isthmus and neck contain dividing cells (stem cells) immature cells and maturing neck mucous cells. The mature cells move up to replace the surface mucous cells. These mucous cells are very pale staining.
Parietal (oxyntic) cells are also concentrated in the isthmus region, but also found in the base and neck of the glands. These are large pale staining cells with a central spherical nucleus. Can you identify them in these sections? (They have a ‘fried egg’ appearance). Parietal cells make hydrochloric acid, and intrinsic factor, which is needed for absorption of vitamin B12 in the terminal ileum.
Chief/Peptic/zymogenic cells are found in the bases of gastric glands. They have a stongly basophilic granular cytoplasm, as they have lots of rER for production of peptin, which is secreted (as precursor pepsinogen), and basally located nuclei.
Neuroendocrine cells in the bases of the glands secrete serotonin and other hormones.
Small Intestine
Structure
Diagram of a plica circulares.
This is a diagram which shows the villi of the small intestine, as indicated by the arrows in the diagram above, at higher magnification.
The main functions of the small intestine are digestion, absorption of food and production of gastrointestinal hormones. The small intestine is 4-6 metres long in humans.
To aid in digestion and absorption:
- the small intestine secretes enzymes and has mucous producing glands. The pancreas and liver also deliver their exocrine secretions into the duodenum.
- The mucosa is highly folded.
- large circular folds called plicae circulares (shown in the diagram to the right), most numerous in the upper part of the small intestine
- smaller folds called villi, which are finger like mucosal projections, about 1mm long.
- the lining columnar epithelial cells have fine projections on their apical surfaces called microvilli.
Together, these folds provide a huge surface area for absorption. Between the villi there are crypts, called crypts of Lieberkuhn, which extend down to the muscularis mucosae. These crypts are short glands.
The lamina propria which underlies the epithelium has a rich vascular and lymphatic network, which absorbs the digestive products, and there is a muscularis mucosae layer immediately at the base of the crypts. The lymphatic capillaries are called lacteals, and absorb lipids. The vascular capillaries are fenestrated to aid absorption.
The muscularis externa layer contains two layers of smooth muscle, an inner circular and outer longitudinal, for continuous peristaltic activity of the small intestine. There are around 200 or so lymphoid aggregations called Peyer’s patches in the mucosa.
Crypts
The crypts additionally contain
- Paneth cells (at the base of the crypts) – they have a defensive function, and stain intensely eosinophilic, due to secretory granules of antimicrobial peptides called defensins, as well as lysozyme and phospholipase A. These cells last for several weeks.
- Endocrine cells, (also eosinophilic) which produce secretin, somatostatin, enteroglucagon and serotonin. One type of endocrine cell for each type of hormone.
- Stem cells, found at the base of the crypts, which divide continuously to replace enterocytes (every 2-3 days), goblet cells, paneth cells and neuroendocrine cells.
Intraepithelial lymphocytes (mostly T-cells).
Duodenum
The first part of the small intestine is the duodenum, and its structure is similar to that seen elsewhere in the small intestine, with some differences. The villi are broader, Peyers Patches are less common, and it has one unique feature: Brunner’s glands, which are found in the sub-mucosa.
Both Brunner’s glands, and the goblet cells in the duodenum secrete mucus. The mucus secreted by Brunner’s glands is alkaline, and helps to neutralise the acid chyme produced by the stomach, to produce chyme with a pH suitable for the digestive enzymes of the small intestine.
Digestion
The chyme is mixed with pancreatic enzymes, and molecules are absorbed by the enterocytes.
Proteins are denatured and chopped up by pepsin from gastric glands, and then further broken down by trypsin, chymotrypsin, elastase and carboxypeptidases in the lumen of the small intestine. Further enzymes in the plasma membrane of the enterocytes complete breakdown into amino acids, and each amino acid is actively transported into the enterocyte.
Carbohydrates are hydrolysed by amylases, and membrane bound enzymes convert sugars to monosaccharides which are absorbed by facilitated diffusion.
Lipids are converted into an coarse emulsion in the stomach, and into a fine emulsion in the duodenum by pancreatic lipases. Small lipid molecules are absorbed by the enterocytes.
Appendix
This is a photomicrograph of the human appendix.
The arrangement of the layers in its walls is similar to that seen elsewhere in the large intestine. However, the outer layer of muscle fibres in the muscularis externa forms a continuous layer.
The most characteristic property of the appendix is the presence of masses of lymphoid tissue in the mucosa and submucosa. There are often follicles containing paler germinal centres similar to the follicles of Peyers patches in the small intestine.
Large Intestine
This is a low power image of the colon. Can you can identify the mucosa, submucosa and muscularis externa.
