Digestion first begins at the mouth where salivary glands completely work on the food breaking carbohydrates down then the tongue send the food down to the esophagus (Slack, 2000). From the esophagus, the food goes through the pyloric sphincter into the stomach, to the duodenum then goes into the small intestine (Marshman, Booth & Potten, 2002). At the small intestine, absorption takes place. From small intestine, it is sent down to the large intestine where food materials are sorted, and some are also absorbed and the remaining products send to the rectum waiting for defecation. According to Marshman, Booth & Potten, (2002), throughout the inner lining of digestive tract referred to as mucosa contains inner epithelium, loose connective tissue later referred to as lamina propria in addition to the thin layer of smooth muscle referred to as the muscular mucosae. Food digestion across the digestive tract passes over different types of epithelia that are adapted differently to enhance digestion by secretion of digestive fluids, mucus and also absorption as well as protection of the tract lining.
According to Marshman, Booth & Potten, (2002), stratified squamous epithelium are available from the oral cavity to the esophagus and lower anal canal, particularly where the digestive tract is highly subjected to abrasion. Further, muscular mucosae tense the mucosa, hence creating grooves and ridges which help in enhancing its surface area and its contact with food (Slack, 2000). This has been established to improve the efficiency of both digestion and nutrient absorption.
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Digestion of food first begins in the mouth. According to Marshman, Booth & Potten, (2002), the mouth is bounded by the lip, tongue, the cheek in addition to the palate. It is linked with oropharynx posteriorly. To sustain potential abrasions, the oral cavity is well covered with the stratified squamous epithelium. In the Oral Cavity, the Stratified Squamous epithelium plays a critical role by preventing possible wear and tear (Slack, 2000). Stratified squamous epithelium is commonly found where there is a higher demand for withstanding both chemical and mechanical wear and tear. They are durable and majorly perform the protection function. According to Marshman. Stratified squamous epithelium is found in most cavity linings in the body and the mouth and plays a key role offering protection against abrasion. In addition to this, the gums and the dorsum of the tongue is also slightly keratinized. Hard palates are normally underlined by the palatine bones aiding the tongue in chewing while soft palate, on the other hand, is formed of skeletal muscle and helps in closing off the nasopharynx during the process of chewing. The tongue grips and reposition food in the mouth during the process of chewing. Further, it mixes the food with saliva and initiates swallowing (Boundless, 2016). Lingual frenulum also plays a major role in securing the tongue to the floor of the oral cavity.
The next important part of digestive tract after the mouth is the esophagus. From the mouth, the food passes down through the esophagus. Like the oral cavity, the esophagus is also covered with the stratified squamous epithelium and mucus glands. In the esophagus, the Stratified Squamous plays an important role in preventing wear and tear. According to Slack (2000), the Stratified squamous epithelium lines the esophagus and plays a key role in protecting against abrasion. The esophagus moves the swallowed food to the stomach through the process of peristalsis. Stratified squamous epithelium that lines the esophagus is very protective, especially when one is eating crusty and hot foods. Stratified squamous epithelium in the esophagus are comprised of two layers, and basal cells divide them pushing towards the apex hence flattening the apical cells (Boundless, 2016). They are comprised of cuboidal, columnar and squamous types.
In the stomach, there is the chemical breakdown of the protein, and further, the food is converted into chyme. Focusing on the microscopic anatomy, the stomach has muscularity that has additional oblique layers responsible for allowing it to churn the food, mix it and then pummel food physically. Here, food is broken down into smaller fragments. According to Slack (2000), the stomach is well lined with the Simple Columnar epithelium that has been shown to perform a critical function of protection, absorption, and secretion. The lining of epithelium comprises of Goblet cells that specifically produces a coat of alkaline mucus. The mucosa, simple columnar epithelium found in the stomach is composed entirely of mucus-producing goblet cells. Simple columnar epithelium is also active in their absorption food nutrients and secretion of mucus for protection and allows easy movement of food (Slack, 2000). Additionally, the mucous surface layer is responsible for trapping bicarbonate-rich fluid beneath it. According to Marshman, Booth & Potten, (2002), the stomach is highly exposed to harsh conditions within the entire digestive system, therefore, to keep it from digesting itself, the stomach has a mucosal barrier with epithelial cells that are held together with a tight junctions, a thick bicarbonate coat that is rich with mucus and the gastric glands with cells that are impermeable to the HCL. The stomach is responsible for holding the swallowed food particles, degrades the food physically and chemically and releases chyme to the small intestine. It further enzymatically breaks down all proteins with pepsin.
