Histology Text Of The Alimentary Canal
The alimentary canal extends from the oral cavity to the anus. The masticated food is delivered into the oral pharynx and then enters the oesophagus and eventually the stomach. In the stomach, the gastric food is broke down by an acidic chyme and passes to the small intestine for absorption. The liquefied food residue passes into the large intestine to complete the digestion and reabsorption and then compaction of faeces. The solidified faeces are then passed to the anus for elimination.
Walls of the Alimentary Canal
The alimentary canal walls is arranged in four layers mucosa, submucosa, muscularis externa, serosa and adventita.
I. Mucosa The mucosa is the innermost layer that directly surrounds the lumen. In the alimentary canal, each region has a distinguished mucosa. The mucosa is composed of three concentric layers:
The mucosa functions include the following: a. Secretion: The alimentary canal lining cells secrete digestive enzymes, hydrochloride acids, mucin, peptides, hormones and water. The mucosa epithelium also secretes antibodies that are produced by the underlying connective tissue. The alimentary canal contains glands developed from invaginations of the luminal epithelium. These glands include the following.
Metabolic substrates, such as digested nutrients, water, electrolytes and other substances essential to the functioning of the body, are absorbed by the epithelium of the alimentary canal mucosa. The mucosa and submucosa contain surface projections into the lumen of the digestive tract, which greatly increase the absorption surface area. These surface projections include the following:
The mucosa of the alimentary canal forms a barrier wall preventing the entry of noxious substances, antigens and pathogenic organisms. Tight junctions presenting between the simple columnar epithelial cells of the gastrointestinal mucosa serve as a selectively permeable barrier. d. Immunologic protection: The mucosa of the alimentary canal possesses lymphatic tissue in the lamina propria, serving as the body’s first line of immune defence. The mucosal lymphatic tissue includes the following.
II. Submucosa
The submucosa is a dense, irregular connective tissue physically supporting the mucosa. It also provides nerve, vascular and lymphatic vessels supplying the mucosa, muscularis externa and serosa. It contains an extensive nerve network representing the enteric nervous system, called submucosal plexus (Meissner’s plexus). They are network of unmyelinated nerve fibres and ganglion cells, which is responsible for innervating the smooth muscle layers of the alimentary canal. The submucosa also houses glands in some regions of the alimentary canal that often aid in identifying the specific segment or region of the tract. For example, the oesophagus and the initial portion of the duodenum contain glands in the submucosa. III. Muscularis Externa
The muscularis externa consists of two concentric relatively thick smooth muscle layers: the inner circular layer (tight spiral layer) and outer longitudinal layer (loose spiral layer). They are wrapped around the alimentary canal in tight and loose helices. In addition, they house a thin connective tissue containing blood vessels, and lymphatic vessels between them. The muscularis externa contains also a nerve network called myenteric plexus (Auerbach’s plexus) between the two muscle layers. This contains nerve cell bodies (ganglion cells) of postganglionic parasympathetic neurons and neurons of the enteric nervous system. The muscularis externa churns and propels the luminal contents along the digestive tract via peristaltic action. The peristalsis is slow, rhythmic waves of contraction of the alimentary canal, which is marked by the constriction and shortening of the tube. It is controlled by the enteric nervous system. Constriction of the lumen, produced by the contraction of the inner circular layer of the muscularis externa, compresses and mixes the contents. Shortening the tube, produced by the contraction of the outer longitudinal layer of the muscularis externa, propels the contents. Not all parts of the digestive tube exhibit the same muscularis externa. For example, the muscularis externa of the proximal portion of the oesophagus (pharyngoesophageal sphincter) and the anal canal (external anal sphincter) contains striated muscle. The muscularis externa of the stomach presents a third, obliquely oriented layer of smooth muscle deep to the circular layer. Finally, teniae coli, three longitudinal bands formed by thickened part of the longitudinal smooth muscle layer, presents in the muscularis externa of large intestine. The teniae contraction facilitates the shortening of the tube to move its contents. The circular muscle layer of the muscularis externa is thickened at several points along the digestive tract to form the sphincter or valves. These structures include the following.
XI: Serosa & Adventitia
Either the serosa or the adventitia is the outermost layer of the alimentary canal. The alimentary canal in the intraperitoneal regions, suspended in the peritoneal cavity, is covered by a serosa (equivalent to the visceral peritoneum). On the other hand, the alimentary canal in the retroperitoneal regions, fixed to the cavity wall, is covered by the adventitia. The serosa is a connective tissue covered by a simple squamous epithelium (mesothelium) to reduce frictional forces during digestive movement. It is continuous with both the mesentery and the lining of the abdominal cavity. Through serosa, large blood and lymphatic vessels and nerve trunks travel to the wall of the digestive tract. However, the adventitia consists of a connective tissue that blends with the connective tissue of the wall cavity. |
Histology Text of Alimentary canal |
Histology Text: Oesophagus - Stomach - Small Intestine - Appendix - Large Intestine - Rectum & Anal Canal
Histology Atlas - Histopathology - Histology Quiz
Histology Atlas - Histopathology - Histology Quiz