Histology of the Upper Digestive Tract
The body is able to absorb and utilize nutrients thanks to the actions of the digestive tract. The digestive tract is a tube-like muscular apparatus that responds to both sympathetic and parasympathetic stimulation in order to alter the rate at which food is processed. This structure commences at the oral cavity, travels through the thoracic and abdominal cavities and terminates at the anus in the pelvic cavity. Essentially, the system reduces macroscopic food particles to their functional components that can be absorbed into, and delivered by, the vascular system.
The tract has been structurally divided into an upper and a lower portion. The upper digestive tract extends from the oral cavity to the ligament of Treitz (suspensory muscle of duodenum) at the duodenojejunal flexure. The lower digestive tract continues from this point to the anus. This boundary helps clinicians to classify pathologies of the digestive tract as either upper or lower digestive anomalies. Furthermore, there are organs that aid in digestion but are situated outside of the alimentary canal. These organs – salivary glands, pancreas, gallbladder and the liver – form the accessory digestive system. The histology of the upper digestive tract (excluding the duodenal histology) will be addressed in this article. The duodenum and remaining lower digestive tract will be addressed in subsequent articles.
Oral Vestibule & Oral Cavity Proper
The oral cavity is the point at which large pieces of food are broken down into smaller, ingestible particles. The oral vestibule (bordered anteriorly and laterally by the inner surface of the lips and cheeks, and posteriorly and medially by the teeth and gingiva) is lined by non-keratinized (NK) stratified squamous epithelium (oral mucosa).
The floor of the oral cavity proper (bounded anteriorly and laterally by the teeth and gingiva, and posteriorly by the oropharyngeal isthmus and oropharynx), the inner surface of the lips and the ventral surface of the tongue also share this kind of epithelium. The hard palates and soft palates (forming the roof of the oral cavity proper) are lined by partially keratinized oral mucosa. There are also taste buds found on the oral surface of the soft palate.
The outer surface of the lips has a very thin cutaneous covering that is made up of keratinized stratified squamous epithelium with hair follicles as well as sweat and sebaceous glands. The vascular supply of the lips is very superficial and gives the lips their characteristic pinkish-red color. The transition from thin keratinized outer layer to the thicker NK inner layer of the lips can be observed histologically.
The core of the lips also has skeletal muscle fibers form orbicularis oris. Labial glands that produce mucus fluids are found deep to the mucous layer of the lips. There are also Krause receptors and Meissener’s receptors present in the dermal layer of the lips. Respectively, they are responsible for thermal and somatic perception.
The tongue is formed from a mixture of skeletal muscle and connective tissue fibers. On the dorsal surface, it has a wide distribution of papillary folds anteriorly, and lymphatic tissue posteriorly. The presence of papillary folds and an irregular distribution of keratinized oral mucosa on the dorsum of the tongue give rise to a significantly rougher surface when compared with its ventral surface. There are four types of papillae that are found on specific parts of the tongue. They are:
- The filiform papillae are narrow, conical papillae that are the smallest of the papillae. They are the most abundant of the papillae and are distributed across the entire dorsal surface of the tongue
- Fungiform papillae are numerically fewer, but structurally larger, broader and taller than the filiform papillae. These papillae have a mushroom like appearance and occupy the anterior region of the tongue.
- The largest of the three papillae has the smallest distribution on the dorsal lingual surface. Eight to twelve circumvallate papillae are found on the posterior part of the tongue. They are surrounded by trenches or furrows into which the ducts of serous von Ebner’s glands drain.
- Finally, the foliate papillae are primitive and not well developed in humans.
The ability to detect taste is made possible by specialized neuroepithelial cells found within barrel-shaped taste buds that span the full thickness of the epithelium. The taste buds are distributed along the surfaces of the aforementioned papillae, soft palate, epiglottis, and pharynx. Food particles come into contact with the microvilli of the neuroepithelial cells by route of the apical opening of the taste buds called taste pores. Additionally, there are non-sensory supporting sustentacular cells and undifferentiated basal stem cells present within the taste buds. The latter is believed to differentiate into, and replenish, both neuroepithelial and sustentacular cells.
Oropharynx & Laryngopharynx
The middle and lower pharyngeal regions are equipped with NK stratified squamous epithelium to assist in protecting the mucosa from friction associated with swallowing. The core of these segments also contains skeletal muscle fibers of the pharyngeal constrictors. Additionally, there are fibrous connective tissue fibers associated with the buccopharyngeal and prevertebral fascial layers that are deep to the pharyngeal constrictors. The epiglottis, which forms part of the anterior wall of the laryngopharynx during swallowing, has elastic cartilage at its core and oral mucosa as its covering.
The esophagus can be described as a tubular structure commencing at the level cricoid cartilage and ending at the cardiac sphincter of the stomach. Viscerally, the esophagus is covered by NK stratified squamous epithelium. There are esophageal cardiac glands and esophageal glands, present in the lamina propria and submucosa (respectively), near the cardiac sphincter that secrete mucus to protect the epithelium from the stomach acids.
There is a transition from purely skeletal muscles, to a mixture of smooth and skeletal muscles, then finally purely smooth muscles from the proximal to distal portion of the esophagus. The muscle fibers are arranged as an inner circular and an outer longitudinal layer, which give rise to longitudinal folding of the esophagus when there is no bolus present in the esophagus. Parietally, the esophagus is covered by connective tissue adventitia in the thorax and simple squamous mesothelium in the abdomen.
The demarcations of the stomach are the gastroesophageal junction proximally, and the pyloroduodenal junction distally. Although the stomach is a continuation of the alimentary canal, its epithelial lining changes precipitously from NK stratified squamous epithelium in the oesophagus to simple columnar epithelium in the stomach.
There are gastric glands that reside in the lamina propria and release their acidic contents into the gastric pits of the visceral surface of the stomach. There are three muscular layers present in the stomach: an inner oblique, a middle circular and an outer longitudinal layer. This arrangement facilitates the mixing of the bolus with gastric juices in the stomach to form chyme (semi fluid mixture of partly digested food passed into the duodenum). When void of food or fluids, the lining of the stomach has significant folding called rugae.
Although there are four segments in the stomach (cardia, fundus, body and pylorus), there are only three histologically distinct parts because the fundus and body are histologically identical. The cardia is the most proximal part of the stomach, while the pylorus is the most distal. Both these regions have deeper gastric pits than the body and fundus. Their glands secrete mainly protective mucus fluids.
In addition to the lymphatic aggregates at the base of gastric glands in the mucosa, there are three types of cells in the gastric glands in the fundus and body that aid digestion. They are:
- Pale-staining mucous neck cells at the base of the gastric pits,
- Eosinophillic parietal cells in the neck of the gastric glands, that produce hydrochloric acid gastric intrinsic factors to aid in digestion and vitamin B12 absorption, respectively
- And basophillic zymogenic cells in the base of the gastric glands, that stores and releases pepsinogen, which is converted into pepsin.