Histology of Alimentary Canal: Understanding the Microscopic Anatomy of Digestive System

The alimentary canal, also known as the digestive tract, plays a crucial role in the process of digestion, absorption, and nutrient transport. Understanding its histology provides valuable insights into the intricate cellular composition and functionality of this vital system.

At the microscopic level, the alimentary canal exhibits a complex arrangement of tissues and cells that work harmoniously to ensure efficient digestion. The canal consists of several organs, including the mouth, esophagus, stomach, small intestine, and large intestine. Each organ has its own unique histological characteristics, tailored to fulfill specific functions.

The mucosa, the innermost layer of the alimentary canal, is lined with specialized epithelial cells that aid in absorption and secretion. The lamina propria, a connective tissue layer, supports the epithelium and contains blood vessels, lymphatic vessels, and immune cells. Additionally, the muscularis mucosae provides a layer of smooth muscle fibers that assist in the movement of substances through the digestive tract.

Beneath the mucosa lies the submucosa, a layer composed of connective tissue, blood vessels, and nerves. It plays a vital role in supporting the mucosal layer and facilitating the transport of absorbed nutrients.

Further, the alimentary canal possesses a muscularis externa, which consists of smooth muscle fibers arranged in an inner circular and outer longitudinal layer. This muscular layer enables peristalsis, the rhythmic contraction and relaxation that propels food along the digestive tract.

Oesophagus

On studying the transverse section of the esophagus the following structures are visible.

• On the outer side, the fibrous coat is situated.
• Below the fibrous coat, there is a layer of external longitudinal muscle, which, in the transverse section is visible in the form of a large number of bundles.
• Circular muscles, forming the third layer, are in the form of long stripes.
• There is also a large inner longitudinal muscle.
• Lying below the inner longitudinal muscle is situated a connective tissue - layer having mucous glands and blood vessels.
• The inner layer consists of muscularis mucosae.
• The innermost layer consists of stratified epithelium, having a lumen on the inner side and connective tissue on its outer side.

Stomach

• A transverse section of the stomach of Rabbit shows that it consists of four coats: (a) Serous, (b) muscular (c) submucous, and (d) mucous coats.
• The serous coat is derived from the peritoneum and is the outermost covering of the intestine. It is made up of an outer layer of squamous epithelium and an inner layer of connective tissue.
• The muscular coat consists of an outer layer of longitudinal muscle and an inner layer of circular unstriped muscle fibers. At the cardiac end of the stomach, there is a third muscular coat, the fibers of which are situated in the inner oblique direction.
• The next inner layer is the submucosa, which is a layer of loose areolar connective tissue having many blood vessels.
• Outside the mucous coat is a layer called muscularis mucosae, which is a double muscular layer, having on the outside, longitudinal muscle fibers and the inner side circular muscle fibers. Muscularis mucosae are highly developed in the stomach rather than in the intestine.
• The Mucosa coat of the stomach consists of gastric glands closely packed together. The walls of the gastric glands are glandular, therefore, they produce gastric juice. When the transverse section of the stomach is through the cardiac part, the gastric glands are called cardiac glands. These contain only mucus-secreting cells. In the fundic part as described by some authors, there are three kinds of cells called
  1. Mucus-secreting cells,
  2. Zymogen or pepsin-secreting cells, and
  3. Oxyntic cells, which produce HCl.

Duodenum

• Histologically the duodenum is also formed of four coats: serous, muscular, sub-mucous, and mucous coats.
• However, the mucous coat consists of columnar epithelial cells covering the inner surface of the intestine and a layer of connective tissue containing lymphoid tissue and numerous blood - vessels.

It has numerous finger-like projections called villi. The function is to increase the absorptive surface. In the mucous coat between the villi are situated simple tubular glands called crypts of Lieberkuhn. Embedded in the mucous coat of the duodenum are small glands known as Brunner's glands, whose ducts pass inwards to open on the surface of the mucous coat. Each Brunner's gland is a convoluted tube lined with columnar epithelium. The crypts of Lieberkuhn and Brunner's gland produce the intestinal juice or saccus entericus.

Large Intestine

The structure of the large intestine resembles in most respects the other parts of the duodenum, but there are neither villi nor Brunner's glands.

Histology of Liver

The liver is one of the most important digestive glands in the body. Histologically liver consists of groups of polygonal lobules. These are separated from one another by their membranes or septa possessing connective tissue, thus, forming Glisson's capsule.

Liver cells in the lobules are located to form radial hepatic cords between which blood capillaries or sinusoids are situated. Branches of the hepatic portal vein are found around the lobules to form interlobular veins. In the center of each lobule is an intralobular vein, which joins to form the hepatic vein. There are also some special types of cells called Kupffer cells, situated in the wall of the sinusoids. These cells serve to remove the bacteria from the liver.

Functions of Liver

The structure of the liver and its role in digestion has already been dealt with. However, the functions of the liver can be summarised as follows.

• As already mentioned, it secretes bile, which is a bitter alkaline fluid and is dark green due to the presence of decomposed products of hemoglobin of the old and worn-out red blood corpuscles. Bile has several salts both organic and inorganic as well as some waste substances. The functions of the bile are as follows:
  1. Bile is responsible for emulsification of fats. Thus, fat is broken into minute droplets, which can remain suspended in a fluid medium.
  2. Bile contains bile salts and bile pigments.
  3. The bile salts are bicarbonate, glycocholate, taurocholate of sodium.
  4. Sodium bicarbonate neutralizes the acidity of chyme.
  5. Glycocholate and taurocholate of sodium break down fats into smaller globules to form emulsions.
  6. Bile pigments are bile verdin and bile rubin.
  7. The green color of bile juice is only due to the presence of bile pigments.
  8. Bile salts are necessary for the absorption of vitamin 'K' and other fat-soluble vitamins.
• The liver helps with the storage of vitamins 'A' and 'D' out of which the former is manufactured in the liver.
• The liver forms lymph and it is responsible for the destruction of worn-out red blood corpuscles. So it is a hemolytic organ.
• It is the heat-producing center of the body. The heat is well maintained in the body.
• The liver also produces fibrinogen, which is responsible for the clotting of blood. Without clotting of blood, we cannot recover from injuries. Clotting is possible only by vitamin 'K' which reaches the liver where it is converted into prothrombin, responsible for clotting.
• The liver produces heparin, to prevent clotting and thus block the arteries of the heart and blood vessels of the brain.
• Poisonous ammonia is converted into urea, and it is ultimately removed by the kidneys.
• It changes amino acids into albumins, which regulate the balance of salt water without which we cannot survive.
• The liver removes the excess sugar by changing it into glycogen, which splits up and is used during days of food scarcity.
• Kupffer cells found in the liver capture, engulf and destroy germs of diseases.
• The liver is a storage organ of iron and copper.

Thus the liver is of great importance to the animal as it serves as a digestive gland, as a storage organ, and helps the removal of excretory waste products.