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The Gastrointestinal Tract



Welcome to the wonderful world of histology. Histology is how we study the body on a cellular level. This week we will be delving into the gastrointestinal (GI) tract.


The gastrointestinal tract begins in the mouth. Here the oral cavity and pharynx are covered in stratified squamous epithelium. This differs from the nasal cavity. Although right beside each other this is part of the respiratory system and is covered in respiratory epithelium.


From the oesophagus to the anal canal, the digestive tract can be divided into four layers. First is the mucosa, which has three sublayers, the epithelium, lamina propria and muscularis mucosae. Next is the submucosa which is composed of loose connective tissue and then is the muscularis externa which is two thick layers of smooth muscle, an inner circular layer and an outer longitudinal layer. Last is the serosa, an outer layer of connective tissue that suspends the digestive tract or attaches it to other organs.


There is an abrupt transition from the stratified squamous epithelium of the oesophagus to the simple columnar epithelium of the stomach cardia. This can clearly be seen under a microscope. Under the microscope, holes can be seen in the stomach, these are gastric pits. At the bottom of these pits are many gastric glands. The gastric pits are lined by surface mucous cells. These pits are essential for the secretion of hydrochloric acid, stomach acid. The muscularis externa of the stomach has an extra oblique layer. This is to aid in the churning action of the stomach.


Again, there is an abrupt change from the stomach mucosa once we reach the duodenum of the small intestine. The inner, circular layer of smooth muscle is now much thicker to form the pyloric sphincter. On the surface of the small intestine are finger-like projections called villi that assist in absorption. The duodenum contains Brunner’s glands in the submucosa. They secrete an alkaline fluid containing mucin, which protects the mucosa from the acidic stomach contents entering the duodenum. The jejunum has tall villi on the folds of the mucosa and submucosa, the plicae circularis. The ileum has very short villi and has Peyer’s patches, lymphoid follicles extending into the lamina propria.


The two main cells in the large intestine are absorptive cells for the removal of salts and water, to help in the formation of faeces as well as goblet cells for the secretion of mucous to lubricate the colon.


There is a distinction between the mucosa of the rectum and the non-keratinised stratified squamous epithelium of the anal. The anal canal is then continuous with the keratinised stratified squamous epithelium of the surrounding skin.

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