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Histology of the digestive system

Recommended video: Organs of the digestive system [12:29]
Anatomy and function of the main organs of the digestive system.

The digestive tract is the pathway by which food enters the body and solid wastes are expelled. It is a tube-like muscular apparatus that commences at the oral cavity, travels through the thoracic and abdominal cavities and terminates at the anus in the pelvic cavity.

The digestive tract can be structurally divided into upper and lower parts. The upper digestive tract extends from the oral cavity to the suspensory muscle of duodenum at the duodenojejunal flexure. The lower digestive tract continues from this point distally to the anus. Furthermore, there are organs that aid in digestion but are situated outside of the digestive tract. These organs which include the salivary glands, pancreas, gallbladder and the liver, form the accessory digestive system.

This article provides an overview of the histological features of the organs of the digestive system.

Tongue and epiglottis

The tongue is a muscular organ with intrinsic and extrinsic muscles. The tongue has keratinized squamous epithelium on its dorsal surface, specifically the fungiform/filiform papillae. Its ventral surface is largely lined with multi-layered, nonkeratinized squamous epithelium and characterized by numerous papillaethat house taste buds to facilitate gustation as well as sensory detection of touch and temperature.

The epiglottis is located at the entrance to the larynx and is made of elastic cartilage. It closes the laryngeal inlet during swallowing to prevent the passage of food and liquid into the lungs via the trachea
The lingual or upper surfaces of the epiglottis faces the tongue and the laryngeal surface faces the larynx. Both sides have a different epithelial lining. The lingual surface is covered with thick, stratified, nonkeratinized squamous epithelium, while the laryngeal surface is lined with respiratory ciliated epithelium. There are many seromucous glands in the lamina propria of the mucosa that keep the epithelium moist.

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Gingiva and teeth

Teeth are a major component of the oral cavity and are essential for the beginning of the digestive process.

Teeth are made up of three specialized tissues:

  • Enamel, which is a hard, thin, translucent layer of acellular mineralized tissue covering the crown of the tooth. 
  • Dentin, which is the most abundant dental tissue, lying deep to the enamel in the crown and cementum in the root. 
  • Cementum, which is a thin, pale-yellowish layer of mineralized tissue covering the dentin of the root of the teeth. 

The gingivae (sg. gingiva) (gums) are found in the oral cavity and comprise the surrounding part of the teeth. They consist of mucosal tissue that covers the alveolar processes of the maxilla and mandible and finish at the neck of each tooth. 

There are two types of gingivae: the marginal gingiva which is mobile and surrounds the neck of the tooth as well as the attached gingiva, which lies between the mobile gingiva and the alveolar gingiva.

The gingival epithelium encompasses the external surface of the gingiva including the mobile and fixed areas as well as the gingival sulcus and the junctional epithelium. It is divided into the following major sections: 

  • Oral epithelium
  • Sulcular epithelium
  • Junctional epithelium.

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Salivary glands

The salivary glands are exocrine glands responsible for the production and secretion of saliva. There are three large pairs of salivary glands: the parotid gland, the sublingual gland and the submandibular gland.

The salivary glands are composed of two main cell types: serous and mucous cells. Serous cells produce a watery secretion rich in enzymes such as amylase, while mucous cells produce a viscous, mucous secretion. The secretory cells are organized into secretory units (acini), small gland-like structures that empty into branched intralobular circuits as well as into extralobular ducts.

The parotid gland is purely serous with distinct striated and intercalated ducts. The sublingual gland is a mixed seromucous gland, with a predominance of mucous cells. The submandibular gland is mixed seromucous, but has a higher proportion of serous cells

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The esophagus is a hollow muscular organ in the digestive system that connects the throat and the stomach. The esophagus has a histological structure typical of the digestive system.

The inner mucosa consists of a multilayered, nonkeratinized squamous epithelium. This special epithelial layer protects the esophagus from mechanical wear and tear caused by the food bolus during swallowing. The lamina propria contains collagen fibers, numerous blood vessels and lymphatic vessels. There are also smooth muscle fibers of the lamina muscularis mucosae, which form longitudinal folds in the mucosa and make the lumen appear star-shaped.

