The oropharynx is the smallest and central part of the pharynx.
Please note that there seems to be a great deal of conflict over the borders of the oropharynx, between different doctors and different textbooks.
The most reliable boundaries I have found are:
- The inferior extent of the uvula
- The palatoglossal fold
- The superior extent of the epiglottis
- Whether this makes the vallecular part of the oropharynx or laryngopharynx is tricky, I'm inclined to say oropharynx because the posterior aspect of the tongue is usually considered with the oropharynx.
The oropharynx (including tonsils and arches) is covered by mucous membrane of a pink-red colour. The tonsils are distinguishable by a pitted appearance and vary greatly in size between different people and with age.
The oropharynx communicates with the nasopharynx through the pharyngeal isthmus, extending posteriorly from the posterior edge of the soft palate. It communicates with the oral cavity anteriorly through the oropharyngeal isthmus, demarcated by the palatoglossal arch, and includes the posterior aspect of the tongue. It communicates with the laryngopharynx, whose boundary is demarcated by the upper border of the epiglottis.
The lateral walls of the oropharynx are distinct for the presence of two archs. The palatoglossal arch, anteriorly, contains the palatoglossus muscle and forms the anterior border of the oropharynx. Behind this arch is the palatopharyngeal arch, containing palatopharyngeus. The space between these two arches is the tonsillar fossa. The volume containing the arches and tonsillar fossa bilaterally is known as the fauces.
The soft palate forms the boundary between the nasopharynx and oropharynx. It is attached laterally to the walls of the nasopharynx / oropharynx. The anterior part is continuous with the hard palate. The posterior border of the soft palate hangs freely and is distinguished by a conical inferior projection, the uvula. The soft palate curves inferiorly as it passes posteriorly.
The soft palate is covered on all surfaces by epithelium, which is of the respiratory pseudostratified type superiorly and stratified squamous inferiorly. A large submucosal layer contains many glands. Centrally, several muscle attachments and the palatine aponeurosis give form to the palate. The palatine aponeurosis is formed by the tendons of tensor veli palatini, and is attached to the palatine bone anteriorly. It is thicker anteriorly and all other palatal muscles attach to it.
The palatine tonsils are paired structures located in the tonsillar fossa, between the two arches of the oropharynx. They are ovoid in shape, usually largest in childhood, before involuting in middle age.
The tonsils rest on the tonsillar hemicapsule, a fibrous layer which is distinct from the underlying pharyngobasilar fascia and superior constrictor.
The tonsillar fossa is the space between the two arches of the oropharynx. It contains the palatine tonsil, tonsillar hemicapsule as well as the mucous membrane and underlying tissues. The floor of the tonsillar fossa is formed by the pharyngobasilar fascia and the superior constrictor.
The tonsillar fossa is important for its close relations to vascular and neural structures. From superior to inferior, these are the facial artery and tonsillar branch, the external palatine vein, glossopharyngeal nerve and the pharyngeal plexus of veins. The descending palatine vein runs vertically in the posterior part of the fossa.
The styloglossus and hyoglossus lie deep to the pharyngobasilar fascia and superior constrictor.
The epithelium of the oropharynx (including soft palate and over the tonsils) is of a non-keratinising, stratified squamous type. Muscosal and submucosal glands are frequently seen, particularly in the inferior aspect of the soft palate (below the palatal aponeurosis).
The tonsils are characterised by an outer surface of non-keratinising stratified squamous epithelium with 10 – 20 crypts which penetrate the its substance. The connective tissue forms septa, and is continuous with lamina propria of the mucosa and the underlying pharyngobasilar fascia. The crypts are lined with a mix of non-keratinising stratified squamous epithelium as well as reticulated epithelium. The latter epithelium contains mesh-like cells through which cells of the immune system are able to gain access to the contents of the crypts. The crypts are typically clogged with bacteria, saliva or other material.
The lymphoid tissue of the tonsil is divided into numerous categories. Lymphoid follicles are located along the connective tissue septa. Numerous IgA producing B-lymphocytes as well as T-lymphocytes and antigen presenting cells are located amongst the reticular epithelium.
