Histology of Alimentary Canal

The alimentary canal, also known as the gastrointestinal (GI) tract, is a continuous tube extending from the mouth to the anus, specialized for digestion and absorption of food. The histological structure of the alimentary canal reflects its functions, with distinct adaptations in different regions. 

1.0Introduction

2.0Layers of the Alimentary Canal

1. Serosa 

• Its outermost layer of gut (it is called tunica adventitia in oesophagus) serosa is made up of a thin mesothelium (visceral peritoneum) with some connective tissue. 
• Tunica adventitia is made up of white fibrous connective tissue and areolar connective tissue.

2. Muscle layer

• It is formed by a circular inner layer and longitudinal outer layer of smooth muscle. 
• Thickest layer is found in the stomach (maximum peristalsis) and the thinnest layer in rectum (minimum peristalsis). An oblique muscle layer may be present in some regions.

3. Sub mucosa 

• It is a loose connective tissue layer with blood vessels, lymph vessels and nerves.

4. Mucosa 

• It is the innermost layer of the gut which contains the secretory and absorptive cells. Mucosal epithelium has goblet cells, secretes mucus that help in lubrication. It is differentiated into 3 parts.
• (i) Outer part :- Called mucosa muscularis. It is made up of outer longitudinal and inner circular muscles. It has important role in exposing of surface area for the absorption
• They also provide support to the folds of the alimentary canal.
• (ii) Middle part :- It's called lamina propria it contains few modified lymphatic tissue which provides immunity ex. peyer's patches . It is made up of reticular and fibrous connective tissue.
• (iii) Innermost part :- Called mucosal epithelial layer.  In oesophagus this layer is made up of non keratinised stratified squamous epithelium. Except oesophagus this layer is single layer thick, which is made up of columnar mucous epithelium.
• This layer makes the lining of lumen of the Alimentary canal.
• This layer makes the folds of the alimentary canal.
• Folds of oesophagus are less developed. Invagination in the mucosa of the stomach develops as a gland called gastric gland.
• There are conical shaped projections in the small intestine called Villi. Villi are supplied
• with a network of capillaries and a large lymph vessel called lacteal. Small slit like
• space is found at the base of villi. These spaces are called crypts of Lieberkuhn
• Maximum villi are found in Jejunum.
• All the four layers show modifications in different parts of the alimentary canal.

3.0Peyer's Patches 

• They are small lymph nodes which are found in the mucosa of the small intestine (Jejunum and Ileum more in number). 
• They are also called intestinal tonsils and provide immunity.

4.0Nerve supply 

• Two types of Nerve plexus are found in the muscle of the alimentary canal.
• Auerbach's Nerve plexus (myentric plexus) this nerve plexus is found between
• longitudinal muscles and circular muscles, it control muscle contraction
• Meissner's Nerve plexus found between circular muscles and sub mucosa but in stomach
• It is found between oblique muscle & submucosa.