The following submucosa contains submucous glands, which form the mucus for the epithelial layer. The muscular coat of the esophagus is divided into three sections. The upper third consists of skeletal muscle, the lower third only consists of smooth muscles (external longitudinal and internal circular), and both forms occur in the middle section. The outer adventitia consists of connective tissue and serves to connect the esophagus to the surrounding structures.

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The stomach is a hollow organ between the esophagus and the duodenum and is used to store and process the food pulp (chyme). Macroscopically, it is divided into cardia, body, fundus and pylorus. These differences are reflected in the gastric pits and the associated gastric glands. The stomach has the same structural plan throughout its entirety consisting of a mucosa, submucosa, muscular coat and serosa.

The gastric mucosa is lined with single-layered, mucus-forming columnar epithelium (foveolar epithelium) that forms a protective barrier against acidic gastric juice.

While the glands of the cardia mainly secrete mucins, the glands in the body and fundus contain various exocrine cells. 

The gastric glands are located close together in the lamina propria and extend to the lamina muscularis mucosae. The submucosa contains blood vessels, lymphatic vessels and nerve plexuses that ensure the supply and control of the stomach. The muscular coat of the stomach consists of three layers of smooth muscle that are responsible for the mechanical chopping and mixing of food. The outer serosa anchors the stomach in the abdominal cavity.

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The pancreas is a heterocrine gland that has both exocrine and endocrine functions. It is made up of a body, head and tail and is surrounded by a fine connective tissue capsule.

The pancreatic tissue is divided into many lobules. The majority, about 80-90%, is exocrine (exocrine component) and consists of acinar cells. These cells produce digestive enzymes that are secreted into the acinar tubules. These canals open into larger intralobular and interlobular ducts, which ultimately flow into the pancreatic duct to the duodenum. The enzyme-containing secretion supports the digestion of fats, proteins and carbohydrates in the small intestine.

The endocrine component of the pancreas is primarily found in the tail of the pancreas and consists of the pancreatic islets (of Langerhans). These lighter islet areas contain B cells (insulin cells) that produce insulin, A cells (glucagon cells) that produce glucagon, and D (somatostatin cells) and PP cells (pancreatic polypeptide cells) for somatostatin and pancreatic polypeptide.

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Small intestine

The small intestineis an approximately 6m long organ of the digestive tract and is responsible for the absorption of nutrients. It is divided into three sections: duodenum, jejunum and ileum. The three sections of the small intestine can be distinguished from each other with the help of certain characteristics. circular folds (of Kerckring), intestinal villi, crypts and microvilli, which significantly increase the resorption surface and optimize nutrient absorption.

The wall of the small intestine consists of the typical layer arrangement of the digestive tract.

The mucosa is lined by a single-layered columnar epithelium characterized by enterocytes, goblet cells and enteroendocrine cells. Deep to the mucosa lies the lamina propria with many lymph follicles and the muscularis mucosae. The submucosa contains glands, blood and lymphatic vessels, and the submucosal plexus (of Meissner). The muscular coat consists of an inner circular muscle layer and an outer longitudinal muscle layer, between which the myenteric plexus (of Auerbach) is located. The outer serosa surrounds the small intestine in the peritoneal space.

The three sections of the small intestine can be distinguished from each other with the help of certain characteristics.

  • The duodenum contains duodenal submucosal glands (of Brunner) in the submucosa that secrete mucins and bicarbonate.
  • In the jejunum, the circular folds (of Kerckring) are most pronounced.
  • Typical of the ileum are abundant aggregated lymphoid nodules (Peyer's patches), which are part of the gastrointestinal associated lymphatic tissue of the intestine (GALT) and are collections of lymph follicles. The circular folds are less pronounced here and the villi are also shorter in this region.

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Large intestine (colon)

The large intestine (colon) is responsible for the absorption of water and electrolytes as well as the formation and elimination of stool. It can be divided into several sections: the cecum, the ascending, transverse, descending and sigmoid colon, followed by the rectum.