Swallowing is a major function of the oropharynx.
The first phase is voluntary, where the tongue pushes a bolus of food posteriorly into the oropharynx. The bolus is held in the space between the epiglottis, soft palate and base of tongue before the second phase begins.
The second phase is rapid and involuntary. It involves elevation of the soft palate and widening of the oropharynx / laryngopharynx. The suprahyoid and longitudinal pharyngeal muscles contract, elevating the larynx. These movements effectively occlude the nasopharynx at the pharyngeal isthmus, the oral cavity at the oropharyngeal isthmus, and the larynx at the laryngeal inlet.
The third phase is also involuntary, involving sequential contraction of the constrictor muscles from superior to inferior. This pushes the bolus of food into the oesophagus.
Tonsillectomy involves removing the palatine tonsils. The tonsillar hemicapsule is divided from the underlying constrictor muscles and pharyngobasilar fascia. Vessels are ligated as the hemicapsule is removed.
Arteries to the oropharynx are similar to those for the rest of the pharynx. Blood is sourced from branches of the external carotid artery, particularly the ascending pharyngeal and ascending palatine arteries.
Blood to the tonsil travels in the tonsillar artery, a branch of the facial artery. A small amount of supply is derived from the dorsal lingual arteries and a branch of the ascending palatine artery.
Venous blood from the oropharynx collects in a plexus external to the pharynx. Pharyngeal veins usually drain directly into the internal jugular, or alternatively to the maxillary or facial veins.
Veins from the soft palate usually pass laterally to empty into the pterygoid venous plexus.
Lymphatic vessels arising from the palatine tonsils pass through the superior constrictor and either terminate directly into upper deep cervical nodes (II) or retropharyngeal nodes (which then usually direct lymph to level II or III). Lymph from the remainder of the pharynx follow a similar path. Lymph from the soft palate may drain bilaterally to submandibular or upper deep cervical nodes.
The posterior aspect of the tongue, frequently considered part of the oropharynx, drains bilaterally to upper deep cervical nodes (level II). Invasion of this structure by tumours of the pharynx requires consideration of bilateral neck irradiation.
The oropharynx has branchial motor, visceral sensory and special sensory innervation.
Branchial Muscles of the Soft Palate and Oropharynx
The muscles of the oropharynx are innervated by somatic branchial motor fibres from the vagus nerve (X), aside from stylopharyngeus which is supplied by the glossopharyngeal nerve (IX).
The branchial muscles of the soft palate also receive somatic branchial motor supply from the vagus nerve, except for tensor veli palatini which is supplied by a branch of the maxillary division of the trigeminal nerve.
Branches from the pterygopalatine ganglion innervate small salivary glands of the soft palate and oropharynx. These initially arise from the greater petrosal nerve, a division of the facial nerve (VII).
Visceral sensory supply to the oropharynx (posterior and lateral walls, base of tongue) is also a function of the vagus nerve, except for the posterior third of the tongue which receives visceral supply from the glossopharyngeal nerve (IX).
Taste receptors in the soft palate are innervated by the vagus nerve. In the posterior third of the tongue, this function is performed by the glossopharyngeal nerve.
Routes of Cancer Spread
Oropharyngeal tumours may have extensive local invasion into surrounding structures. This may include the laryngopharynx, oral cavity or nasopharynx. Alternatively, lateral spread into tissue deep of the tonsillar fossa can lead to impingement of the glossopharyngeal nerve (IX).
More extensival local invasion (stage T4) includes invasion of the palatine bone or mandible, larynx, pterygoid muscles. Tumour may encase the carotid artery (Stage T4b).
The most commonly involved lymph nodes in oropharyngeal carcinoma are the retropharyngeal nodes and the superior deep cervical nodes (up to 75% of cases). Level III and IV nodes are also involved with greater frequency than Level I or V.
Distant haemotogenous spread of malignancy is uncommon in the early stages of oropharyngeal carcinoma, but generally affects the lungs if it occurs.
The oropharynx is innervated extensively by visceral sensory and motor nerves arising from cranial nerves IX and X. Cancer may infiltrate these nerves and pass posteriorly into the posterior cranial fossa.