Pharyngeal tonsil

Location and relations

The pharyngeal tonsil is a midline pharyngeal structure, located posterior to the soft palate. It is shaped like a pyramid with an apex and a base. The apex is directed towards the nasal septum and nasal choana, and the base rests on the nasopharyngeal wall at the junction between the roof and posterior wall of the nasopharynx. Lateral to the pharyngeal tonsil are the tori tubarii, cushions that surround the auditory tube (Eustachian tube).

The free surface of the pharyngeal tonsil has folds and fissures which surround a median blind recess known as the pharyngeal bursa (Bursa of Luschka), a remnant of the rostral end of the notochord.

Histology

Microscopically, the pharyngeal tonsil is similar to the palatine tonsil, consisting of parenchyma with diffuse lymphoid tissue and lymphoid nodules, some of which may have germinal centers which are pale in the middle with dark margins. The tonsil also contains seromucous glands, whose ducts travel through the parenchyma to reach the surface. The secretions of these glands keeps the surface moist, possibly aiding with antigen capture.

However, the pharyngeal tonsil differs from the palatine tonsil in a few aspects:

• It has few to no crypts, but is instead invaginated by shallow folds known as pleats.
• It has a thin connective tissue hemicapsule that separates the superior surface from the sphenoid and occipital bones. Reticular connective tissue fibers from the capsule divide the tonsil into 4 to 6 lobes.
• It has pseudostratified ciliated columnar epithelium (similar to respiratory epithelium) which lines the folds, with only scattered patches of non-keratinised stratified squamous epithelium.

Try and identify the microscopic structures of the pharyngeal tonsil with this quiz!

Arterial supply

The pharyngeal tonsil is supplied by 6 blood vessels, which include the:

• Ascending pharyngeal artery
• Ascending palatine artery
• Pharyngeal branch of the maxillary artery
• Tonsillar branch of the facial artery
• Artery of the pterygoid canal
• Basisphenoidal artery (branch of the inferior hypophyseal artery)

Venous drainage

The pharyngeal tonsil is drained by a large number of veins, which emerge from the surface and usually drain into the pharyngeal venous plexus, which in turn empties into the internal jugular veins.

Lymphatic drainage

The efferent lymphatics from the parenchyma drain into lymph nodes in the parapharyngeal space and the retropharyngeal nodes, ultimately reaching the upper deep cervical lymph nodes.

Nerve supply

The pharyngeal tonsil is innervated by the pharyngeal plexus, receiving fibers from the:

• Glossopharyngeal nerve (CN IX)
• Vagus nerve (CN X)

The glossopharyngeal nerve is the ninth of 12 cranial nerves. Refresh and solidify your knowledge of all 12 nerves with our cranial nerve quizzes and labeling exercises!

Variations

During childhood, the pharyngeal tonsil grows rapidly, reaching the largest size at around 5 years of age. It begins to undergo involution from 8 to 10 years of age, slowly atrophying and is generally hypoplastic or absent in adults.

The pharyngeal tonsil is a single structure, however it is commonly pluralized as “adenoids”. Occasionally, the tonsil can be split into two by a fissure that extends from the pharyngeal bursa, or by a fold in the midline extending from the bursa to the nasal septum.

Functions

The pharyngeal tonsils are placed at the entrance to the respiratory and gastrointestinal tracts, conducive to receiving antigens entering from the external environment via inhalation and ingestion. The oral cavity and pharynx contain an incomplete tonsillar ring, known as the pharyngeal lymphoid ring (Waldeyer’s ring). This includes:

• The lingual tonsils (anteroinferiorly)
• The palatine tonsils (laterally)
• The tubal tonsils (laterally)
• The pharyngeal tonsil (posterosuperiorly)

These lymphoid structures form a part of the mucosa-associated lymphoid tissue (MALT). They are involved in primary and secondary immune responses, depending on whether the antigen has been encountered previously or not. This process involves antigen presenting cells, T lymphocytes, B lymphocytes and immunoglobulin production.

Don’t stop here, learn about the other lymphoid organs with these study units!