The innermost layer of the colon, the mucosa, consists of single-layered columnar epithelium with short microvilli and many goblet cells that produce mucus. In contrast to the small intestine, the large intestine lacks circular folds (of Kerckring) and villi, instead showing deep crypts. The lamina propria and the lamina muscularis mucosae support this structure. The submucosa houses blood vessels, nerves and the submucosal plexus (of Meissner). The muscular coat consists of two distinct muscle layers, with the outer longitudinal muscles running in three characteristic bands, the teniae coli. The serosa forms the outermost layer of the colon wall. In addition to the taeniae coli, omental appendices and segmental protrusions (haustra) are characteristics of the colon.

There are also numerous lymph follicles in the large intestine, which are particularly common in the vermiform appendix area. At the end of the digestive tract, in the anal canal, the colonic epithelium is gradually replaced by stratified squamous epithelium with eccrine and apocrine sweat glands as well as hair and sebaceous glands.

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The liver is the largest internal organ in humans and plays a central role in metabolic processes and removing toxins from the blood. It is covered by a fibrous capsule, also known as Glisson's capsule. Histologically, it is made up of functional units known as liver lobules, which have a central vein and are bordered by extensions of the fibrous capsule of the liver (interlobular septae). Where several lobules meet, a portal area is formed, which contains connective tissue, nerves and lymphatic vessels as well as a portal triad, consisting of a branch of the hepatic portal vein and hepatic artery as well as an interlobular bile duct. It is important to be aware that the flow of bile is countercurrent to the flow of blood.

The parenchyma of the liver consists of specialized epithelial cells called hepatocytes. These store glycogen and synthesize bile and fatty acids as well as various proteins. The hepatocytes are arranged in bar-like plates known as hepatic cords, between which there are hepatic sinusoids and bile canaliculi.

Hepatic sinusoids
are dilated capillaries that contain mixed blood from the portal vein and the hepatic artery. Stellate macrophages, also known as Kupffer cells, are specialized macrophages that phagocytose foreign particles and dead cells are also found here. There is a perisinusoidal space (of Disse) around the sinusoids, which is important for the exchange of substances between hepatocytes and the blood.

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The gallbladder is a small, pear-shaped organ which is located beneath the liver. It functions to collect, store and concentrate bile received from the liver and expel it to the small intestine as needed for the emulsification of fats.

The wall of the gallbladder consists of a mucosa, muscular coat and serosa. The mucosa is formed by a layer of simple columnar epithelium (with microvillous border. The epithelium is responsible for concentrating the bile as well as secreting mucus to protect the internal wall of the gallbladder. In a nondistended state, the mucosa forms distinct mucosal folds (rugae), between which are pocket-like mucosal crypts, or diverticula. The lamina propria mucosae, which underlies the epithelium, is composed of loose connective tissue. 

The muscular coat consists of a single layer of randomly oriented smooth muscle, which functions to propel bile into the bile duct. Externally the gallbladder is surrounded by a serosa except where the gallbladder attaches to the liver; here the serosa is replaced by an adventitia.

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Vermiform appendix

The vermiform appendix is a tube-like appendage of the cecum (large intestine) in the right lower abdomen. It consists of the same histological layer arrangement as the entire intestine which consists of the mucosa, submucosa, muscular coat and the serosa.

The appendix is lined with single-layered columnar epithelium that has deep intestinal crypts but no villi. The lymphatic tissue is particularly prominent in the lamina propria and submucosa with many lymphoid follicles and large germinal centers. The crypts are particularly deep so that the follicles are in close contact with the intestinal lumen. The epithelium of the intestinal crypts is absent above the peaks of the lymph follicles. There it is replaced by specialized M cells (microfold cells).

The muscular coat of the vermiform appendix consists of an inner circular and an outer longitudinal muscle layer. In contrast to the cecum and colon, the longitudinal muscle layer of the appendix is continuous and not divided into teniae. Since the appendix is usually located intraperitoneally, it is surrounded by a serosa.

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