The large intestine completes absorption, and retrieves water and sodium from the luminal contents which become fecal residue. It secretes large amounts of mucus, and some hormones, but no digestive enzymes.
The thick mucosa has deep crypts, but there are no villi. The epithelium is formed of columnar absorptive cells with a striated border, many goblet cells, endocrine cells and basal stem cells, but no Paneth cells. The surface epithelial cells are sloughed into the lumen, and have to be replaced around every 6 days.
The lamina propria and submucosa are similar to the small intestine.
The longitudinal smooth muscle in the muscularis externa is arranged in three longitudinal bands called taenia coli. At the anus, the circular muscle forms the internal anal sphincter.
Liver
Functions of the Liver
The liver is the body’s largest compound gland. It is a major metabolic organ, and is important for degrading alcohol and drugs. It stores glycogen, secretes glucose, plasma proteins and lipoproteins into the blood, and secretes bilirubin (by-produce of haemoglobin), secretory IgA, and bile salts (which emulsify fats) as components of bile (endocrine secretion of the liver).
It is unusual because it has a dual blood supply. Not only does it receive arterial blood from the hepatic artery (about half of the total blood flow), it also receives blood from the hepatic portal vein, which contains nutrients absorbed from the GI tract. After passing through the liver, the blood collects and leaves in hepatic veins. These major blood vessels, enter and leave the liver at the porta hepatis. Also emerging from the porta hepatis are the left and right hepatic ducts which contain the collected bile, and the efferent lymphatics.
Pancreas
pancreas shows the secretory acini of the exocrine pancreas
Exocrine Pancreas
The exocrine part of the pancreas has closely packed serous acini, similar to those of the digestive glands. It secretes an enzyme rich alkaline fluid into the duodenum via the pancreatic duct. The alkaline pH is due to the presence of bicarbonate ions, and helps to neutralise the acid chyme from the stomach, as it enters the duodenum. The enzymes digest proteins, carbohydrates, lipids and nucleic acids. These enzymes include: trypsin and chymotrypsin (secreted as inactive precursors, and activated by the action of enterokinase, an enzyme secreted by the duodenal mucosa). An enzyme called CCK stimulates the release of these enzymes, from stored granules in the secretory cells of the acini. Secretin (from neuroendocrine cells in the small intestine) stimulate the release of watery alkaline secretions.
The endocrine part of the pancreas, consists of isolated islands of lighter staining cells called islets of Langerhans. The secretions of the acini empty into ducts lined with a simple low cuboidal epithelium, which becomes stratified cuboidal in the larger ducts.
Endocrine Pancreas
The islets of Langerhans are clumps of secretory cells (up to around 3000) supported by reticulin fibres, and containing numerous fenestrated capillaries. There is a delicate capsule around each islet. They are paler than the surrounding exocrine cells, as they have less rER. These islets do not have an acinar organisation.
The islet cells are indistinguishable from each other in sections, but in fact three secretory cells types are present:
- alpha – secrete glucagon,
- beta – secrete insulin
- delta – secrete somatostatin
The islets are supplied by up to three arterioles, which form a branching network of fenestrated capillaries, into which the hormones are secreted. The islet is drained by about six venules, which pass between the exocrine acini to the interlobular veins.
The islets are supplied by up to three arterioles, which form a branching network of fenestrated capillaries, into which the hormones are secreted. The islet is drained by about six venules, which pass between the exocrine acini to the interlobular veins.
Gall Bladder
The gall bladder is a simple muscular sac, lined by a simple columnar epithelium. It receives and stores bile from the liver via the hepatic and then cystic duct, and can store about 50 to 100ml in humans. It is attached to the visceral layer of the liver.
The gall bladder is stimulated to contract and expel the bile into the duodenum, by the hormone cholecytoskinin pancreozymin (CCK) produced by the endocrine cells of the duodenal mucosa. The production of this enzyme is stimulated by the presence of fat in the proximal duodenum. The contractions expel bile into the common bile duct, and the bile is then carried to the duodenum.
The inner surface of the gall bladder is covered by the mucosa. The sufrace is made up of a simple columnar epithelium. The epithelial cells have microvilli, and look like absorptive cells in the intestine. Underneath the epithelium is the lamina propria. The wall of the bladder does not have a muscularis mucosae and submucosa.
The muscularis externa (muscle layer) contains bundles of smooth muscle cells, collagen and elastic fibres. Underneath this, on the outside of the gall bladder is a thick layer of connective tissue, which contains large blood vessels, nerves and a lymphatic network. Where this layer is attached to the liver, it is called the adventia. In the unattached region, there is an outer layer of mesothelium and loose connective tissue (the serosa).