Marshman, Booth & Potten, (2002) notes that small intestines starts from the pyloric sphincter to ileocecal valve and has three subdivision including duodenum, ileum, and jejunum. Focusing on their microscopic anatomy, there are several plicae circulars of mucosa and the submucosa. According to Slack (2000), the villi are fingerlike extensions of the mucosa, the microvilli are tiny projections of the absorptive mucosal cell plasmas membrane. In the small Intestine particularly in the jejunum, ileum, duodenum, there is Simple Columnar epithelium that performs the following major functions, protection, absorption and even secretion. Simple Columnar epithelium is common in areas that have high secretion functions such as the small intestine. Simple Columnar epithelium found in the small intestine is in a lone row of tall and closely arranged cells that are aligned within a row. They are also active in the absorption and secretion of the mucosa. Its primary role is to take in fluids and secretes mucus and enzymes for digestion and protection purposes respectively. The epithelium of the mucosa comprises of, the absorptive cells and the goblet cells, the enteroendocrine cells in addition to the interspersed T cells referred to as intraepithelial lymphocytes (Marshman, Booth & Potten, 2002).
From the small intestines, food goes down to the large intestine. The large intestine has three unique features including, teniae coil, haustra, and the epiploic appendages. It is further divided into cecum, appendix, colon, rectum and the anal canal. Marshman, Booth & Potten, (2002) also pointed out that the large Intestine including the ascending, transverse, descending, sigmoid colons, and even the rectum all have the Simple Columnar epithelium which also performs critical functions of protection, absorption, and secretion. The Colon mucosa is the simple columnar epithelium. Anal canal mucosa is also stratified squamous epithelium.
The food materials move down to the anus from the large intestine. At this place, food materials are stored waiting for elimination from the body. The anal mucosal membrane contains the stratified columnar epithelium that provides extra protection and padding. The stratified columnar epithelium found in the anal mucosal membrane main role is to protect against abrasion (Boundless, 2016)
In conclusion, the epithelium is the largest tissue cells that line up the digestive tract. It is a continuous thin layer that runs from the mouth, through to the esophagus, intestines and ends up in the anal canal. Epithelial composition and properties changes from one part of the digestive tract to the other and this explains their differing functions respectively. Epithelium acts as a protective layer in some areas and in other parts, it performs the absorption role. For instance, the mucous secretions from epithelial cells offer protection and lubrication to the epithelial membrane along the digestive tract. Digestion process entails changing the forms of food to a subtle substance that can further be absorbed and assimilated into the body as performed by glands located along the digestive tract. Epithelium help in the absorption of substances within the digestive tract but with distinct modification, they regulate and excrete chemicals between tissues and body cavities. Lastly, it has also been established that epithelium tissues perform the secretion of hormones function.
References
Marshman, E., Booth, C., & Potten, C. S. (2002). The intestinal epithelial stem cell. Bioessays , 24 (1), 91-98. http://onlinelibrary.wiley.com/doi/10.1002/bies.10028/epdf?r3_referer=wol&tracking_action=preview_click&show_checkout=1&purchase_referrer=www.ncbi.nlm.nih.gov
Slack, J. M. (2000). Stem cells in epithelial tissues. Science , 287(5457), 1431-1433. http://science.sciencemag.org/content/287/5457/1431.full
Boundless. (2016). Epithelial Tissues. Boundless Biology . Retrieved from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-animal-body-basic-form-and-function-33/animal-primary-tissues-193/epithelial-tissues-737-11967/
Works consulted
Antranik. (2011). Eight Types of Epithelial Tissue. Retrieved from.
http://antranik.org/eight-types-of-epithelial-tissue/
Cliffsnotes. (2016). The structure of the Digestive Tract Wall . Retrieved from https://www.cliffsnotes.com/study-guides/anatomy-and-physiology/the-digestive-system/structure-of-the-digestive-tract-wall
Study.com. (2016). What Is Epithelial Tissue? - Function, Types & Structure - Video & Lesson Transcript. Retrieved from http://study.com/academy/lesson/what-is-epithelial-tissue-function-types-structure.html
