Adenoid, also known as the nasopharyngeal tonsil, is a mass of lymphoid tissue found in the posterosuperior wall of the nasopharynx. Adenoids, along with palatine and lingual tonsils, belong to mucosa associated lymphoid tissue (MALT) and constitute the major part of the Waldeyer’s ring. Located at the gateway of respiratory and alimentary tract, this ring of tissue is the first site of encounter with microorganisms and pathogens, therefore is considered responsible for immune protection. This article will focus mainly on the anatomy of the adenoids with their clinical relevance.
The nasopharyngeal tonsil originates from the pharyngeal endoderm in the posterior midline of the nasopharynx. The development begins during the late first trimester (third month) in association with mucus glands of the pharynx and is completely formed by the seventh month of gestation. The size of the tonsil increases during the early years of childhood, but then usually begins to atrophy at the end of first decade of life.
The adenoid is a pyramidal shaped structure composed of lymphoid tissue. The apex of this pyramid is extended toward to nasal septum, and the base sits at the posterior most wall of the nasopharynx. The adenoidal surface is inavginated by number of folds with some crypts. There is a midline pharyngeal bursa (bursa of Luschka) which extends posteriorly and superiorly. This bursa represents the site of notochordal attachment to the endoderm of the primitive pharynx.
Microscopically, the adenoid is covered by the same epithelium as the respiratory tract i.e. pseudostratified ciliated columnar epithelium. The epithelium covers the organ laterally and inferiorly with some small scattered patches of non keratinised stratified squamous epithelium. This epithelium also lines the series of mucosal folds. Between the upper surface and the adjacent sphenoid and occipital bone there is a layer of connective tissue (hemi-capsule) made up of reticular fibers. This connective tissue covering sends septa inside the lymphoid parenchyma and subdivide it into 4-6 lobes. Many sero-mucous glands are present within the connective tissue with their ducts extending through the lymphoid parenchyma.
The adenoid is supplied by the:
- glossopharyngeal nerve via the pharyngeal plexus
- posterior palatine branch of the maxillary nerve
- fibers from the lingual branch of the mandibular nerve
The lymphatic drainage of the adenoids is to the pharyngomaxillary space and retropharyngeal lymph nodes.
The adenoids receive their blood supply from 6 arteries:
- Ascending pharyngeal artery: This artery arises from the inferior part of the external carotid artery. It has anteriorly directed pharyngeal branches which supply the pharynx and associated structures.
- Ascending palatine artery: This is a branch of the facial artery, and passes superiorly between the styloglossus and stylopharyngeus muscles. In the superior part of its course, it passes between the superior constrictor of the pharynx and the medial pterygoid muscle.
- Tonsillar branch of the facial artery: This branch emerges from between the internal pterygoid muscle and the styloglossus muscle. The artery will also continue to enter the palatine tonsil and posterior most section of the tongue.
- Pharyngeal branch of the maxillary artery: The maxillary artery is the seventh branch of the external carotid artery. The pharyngeal branch is a branch from its third section.
- Artery of the pterygoid canal: This is another branch of the third section of the maxillary artery, and supplies the superior part of the pharynx and the auditory tube.
- Basisphenoid artery, branch of the inferior hypophyseal arteries, supplies the bed of the adenoid
The venous drainage is via the internal submucous and external pharyngeal venous plexus. After emerging from the lateral surface of the tonsils, draining veins join the paratonsillar veins which pierce the superior constrictor to join the pharyngeal venous plexus. They may also drain into the internal jugular and facial veins.